Marine Molluscs from Two Contrasted Algal Habitats in Tossa de Mar (Costa Brava, Ne Spain)

1. Introduction

In the upper infralittoral of all seas and oceans, seaweeds are essential components for the development of animal populations, particularly invertebrates, which find protection, shelter, food and reproductive sites [1-5]. In some cases, they even form true mutualistic symbiotic relationships between algae and animals [6].

In the Mediterranean Sea, many studies emphasize the structural complexity of the various habitats found there, which is why comparisons between different communities and their associated fauna are necessary. Focussing on mollusks in shallow marine environments, Chemello et al. [7] noted that the spatial distribution of the malacological fauna associated with the photophilous algal communities is influenced by factors such as depth and the algal substrate. However, others, like García et al. [8], observe a more homogeneous distribution of gastropod species across all types of algae and habitats, except for certain groups like Sacoglossa gastropods, which are associated with specific species of Caulerpa algae, such as Berthelinia caribbea Edmunds, 1963.

Avila et al. [9] highlight that the floristic composition and algal biomass are crucial in determining the distinctiveness of the micromollusk community, as in the intertidal zone of the Azores, where these factors provide protection from waves, high temperatures, and desiccation. The so-called algal architecture is also critical for the settlement and abundance of epibiont marine invertebrate species, particularly mollusks [10, 11] Various sources also report behavioral patterns shifts and the advantages smaller species have over larger adults when living among algae fronds. The behavioral changes observed by Terlizzi et al. [12] show that the species within the same genus can exhibit differences in juvenile abundance due to differences in their life cicles. This is exemplified by Bittium latreillii (Payraudeau, 1826) and Bittium reticulatum (da Costa, 1778), where the former has an 18 month life cycle with semelparous reproduction, recruiting in October, February and June, while the latter shows a single recruitment peak in May, reflected in its abundance throughout the year. On the other hand, smaller species benefit from reduced dispersal capacity [9] and the protection offered by the interstices within algal fronds [8].

It is well known that infralittoral algal communities can be classified according to hydrodynamism (calm or turbulent) and the light they receive (sciophilous or photophilous). In the Mediterranean Sea, typical algal species that characterize sciophillous habitats in the upper infralittoral zone include, among others, Flabellia petiolata (Turra) Nizamuddin, 1987, Halopteris filicina (Grateloup) Kützing, 1843, Peyssonnelia rubra (Greville) J .Agardh, 1851 and Halimeda tuna (J.Ellis & Solander) J.V.Lamouroux, 1816. Photophilous habitats are mainly defined by the presence of algae such as Halopteris scoparia (Linnaeus) Sauvageau, 1904, Padina pavonica (Linnaeus) Thivy, 1960, Dictyota dichotoma (Hudson) J.V.Lamouroux, 1809 and Corallina officinalis Linnaeus, 1758, the latter being more closely studied in relation to malacological fauna.

In recent dècades, several studies in the Mediterranean Sea have explored the relationship between mollusks and different types of algae. Por example on calcareous algae [13, 14]; on photophilous algae [7, 10, 15, 16]; on Halopteris scoparia (Linnaeus) Sauvageau, 1904 [17]; on Cystoseira [18]; or on the malacofauna of Posidonia oceanica meadows [19], among others.

Tossa de Mar (Girona, NE Spain) is a tourist town on the Spanish Costa Brava very popular among scuba divers. From the beach, divers can access rocky and sandy seabeds, caves and Posidonia oceanica meadows, all teeming with marine biodiversity.

This study aims to contribute to the knowledge of the algal-associated marine malacofauna in this locality. Specifically we aim to determine the role of micromollusks and juvenile forms of larger species in algal communities. We also assess whether significant differences exist in the biodiversity and abundance of mollusk populations between two contrasting algal communities -one photophilous and the other sciophilous- located on two nearby rock walls. Lastly, we examine abundance and diversity patterns in these two contrasting environments over the course of an annual cycle.

2. Material and Methods

2.1. Study Area

The study was conducted in the town of Tossa de Mar, located in the La Selva region of Girona province, on the southern Costa Brava (NE Spain), approximately 100 km north of Barcelona. The sampling site was at the location known as “Sa Banyera de ses Dones” (Figure 1) situated at the northern end of La Mar Menuda beach. This site features a rocky corridor with a thick sandy bottom flanked by nearly vertical rock walls that are about 10 m apart. The bathymetric levels of these walls range from 2 to 11 meters in depth. The southern wall is vertical and faces northwest, giving it distinctly sciophilic characteristics. In contrast, the northern wall has a slope of about 70º and faces southeast, allowing it to receive direct sunlight, which results in a community dominated by photophilous algae (Figure 2).

2.2. The Studied Communities

At depths between 1,5 and 3m, the sciophilous wall is dominated by photophilous algae, including Osmundea pinnatifida, Ellisolandia elongata (J.Ellis & Solander) K.R.Hind & G.W.Saunders, 2013, Padina pavonica and some thalli of Codium vermilara (Olivi) Delle Chiaje, 1829. Additionally the community includes the unicellular green alga Acetabularia acetabulum (Linnaeus) P.C.Silva, 1952, cirriped crustaceans of the genus Semibalanus balanoides (Linnaeus, 1767), sea urchins (Arbacia lixula (Linnaeus, 1758)) and sponges such as Sarcotragus fasciculatus (Pallas, 1766). At depths of 3 to 4 meters, the wall becomes slightly more sciophilic (Figure 3B) with abundant fronds of Halopteris scoparia, the red algae Peyssonnellia squamaria (S.G.Gmelin) Decaisne ex J.Agardh, 1842, and the brown algae Dictyota dichotoma. Here, red encrusting sponges (Crambe crambe Schmidt, 1862) and some Sarcotragus fasciculatus are also observed. From 5 to 6 m. (Figure 3C) calcareous algae such as Mesophyllum lichenoides (J. Ellis) Me. Lemoine, 1928, and the red alga Sphaerococcus coronopifolius Stackhouse, 1797, as well as hydrozoans of the genus Eudendrium, become abundant. Scattered thalli of Halimeda tuna (J. Ellis & Solander) J.V. Lamouroux, 1816, along with the red algae Halopteris scoparia and Peyssonnelia squamaria (S. G. Gmelin) Decaisne ex J. Agardh, 1842, some thalli of Codium bursa (Olivi) C. Agardh, 1817, the sponge Crambe crambe, and the zoantharian cnidarian Parazoanthus axinellae (Schmidt, 1862) are also present. From depths of 7 to 8 m down to the bottom of the wall (11 m) (Figure 3D) the alga Flabellia petiolata predominates, with abundant polychaete spirorbids growing on its fronds. The community also includes encrusting calcareous algae, Peyssonnelia squamaria, Parazoanthus axinellae, the sponges Phorbas tenacior (Topsent, 1925), Petrosia (Petrosia) ficiformis (Poiret, 1789), Chondrosia reniformis Spikenard, 1847 and Clathrina clathrus (Schmidt, 1864), as well as serpulid polychaetes like Serpula vermicularis Linnaeus, 1767.

The photophilous wall exhibits a fairly homogeneous algal community, with few macroscopic invertebrates observed. At depths between 2 and 4 meters, the wall is vertical and dominated by photophilous algae in a wave-beaten mode (Figure 4A), primarily Osmundea pinnatifida, Acetabularia (Acetabularia) acetabulum, filamentous brown algae, Corallina elongata and Padina pavonica. Among these algae, numerous specimens of the cirriped crustacean Semibalanus balanoides (Linnaeus, 1767) can be found. From 4 meters to the bottom (11 meters), the wall becomes slightly inclined and is nearly completely covered by photophilous algae, with abundant thalli of Padina pavonica and Halopteris scoparia (Figure 4D), Acetabularia (Acetabularia) acetabulum (Figure 4B). Scattered specimens of Codium bursa, Dictyota, thalli of Sphaerococcus coronopifolius, and encrusting red algae are present beneath the high thallus algal mat. There are also several isolated thalli of Codium vermilara (Figure 4C). Among the macroscopic fauna, sea urchins (Arbacia lixula), sponges (Sarcotragus fasciculatus) and some polychaete spirographs (Sabella spallanzanii (Gmelin, 1791)) can be observed. Additionally, there are also small sciophilous enclaves featuring Peyssonnelia squamaria, the serpulid polychaete Protula tubularia (Gmelin, 1791), the sponge Aplysina aerophoba (Nardo, 1833) and the equiurid Bonellia viridis Rolando, 1822.

2.3. Sampling and Processing of Samples

Sampling was conducted every two months from January to November 2011 by two divers using scuba gear. Underwater photographs of the main communities present on both walls were taken with an Olympus SP-350 camera inside a waterproof housing.

Algal samples were collected by scraping the existing algal community inside a metal square measuring 20 x 20 cm. During each sampling dive, three replicates were randomly collected from each wall, (photophilous and sciophilous. Samples were introduced in plastic bags in situ. After surfacing and draining most of the water, 70% alcohol was added to each bag. In total, 36 samples were obtained, with 18 corresponding to the photophilous wall (6 samples x 3 replicates) and another 18 to the sciophilous wall (6 samples x 3 replicates).

2.4. Work in the Laboratory

In the laboratory, each sample was washed under running water using two metal filters with mesh sizes of 2 mm and 315 µm. This process allowed the separation of the algal and invertebrate specimens into two fractions: the coarse fraction and the fine fraction respectively. From these two fractions, the mollusk specimens were separated from the other invertebrates using a Carl Zeiss binocular magnifying glass model GMbH STEMI 2000-C.

Next, the mollusk specimens from both fractions were sorted by morphotype in plastic classification racks for subsequent counting and taxonomic identification up to the species or higher level. Various general identification guides were employed [21-27] along with specialized literature for identifiying juvenile mollusk stages [28] For the current taxonomy and nomenclature of the species, the recommendations of World Registrer of Marine Species [29] were generally followed.

Specimens that posed challenges in their taxonomic determination or were unusual were photographed and preserved in Eppendorf tubes for identification by specialists. However, some specimens (especially bivalves) were only identified at the class level due to their early development stages.

2.5. Data Analysis and Statistical Methods

Species abundance data were expressed as number of individuals per replicate sample (no. individuals / 400 cm²). Species richness (S), total abundance of individuals (N), Shannon-Wiener diversity index (H‘) and Pielou evenness (J’) were calculated for each sampling site. For diversity and evenness, permutation tests (within the PERMANOVA framework) were performed to determine whether there were significant differences between habitats, fractions and months.

To analyse and compare community composition in relation to habitat, fraction, and seasonal variation, a proximity matrix was constructed using Bray-Curtis similarity. Prior to this, a fourth root transformation was applied to reduce the influence of extreme values and obtain less asymmetric distributions of abundances. PERMANOVA designs were then used to assess significant differences based on habitat, fraction, and months. Two-dimensional representations using the NMDS technique (Kruskal's algorithm) were created to visualise relationships across habitats, fractions and months.. To identify potential clusters, dendrograms were constructed using the UPGMA method, and significant branches were tested with the SIMPROF test (α=5%). The clusters identified through this methodology were then represented as ellipsoids on NMDS plots. Finally, species abundance data were grouped by trophic level, and some of the above techniques were re-applied.

All analyses were carried out using PRIMER 7 with PERMANOVA (Plymouth Routines In Multivariate Ecological Research).

3. Results

3.1. General Abundance and Specific Richness

A total of 7,913 mollusc specimens, representing 148 different species, were counted across all samples. Of these, 32 belong to the class Bivalvia, 112 to Gastropoda, and 4 to Polyplacophora. Some specimens, due to their small size and immaturity, were identified only to the genus level, higher taxon or simply as morphotypes. Table I lists the species found in the samples, ordered by total abundance, along with their abundance in the two environments (photophilous and sciophilous) and their trophic categories. Supplementary Table I shows the species’ abundance according to their presence in the two studied fractions, fine and coarse. Regarding the two habitats, the photophilous environment proved to be the most abundant, with 6,244 mollusk specimens (6,017 in fine fraction and 227 in coarse fraction), while the sciophilic algae environment contained 1,669 specimens (1,559 in fine fraction and 110 in the coarse fraction). In terms of specific richness, the photophilous environment again stood out, with 133 different species (105 in the fine fraction and 58 in the coarse fraction), compared to 84 species in the sciophilous environment (62 in the fine fraction and 38 in the coarse fraction). Of the total number of species, 15 were exclusive to the sciophilous environment, 65 were found only in the photophilous environment, and 68 were present in both environments. Of all species, 35 were found only in the coarse fraction, 76 only in the fine fraction, and 37 species were present in both fine and coarse fractions. Among the mollusks of the fine fraction, 57 species were found in both the photophilous and sciophilous samples, 50 species were exclusive to the photophilous environment, and 6 were exclusive to the sciophilous environment. In the coarse fraction, 25 species were present in both sciophilousenvironments, 33 were found only in the photophilous samples, and 12 species appeared only in sciophilous samples. A total of 72 species were identified in the coarse fraction samples, and 111 in the fine fraction.

Table 1. List of species according to their abundances and trophic categories. “C”: carnivore; “EC”: ectoparasite; “F”: filter feeder; “HE”: herbivore; “SC”: scavenger; “SDF”: surface depot feeder; “UN”: unknown. “Sc”: sciophilous environment; “Ph”: photophilous environment. Note: Some authors classify the species from the Pyramidelloidea group as ectoparasites of echinoderms or other invertebrates, while others consider them microcarnivores. In this study, they are classified as ectoparasites.

Species	Trophic categories	Total abundances	Sc	Ph
Bittium latreillii	HE	2106	55	2,051
Musculus costulatus	F	1282	486	796
Pusillina philippi	C	817	272	545
Pusillina radiata	F	546	179	367
Hiatella arctica	HE	404	162	242
Rissoella diaphana	HE	228	0	228
Odostomia striolata	HE	217	50	167
Alvania lineata	HE	176	49	127
Chauvetia mamillata	C	130	22	108
Parvicardium scriptum	F	130	29	101
Triphoridae indet.	C	130	16	114
Bittium reticulatum	HE	96	10	86
Clanculus cruciatus	HE	88	8	80
Striarca lactea	F	88	13	75
Tritia incrassata	SC	80	20	60
Ammonicera fischeriana	HE	79	13	66
Modiolula phaseolina	F	67	11	56
Vitreolina perminima	C	65	6	59
Jujubinus ruscurianus	HE	62	13	49
Mytilus galloprovincialis juv.	F	57	11	46
Crisilla semistriata	HE	53	29	24
Cerithium vulgatum	SDF	41	2	39
Parvamussium fenestratum	F	41	22	19
Aplysia punctata juv.	HE	40	10	30
Arca noae	F	40	1	39
Runcina sp.	HE	39	4	35
Aequipecten opercularis	F	36	23	13
Parthenina emaciata	C	36	0	36
Alvania cancellata	HE	35	3	31
Runcina adriatica	C	30	3	27
Cyrillia linearis	C	29	1	28
Cerithiopsis tubercularis	C	25	2	23
Limaria tuberculata	F	24	13	11
Irus irus	F	22	5	17
Mimachlamys varia	F	22	13	8
Scissurella costata	HE	22	1	21
Gibberula turgidula	C	20	0	20
Odostomia plicata	EP	20	11	9
Tricolia sp.	HE	20	3	17
Chauvetia brunnea	C	17	9	8
Cerithiopsis jeffreysi	C	16	0	16
Gregariella semigranata	F	16	8	8
Mitromorpha columbellaria	C	16	5	11
Ocinebrina aciculata	C	16	2	14
Odostomella doliolum	EC	16	1	15
Bulla striata	C	15	3	12
Setia pulcherrima	HE	15	5	10
Haminoea navicula	HE	13	0	13
Petricola lithophaga	F	13	0	13
Chama gryphoides	F	12	8	4
Rissoa lia	HE	11	1	10
Monophorus erythrosoma	C	10	2	8
Alvania nestaresi	HE	9	0	9
Bittium sp.	HE	9	0	9
Cerithiopsis sp.	C	9	0	9
Modiolus barbatus	F	9	1	8
Rissoa variabilis	HE	9	1	8
Bivalvia morph. 1	F	8	0	8
Doridina nudibranch	C	8	3	5
Hexaplex trunculus	C	8	4	4
Pusia tricolor	C	8	2	6
Raphitoma echinata	C	8	2	6
Anomia ephippium	F	6	4	2
Diodora graeca	HE	6	2	4
Marshallora adversa	C	6	2	4
Muricopsis cristata	C	6	4	2
Philine sp.	C	6	0	6
Pseudochama gryphina	F	6	3	3
Rhyssoplax corallina	HE	6	3	3
Alvania aspera	HE	5	0	5
Alvania sp.	HE	5	0	5
Gastropoda morph. 1	UN	5	0	5
Gibberula miliaria	C	5	0	5
Mitromorpha olivoidea	C	5	1	4
Tricolia tingitana	HE	5	0	5
Trinchesia genovae	C	5	0	5
Acanthochitona sp.	HE	4	0	4
Chauvetia recondita	C	4	0	4
Nodulus contortus	HE	4	0	4
Smithiela costulata	C	4	1	3
Spiralina alpinoligustica	EC	4	2	2
Acanthochitona fascicularis	HE	3	2	1
Aegires leuckartii	C	3	1	2
Bivalvia morph. 2	F	3	0	3
Gibberula morph. 1	UN	3	0	3
Haliotis tuberculata	HE	3	2	1
Lima lima	F	3	0	3
Limaria hians	F	3	2	1
Pusillina inconspicua	HE	3	0	3
Veneridae indet.	F	3	1	2
Bivalvia morph. 4	F	2	1	1
Brachystomia scalaris	EC	2	0	2
Calliostoma zizyphinum	C	2	0	2
Crisilla simulans	HE	2	0	2
Diodora italica	HE	2	2	0
Doris ocelligera	C	2	2	0
Emarginula pustula	C	2	1	1
Episcomitra cornicula	C	2	2	0
Folinella excavata	EC	2	0	2
Fusinus rudis	C	2	0	2
Gastropoda morph. 2	UN	2	0	2
Jujubinus exasperatus	HE	2	1	1
Polyplacophoran indet.	HE	2	0	2
Raphitoma laviae	C	2	0	2
Raphitoma morph. 1	C	2	0	2
Rissoa guerinii	HE	2	0	2
Weinkauffia turgidula	C	2	0	2
Alvania cimex	HE	1	1	0
Alvania discors	HE	1	0	1
Aplus dorbignyi	SC	1	0	1
Babelomurex cariniferus	C	1	1	0
Bivalvia morph. 3	F	1	0	1
Caloria elegans	C	1	1	0
Cardiidae morph. 1	F	1	0	1
Cardiidae morph. 2	F	1	0	1
Chauvetia morph. 1	UN	1	0	1
Chauvetia morph. 2	UN	1	0	1
Columbella rustica	HE	1	1	0
Doto sp.	C	1	1	0
Felimare sp.	C	1	0	1
Gari tellinella	F	1	0	1
Gastropoda morph 3	UN	1	1	0
Gibberula morph. 2	UN	1	0	1
Idas sp.	F	1	1	0
Ischnochitonidae indet.	HE	1	0	1
Lithophaga lithophaga	F	1	1	0
Mangelia multilineolata	C	1	0	1
Mangelia morph. 1	UN	1	0	1
Mangelia morph. 2	UN	1	0	1
Mitrella broderipii	C	1	0	1
Muricidae indet.	C	1	0	1
Nodulus spiralis	HE	1	0	1
Otina ovata	UN	1	0	1
Paradoris indecora	C	1	0	1
Patella sp.	HE	1	0	1
Philine intricata	C	1	0	1
Phorcus richardi	HE	1	0	1
Pseudomangelia vauquelini	C	1	0	1
Pusia ebenus	C	1	0	1
Raphitoma morph. 2	C	1	0	1
Retilaskeya horrida	C	1	1	0
Retusa mammillata	C	1	0	1
Runcina coronata	HE	1	0	1
Spondylus gaederopus	F	1	1	0
Talochlamys multistriata	F	1	0	1
Tarantinaea lignaria	C	1	0	1
Tectura virginea	HE	1	1	0
Williamia gussoni	HE	1	1	0

Table 1. List of species according to their abundances and trophic categories. “C”: carnivore; “EC”: ectoparasite; “F”: filter feeder; “HE”: herbivore; “SC”: scavenger; “SDF”: surface depot feeder; “UN”: unknown. “Sc”: sciophilous environment; “Ph”: photophilous environment. Note: Some authors classify the species from the Pyramidelloidea group as ectoparasites of echinoderms or other invertebrates, while others consider them microcarnivores. In this study, they are classified as ectoparasites.

Species	Trophic categories	Total abundances	Sc	Ph
Bittium latreillii	HE	2106	55	2,051
Musculus costulatus	F	1282	486	796
Pusillina philippi	C	817	272	545
Pusillina radiata	F	546	179	367
Hiatella arctica	HE	404	162	242
Rissoella diaphana	HE	228	0	228
Odostomia striolata	HE	217	50	167
Alvania lineata	HE	176	49	127
Chauvetia mamillata	C	130	22	108
Parvicardium scriptum	F	130	29	101
Triphoridae indet.	C	130	16	114
Bittium reticulatum	HE	96	10	86
Clanculus cruciatus	HE	88	8	80
Striarca lactea	F	88	13	75
Tritia incrassata	SC	80	20	60
Ammonicera fischeriana	HE	79	13	66
Modiolula phaseolina	F	67	11	56
Vitreolina perminima	C	65	6	59
Jujubinus ruscurianus	HE	62	13	49
Mytilus galloprovincialis juv.	F	57	11	46
Crisilla semistriata	HE	53	29	24
Cerithium vulgatum	SDF	41	2	39
Parvamussium fenestratum	F	41	22	19
Aplysia punctata juv.	HE	40	10	30
Arca noae	F	40	1	39
Runcina sp.	HE	39	4	35
Aequipecten opercularis	F	36	23	13
Parthenina emaciata	C	36	0	36
Alvania cancellata	HE	35	3	31
Runcina adriatica	C	30	3	27
Cyrillia linearis	C	29	1	28
Cerithiopsis tubercularis	C	25	2	23
Limaria tuberculata	F	24	13	11
Irus irus	F	22	5	17
Mimachlamys varia	F	22	13	8
Scissurella costata	HE	22	1	21
Gibberula turgidula	C	20	0	20
Odostomia plicata	EP	20	11	9
Tricolia sp.	HE	20	3	17
Chauvetia brunnea	C	17	9	8
Cerithiopsis jeffreysi	C	16	0	16
Gregariella semigranata	F	16	8	8
Mitromorpha columbellaria	C	16	5	11
Ocinebrina aciculata	C	16	2	14
Odostomella doliolum	EC	16	1	15
Bulla striata	C	15	3	12
Setia pulcherrima	HE	15	5	10
Haminoea navicula	HE	13	0	13
Petricola lithophaga	F	13	0	13
Chama gryphoides	F	12	8	4
Rissoa lia	HE	11	1	10
Monophorus erythrosoma	C	10	2	8
Alvania nestaresi	HE	9	0	9
Bittium sp.	HE	9	0	9
Cerithiopsis sp.	C	9	0	9
Modiolus barbatus	F	9	1	8
Rissoa variabilis	HE	9	1	8
Bivalvia morph. 1	F	8	0	8
Doridina nudibranch	C	8	3	5
Hexaplex trunculus	C	8	4	4
Pusia tricolor	C	8	2	6
Raphitoma echinata	C	8	2	6
Anomia ephippium	F	6	4	2
Diodora graeca	HE	6	2	4
Marshallora adversa	C	6	2	4
Muricopsis cristata	C	6	4	2
Philine sp.	C	6	0	6
Pseudochama gryphina	F	6	3	3
Rhyssoplax corallina	HE	6	3	3
Alvania aspera	HE	5	0	5
Alvania sp.	HE	5	0	5
Gastropoda morph. 1	UN	5	0	5
Gibberula miliaria	C	5	0	5
Mitromorpha olivoidea	C	5	1	4
Tricolia tingitana	HE	5	0	5
Trinchesia genovae	C	5	0	5
Acanthochitona sp.	HE	4	0	4
Chauvetia recondita	C	4	0	4
Nodulus contortus	HE	4	0	4
Smithiela costulata	C	4	1	3
Spiralina alpinoligustica	EC	4	2	2
Acanthochitona fascicularis	HE	3	2	1
Aegires leuckartii	C	3	1	2
Bivalvia morph. 2	F	3	0	3
Gibberula morph. 1	UN	3	0	3
Haliotis tuberculata	HE	3	2	1
Lima lima	F	3	0	3
Limaria hians	F	3	2	1
Pusillina inconspicua	HE	3	0	3
Veneridae indet.	F	3	1	2
Bivalvia morph. 4	F	2	1	1
Brachystomia scalaris	EC	2	0	2
Calliostoma zizyphinum	C	2	0	2
Crisilla simulans	HE	2	0	2
Diodora italica	HE	2	2	0
Doris ocelligera	C	2	2	0
Emarginula pustula	C	2	1	1
Episcomitra cornicula	C	2	2	0
Folinella excavata	EC	2	0	2
Fusinus rudis	C	2	0	2
Gastropoda morph. 2	UN	2	0	2
Jujubinus exasperatus	HE	2	1	1
Polyplacophoran indet.	HE	2	0	2
Raphitoma laviae	C	2	0	2
Raphitoma morph. 1	C	2	0	2
Rissoa guerinii	HE	2	0	2
Weinkauffia turgidula	C	2	0	2
Alvania cimex	HE	1	1	0
Alvania discors	HE	1	0	1
Aplus dorbignyi	SC	1	0	1
Babelomurex cariniferus	C	1	1	0
Bivalvia morph. 3	F	1	0	1
Caloria elegans	C	1	1	0
Cardiidae morph. 1	F	1	0	1
Cardiidae morph. 2	F	1	0	1
Chauvetia morph. 1	UN	1	0	1
Chauvetia morph. 2	UN	1	0	1
Columbella rustica	HE	1	1	0
Doto sp.	C	1	1	0
Felimare sp.	C	1	0	1
Gari tellinella	F	1	0	1
Gastropoda morph 3	UN	1	1	0
Gibberula morph. 2	UN	1	0	1
Idas sp.	F	1	1	0
Ischnochitonidae indet.	HE	1	0	1
Lithophaga lithophaga	F	1	1	0
Mangelia multilineolata	C	1	0	1
Mangelia morph. 1	UN	1	0	1
Mangelia morph. 2	UN	1	0	1
Mitrella broderipii	C	1	0	1
Muricidae indet.	C	1	0	1
Nodulus spiralis	HE	1	0	1
Otina ovata	UN	1	0	1
Paradoris indecora	C	1	0	1
Patella sp.	HE	1	0	1
Philine intricata	C	1	0	1
Phorcus richardi	HE	1	0	1
Pseudomangelia vauquelini	C	1	0	1
Pusia ebenus	C	1	0	1
Raphitoma morph. 2	C	1	0	1
Retilaskeya horrida	C	1	1	0
Retusa mammillata	C	1	0	1
Runcina coronata	HE	1	0	1
Spondylus gaederopus	F	1	1	0
Talochlamys multistriata	F	1	0	1
Tarantinaea lignaria	C	1	0	1
Tectura virginea	HE	1	1	0
Williamia gussoni	HE	1	1	0

76.27% of the total abundance is represented by 10 species: Bittium latreillii (Payraudeau, 1826), Musculus costulatus (Risso, 1826), Pusillina philippi (Aradas & Maggiore, 1844), Pusillina radiata (R. A. Philippi, 1836), Hiatella arctica (Linnaeus, 1767), Rissoella diaphana (Alder, 1848), Odostomia striolata Forbes & Hanley, 1850, Alvania lineata Risso, 1826, Parvicardium scriptum (Bucquoy, Dautzenberg & Dollfus, 1892) and Chauvetia mamillata (Risso, 1826), each with more than 100 specimens counted across all samples. Notably, a large number of juvenile gastropods from the family Triphoridae (130 specimens) were also recorded but could not be identified at the species or genus level.

Table A1 in Appendix A details the abundances of each species by environment and by their presence in either the fine or coarse fraction. In the sciophilous environment the most abundant species -those with over 50 observed specimens- were Musculus costulatus with 486 specimens (all but two in the fine fraction), Pusillina philippi with 272 specimens (all in the fine fraction), Pusillina radiata with 179 specimens (all in the fine fraction), Hiatella arctica with 162 specimens (153 in the fine fraction and 9 in the coarse fraction), Bittium latreillii with 55 specimens (all but one in the fine fraction) and Odostomia striolata with 50 specimens (all in the fine fraction).

The samples from the photophilous wall, due to their greater specimen abundance, displayed a higher species diversity, with 16 species exceeding 50 observed specimens. Among those with more than 100 observed specimens, the most abundant by far was Bittium latreillii with 2,051 specimens (only one in the coarse fraction), followed by the bivalve Musculus costulatus with 796 specimens (only 8 in the coarse fraction), Pusillina philippi with 545 specimens (all in the fine fraction), Pusillina radiata with 367 specimens (all in the fine fraction), Hiatella arctica with 242 specimens (only three in the coarse fraction), Rissoella diaphana with 228 specimens (all in the fine fraction), Odostomia striolata with 167 specimens (all in the fine fraction), Alvania lineata with 127 specimens (only 11 in the coarse fraction), Chauvetia mamillata with 108 specimens (only 16 of them in the coarse fraction), and Parvicardium scriptum with 101 specimens (only one in the coarse fraction).

Regarding the two studied fractions, in the coarse one the most abundant species was Tritia incrassata with 40 specimens (35 in the photophilous environment), followed by Alvania lineata with 33 specimens (16 in the photophilous environment and 17 in the sciophilous environment), Chauvetia mammillata with 21 specimens (16 in the photophilous environment and 5 in the sciophilous) and Chauvetia brunnea with 17 specimens (8 in the photophilous environment and 9 in the sciophilous). In the fine fraction, the highest abundance corresponded to Bittium latreillii, with 2,104 specimens (2,050 in the photophilous environment and 54 in the sciophilous), followed by Musculus costulatus with 1,272 specimens (788 in the photophilous environment and 484 in the sciophilous), Pusillina philippi with 817 specimens (545 in the photophilous environment and 272 in the sciophilous), Pusillina radiata with 546 specimens (367 in the photophilous environment and 179 in the sciophilous) and Hiatella arctica with 242 specimens (239 in the photophilous environment and only 3 in the sciophilous).

3.2. Species Richness, Abundance, Diversity and Evenness Throughout the Year

The families Cerithiidae, Mytilidae and Rissoidae show a clear contrast in terms of abundance compared to the other families in almost all the samples from the two studied environments. Mytilidae were the most abundant in nearly all samples, both photophilous and sciophilous, except in July and September, when Cerithidae dominated, and in November, when Rissoidae dominated both environments. The Rissoidae family exhibited a consistent presence throughout the year in both habitats, with their abundance peaking in the winter months (January and November).

The species found in the photophilous and sciophilous habitats showed different fluctuations over the study period (Figure 5). In the photophilous community, mollusk species richness was lowest from January to March, while the highest richness was observed from May to September, peaking in July. In the sciophillous environment, species richness was generally lower, but followed a similar pattern, with slight increases from July to September.

The seasonal abundance (Figure 6) of individuals in the sciophillous habitat remained relatively low throughout the sampling period, with November showing the highest abundance, with almost 500 specimens. The increase in the number of specimens from July to November was primarily driven by the abundance of species from the families Rissoidae and Mytilidae. In the photophilous community, the greatest abundance of individuals occurred in July, mainly due to species from the Cerithiidae Family.

Overall, it was observed that for both environments, the peak in species richness and individuals abundance occurred during the summer months.

The Shannon-Wiener diversity index (H') values (Figure 7) obtained from the samples (combining the three replicates) are not particularly high, ranging from 1.087 (March) to 2.765 (September) in the sciophilous environment, and from 2.084 (March) to 3.091 (July) in the photophilous environment. A 3-factor PERMANOVA analysis shows significant differences between environments (p=0.0079) and fractions (p=0.0266), although the month factor approaches significance (p=0.0850). No interactions between the factors are detected.

Regarding evenness (Pielou’s J´ index) (Figure 8), the values for the fine fraction range from 0.392 (March) to 0.760 (January and September) in the sciophilous environment, and from 0.540 (July) to 0.714 (May) in the photophilous environment. Evenness is higher in the coarse fraction due to the small number of individuals, which are relatively evenly distributed across several species (Figure 8). A 3-factor PERMANOVA analysis shows significant differences only between fractions (p=0.0001).

3.3. Community Composition According to Environment, Fraction and Changes over the Year

A 3-factor PERMANOVA design (En=environment, Fr=fraction and Mo=month) shows that all main effects and two-way interactions are significant (En, p=0.0001; Fr, p=0.0001; Mo, p=0.0001; En x Fr, p=0.0001; En x Mo, p=0.0238; Fr x Mo, p=0.0001), with only the three-way interaction being non-significant (En x Fr x Mo, p=0.2539). These results indicate that the composition and evolution of the community throughout the year varies depending on environment and fraction.

Based on these findings, a SIMPER analysis was conducted to examine the En x Fr groups (i.e., PhF, PhC, ScF and ScC) and months (Table II and Table 3A). The results show clear differences between groups (environments and fractions) and across months.

Species

3.4. Trophic Level According to Environment, Fraction and Evolution over the Year

For each sampling site and replicate, species abundance data were aggregated according to trophic category. Subsequently, the fourth-root transformation was applied, and a Bray-Curtis similarity matrix was calculated. Running a 3-factor PERMANOVA design (En=environment, Fr=fraction and Mo=month) showed that all main effects and the En x Fr interaction and the Fr x Mo interaction were significant (En, p=0.0001; Fr, p=0.0001; Mo, p=0.0392; En x Fr, p=0.0104; Fr x Mo, p=0.0329). The double interaction En x Mo and the triple interaction were not significant (p=0.8155 and p=0.9902, respectively).

The NMDS representation (Figure 11) again shows a clear separation between the fine and coarse fraction. Differences between habitats and months are also evident. The community composition by trophic category is shown in more detail in a bar chart (Figure 12), notice that surface depot feeders (SDF) and ectoparasites (EC) were absent in the coarse fraction.

The NMDS representation (Figure 9) reveals a distinct separation between the fine and coarse fraction, highlighting differences in species composition. Differences between habitats are also observed, although in the coarse fraction, the separation between September-October and the rest of the year appears to be more pronounced.

To further investigate differences in community composition within the fine fraction, a new NMDS was performed, excluding data from the coarse fraction (Figure 10). This allowed for a clearer separation between environments, particularly at certain times of the year, highlighting distinct differences in community composition.

4. Discussion

The structural complexity of a marine habitat is influenced by various factors and is a dynamic process. Over time, local alterations can lead to changes, which may modify the community’s fauna and flora [3]. Additionally, fluctuations in abundance, particularly in smaller species, can be affected by various disturbances. These include catastrophic events like large waves or high sea temperatures, predation, the emergence of juveniles from new generations, and the availability of nutrients provided by the algae in which they live, or by epibiont algae. This has been observed in the gastropods of the families Trochidae, Tricoliidae and Rissoidae in the present study.

This study identified rich malacological populations (148 species) associated with algae, comparable to other studies such as on the Spanish Costa Brava (131 species) [14] and on the island of Formentera (118 species) [30]. Studies from other regions also report similar specific richness associated with certain types of algae, such as La Goulette Bay, Tunisia (93 species) associated with photophilous algae with Corallina officinalis being the most abundant [31]; Lampedusa Island, Italy (176 species) in the algae community of the genus Cystoseira [32]; Algeciras, Spain (98 species) [17] and the Alboran Sea, Spain (193 species) [16] both in algal communities dominated by Halopteris; and Aegean Sea (184 species) [33] associated with photophilous algal turfs. However, these studies are not strictly comparable to ours due to differences in methodology and sampling effort.

Considering previous records from Tossa de Mar [34, 35] the observations of amateur naturalists (published with photos on the internet), and our own study, it can be said that the marine mollusks of Tossa de Mar are well-known. The only published work focusing on mollusks observed in Tossa de Mar [34] reports 98 species of gastropods (58 previously classified as “prosobranchs” and 40 as traditional “opisthobranchs”) from the area, including the coast and nearby coves. Of these, 73 species were also recorded in our study area. Among the approximately 1,300 species of marine mollusk species documented in Catalonia (NE Spain) (MB, personal observation), 296 species are known from Tossa de Mar (Table A2). This means that 22.3% of all known mollusk species from the 580 km of the Catalan coast are found at this single location.

Notably, sea slugs belonging to the marine Heterobranchia group (formerly "Opisthobranchia"), have attracted significant interest over several decades on the Catalan coast, both from scientists and amateur divers. As a result, the Heterobranchia are particularly well documented in Tossa de Mar, with about 130 species cited, representing around 52% of the total species listed in the Catalan coast’s “opisthobranch” catalog [36]. Moreover, two regional Catalan platforms on the Internet [37, 38] are dedicated to these gastropods. However, only a few species of Polyplacophora and Bivalvia have been recorded, and there is no data available on the observation of species from other mollusk groups in Tossa de Mar.

Regarding specimen abundance, our findings confirm the importance of algal communities for the establishment of mollusk populations. By extrapolating the abundances obtained from the three replicates (surface 20 cm x 20 cm x 3 = 1,200 cm²) per month to a surface area of 1m² of the algal community, we found that mollusk abundances in the photophilous habitat range from 2,425 specimens/m² in January to 22,133 specimens/m² in July. In the sciophilous environment, abundance values ranged from 650 specimens/m² in January to 3,883 specimens/m² in November. These data suggest that algal communities provide various resources, including food, shelter, protection and reproductive support, for a wide variety of medium- and small-sized mollusk species.

The low abundance of specimens in January (296 in the photophilous environment and 78 in the sciophilous one) compared to the other sampling days could be attributed to strong wave action in the days prior to the sampling, which may have negatively impacted the communities of the two rock walls studied.

The abundance values of mollusks per square meter in this study may appear exceedingly high and possibly questionable. However, similarly high abundance figures have been reported in other research studies. For instance, have been observed between 2,400 and 6,300 individuals per square meter in photophilous algae communities dominated by Cystoseira species on the island of Ustica (Tyrrhenian Sea, Italy), with species richness ranging from 101 to 121 [39]. In Antarctica, have been recorded up to 7,358 small invertebrates per square meter in the intertidal zone of Adelaide Island [40]; other authors [41] reported a staggering 130,000 invertebrates per square meter in the intertidal soft substrates of King George Island (South Shetland) while in the same island have been [42] found abundances as high as 100,000 invertebrate per square meter in intertidal algae on hard substrate, highlighting specific mollusk species such as the Littorinidae Laevilittorina caliginosa (3,933 specimens per square meter), the bivalve Mysella subquadrata (12,250 specimens per square meter) and the extraordinary count of 42,268 individuals per square meter of the littorinid Eatoniella kerguelenensis.

These findings illustrate that small mollusks can thrive in algae fronds, establishing highly dense populations across diverse geographical regions, from the western Mediterranean to the icy waters of Antarctica.

Most of the species identified in this study are well-known in the western Mediterranean and align with findings from similar research. For instance, a study on the seasonal dynamics of mollusks living among photophilous algae in the Alboran Sea [16], reported several abundant species, including Musculus costulatus, Mytilus galloprovincialis, Hiatella arctica, Bittium reticulatum, Pusillina philippi, Chauvetia mamillata, Parvicardium scriptum, and Tritia incrassata. These species also appear frequently in our samples. However, species such as Eatonina fulgida, Rissoa guerini, Nodulus contortus, Setia amabilis or Rissoa similis, which were abundant in their study, were either absent or present in very low numbers in our findings. The same work also observed that the highest abundances of individuals and species occurred during the summer months, coinciding with the peak growth of photophilous algae thalli -a pattern similarly noted in our study.

The most abundant species identified in this study was Bittium latreillii with 2,106 specimens, the vast majority (2,051) found in samples from the photophilous environment. Along with its congener B. reticulatum (96 specimens), these two species account for 27.8% of all mollusks found in our study. Their significant presence, particularly during the summer months, plays a role in determining the seasonal peak in mollusk abundance. Both species of Bittium are generalist herbivores distributed throughout the Mediterranean and nearby Atlantic regions, including the North Sea, the Cantabrian Sea, the coasts of Portugal, the Azores, Madeira, the Canary Islands and Mauritania [29, 45]They are often abundant in various algal environments, and our study confirms their preference for photophilous algae-dominated habitats.

The second most abundant species, the mytilid bivalve Musculus costulatus, totaled 1,282 specimens. Unlike the Bittium species, M. costulatus was found in photophilous and sciophilous environments, with 486 specimens in the sciophilous and 796 in the photophilous samples, indicating no strong preference for well-lit or darker environments. A characteristic feature of this bivalve genus is its well-developed muscular foot, which facilitates movement and likely contributes to its wide distribution across different algal communities.

Some species identified in this study, though low in abundance, are particularly interesting despite not contributing significantly to the characterization of their respective habitats. For instance, Otina ovata (T. Brown, 1827) (Heterobranchia, Eupulmonata, Family Otinidae) and Williamia gussoni (O.G. Costa, 1829) (Heterobranchia, Siphonariomorpha, Family Siphonariidae), with only one specimen of each found, belong to groups of mollusks that breathe atmospheric air and are typically found in the intertidal zone of the Mediterranean. They are often found under stones, among algae, and associated with cirriped crustaceans [42, 43]. However, in our study, these species were found at depths that do not align with the usual habitat described by most authors.

Species of the Siphonariidae family are generally considered intertidal and air-breathing [46]. However, A study that redescribes Williamia gussoni and performs a 3D analysis of its morphology [47], notes that this species typically inhabits subtidal zones and never comes into contact with atmospheric air. He further suggests that its pallial cavity does not function as a lung, making it an anomaly within its family. Based on this, the presence of W. gussoni at several meters depth in our study is not unusual.

As for Otina ovata, we speculate that its presence in deeper habitats could be attributed to transport by waves and currents. Interestingly, O. ovata had not been recorded previously in Catalonian waters, along the Spanish Levant coast, or the Balearic Islands, according to the marine mollusk census for the Spanish coast [48]This discovery may represent a new record for this species in these regions.

In this study, we found a distinct preference among the algal mollusks of Tossa de Mar for photophilous environments over sciophilous ones, with 6,244 specimens and 133 species in the former, compared to 1,669 specimens and 84 species in the latter. This suggests that rocky substrates dominated by photophilous algae offer a wider variety of ecological niches, where mollusks and other marine invertebrates can access food resources [49].The dense fronds and large biomass in certain months create numerous microhabitats, including small sciophilous enclaves, where diatoms and particulate organic matter accumulate. These deposits serve as food for micromollusks, such as Rissoa guerini and other rissoaceans, which “graze” on the periphyton of the algae. The presence of these herbivorous mollusks and invertebrates also influences the abundance of carnivorous mollusk species that feed on them.

Regarding specimen size, we observed a significant abundance of smaller mollusks in the fine fraction than in the coarse fraction. A total of 7,576 specimens smaller than 2 mm (fine fraction) were counted, compared to just 337 specimens larger than 2 mm (coarse fraction) (Table A1). This indicates that micromollusks and juvenile forms smaller than 2 mm from larger species find an ideal habitat among algae, both in photophilous environments (6,017 specimens, 105 species) and sciophilous environments (1,559 specimens, 62 species).

Regarding the statistical analysis of biodiversity, the results indicate significant differences between the two environments (photophilous and sciophilous) and between the two fractions (coarse and fine). Additionally, the sampling month factor is close to reaching statistical significance, as shown in Figure 7.

When analyzing evenness, the coarse fraction exhibits notably higher values compared to the fine fraction. This is likely due to the fact that, although there are fewer specimens in the coarse fraction, they tend to be more evenly distributed among the various species. In other words, no single species dominates the abundance. In contrast, no clear temporal variations were detected across different months, as illustrated in Figure 8.

Regarding the species community composition, a clear distinction emerges between the fine and coarse fractions, as illustrated in the NMDS representation (Figure 9). Notably, the coarse fraction samples from both photophilous and sciophilous environments during September and November occupy a space between the two groups. This positioning may be attributed to the low number of specimens and limited species richness observed in those months. In contrast, samples from the other months in the coarse fraction appear to form a separate group, although some differences between photophilous and sciophilous environments are still evident.

Focusing on the fine fraction, the months of January and March are distinctly separated from the sciophilous environment, indicating a unique composition compared to the other months (Figure 9 and Figure 10). This differentiation is likely due to these months having the fewest total specimens and the lowest species richness. Overall, a clear distinction between environments is apparent, with the months of May to November clustering at the top of the graph (Figure 10), correlating with a higher number of individuals and greater species richness. Specifically, July in the photophilous environment stands out at the top of the graph, reflecting the peak in both species’ richness and total specimens during that month.

The analysis of trophic categories reveals noteworthy findings, particularly in terms of the distinct separation between the fractions (Figure 11). This separation is attributed not only to the lower species richness and number of specimens in the coarse fraction, but also to the different compositions of the trophic categories. In the bar plot representation (Figure 12), it is evident that the coarse fraction lacks the presence of Species Feeding on Detritus (SDF) or Epifaunal Consumers (EC), exhibiting a higher relative proportion of Collector (C) and Scraper (SC) species. The reduced number of specimens found in the sciophilous environment, along with slight compositional differences between environments, results in a slight differentiation in the graph. March stands out as particularly unique within the sciophilous environment, characterized by a lower number of individuals and species. Notably, no herbivorous (HE) species were detected during this month, a finding that is challenging to explain. This anomaly may stem from undetected sampling issues or complications in the laboratory analysis of the samples.

5. Conclusions

The algal communities of the upper infralittoral zone, subjected to different light intensities allow the characterization of two contrasting environments that differ in terms of structure, abundance and species biodiversity. This has been demonstrated in this study by examining the molluscs on algal populations.

The photophilic algal community exhibits greater spatial heterogeneity compared to poorly lit bottoms, with well-lit areas alternating with small sciophilous microenvironments. This variation enhances habitat diversity. In contrast, the sciophilous wall excludes much of the flora and fauna typically found in photophilous zones. This is confirmed by the significantly higher abundance and species richness of molluscs in samples from the photophilous environment compared to the sciophilous one.

Although both communities are similar at the trophic level, due to the high contribution of herbivores and filter feeders, specific species distinguish the sciophilous community, such as: Musculus costulatus, Hiatella arctica, Pusillina philippi, Alvania lineata and Pusillina radiata. In the photophilous community, species like Musculus costulatus, Bittium latreillii, Pusillina philippi, Hiatella arctica, Chauvetia mamillata, Parvicardium scriptum, Alvania lineata, Bittium reticulatum, Odostomia striolata and Pusillina radiata are predominant.

The results in terms of abundance and specific richness indicate that in Tossa de Mar, micromollusks (less than 2 mm) and juvenile individuals of larger species find an extremely suitable environment for living and carrying out all their vital functions within both photophilous and sciophilous algal communities.

Based on our data and other studies published in journals and online platforms, we can conclude that the malacofauna of Tossa de Mar is the best documented along the Catalan coast, with a total of 296 species recorded (Table A2). Notably, the sea slugs (in the broad sense) of the subclass Heterobranchia are particularly well-represented, with around 130 species cited in this locality. However, there is still much to be explored in terms of biodiversity of Bivalves, Cephalopods and other minor mollusk groups.