A Knowledge Update of Rarely Reported Crustacean Species, Rissoides pallidus (Giesbrecht, 1910) (Stomatopoda, Squillidae), Caught in the Strait of Sicily Waters (Central Mediterranean Sea)

1. Introduction

The Strait of Sicily is considered a very important biogeographical area [1,2] and remains representative as a barrier to some species [3,4], whereas some others an “ecological corridor” [5,6,7]. These features make it a biodiversity and productivity hotspot of marine edible resources, which are exploited by national and international fleets [8,9,10,11]. Despite that stomatopod crustaceans occupy some space within the Mediterranean waters, there belong to the fauna that has been studied less [12]. Not only is the commercial interest for Stomatopods amply minimal, information regarding the majority of its species are not always available. For instance, the stomatopod genus Rissoides comprise 5 species [13] mainly distributed in the shallow waters along the burrows or crevices on different types of substrate [14]. Among these, only the Rissoides desmaresti (Risso, 1816) and Rissoides pallidus (Giesbrecht, 1910) appear recorded in the Mediterranean Sea [15]. These stomatopod species show a patchy distribution with low densities. In particular, R. pallidus is rarely recorded, though believed to be widespread in the Mediterranean, noting the records shown from the Levant [16] to the western Alboran Sea [17], as well as north-western Africa, Canary Islands, and Madeira [18,19,20]. To supplement existing information, this current note updates the existing knowledge of R. pallidus in the Strait of Sicily. Additional information specific to biometrics, bottom types, depth preference, habitats, and geographical distribution of this crustacean species is provided.

2. Materials and Methods

On December 2020, two specimens of R. pallidus (trawl haul points: 37.515°N, 12.4025°E and 37.493°N, 12.4433°E) were captured as part of the discarded fraction of a selectivity trawl survey (see [21,22] for technical details) carried out off Mazara del Vallo harbour (Northern sector of the Strait of Sicily). The depth of the sampling site ranged between 132 and 152 m. Figure 1 shows the map of how the R. pallidus has been distributed based on previous and current studies within the Mediterranean Basin.

Thereafter, biological samples were transported to the laboratory of CNR-IRBIM of Mazara del Vallo where the R. pallidus specimens were identified and biometric data were collected. Specifically, mantis shrimp individuals were photographed, weighted (accuracy 0.1 g) and measured as carapace length (CL – distance between the centre of the anterior interorbital margin and the centre of the posterior margin, excluding the rostral plate), carapace width (CW –carapace at widest point) as well as total length (TL – distance between the anterior edge of the rostral plate and posterior extremity of the telson) with the help of a calliper to the lowest millimetre. The sex of R. pallidus specimens was determined by the presence or absence of penes at the base of the pereopods on the eighth thoracic somite following the method of KUBO et al. [40]. Furthermore, a summary of published records, showing biometrics, bottom types, habitat, and depth of R. palllidus previously found within the Mediterranean Basin was carried out to enable robust comparisons of emergent data. Lastly, the geographical distribution of this uncommon species in the Mediterranean basin was mapped using Quantum GIS software (QGIS, 2020).

3. Results and Discussion

The R. pallidus specimens from the Strait of Sicily were female with 10 and 8 mm CL, 8 and 4 mm CW, and 48 and 33 mm TL, respectively (Figure 2).

As for weight, these specimens were 1.26 and 0.26 g, respectively. The biometrics of the specimens in the present note resembled those reported by previous authors [12,16,25,28,39]. For instance, Lewinsohn and Manning [16] reported TL of females 47 mm, and males 45 mm. Kocataş and Kataǧan [31] reported R. pallidus obtained from the Aegean Sea with TL of 56 mm. Indeed, the female specimens recorded in the Mediterranean showed a TL from 32 to 67 mm. From our conducted literature summary in Table 1, we can see that the R. pallidus specimens found within the Mediterranean can have a reasonable TL range of between 32 and 67 mm.

Compared to other species that belong to Squilidae, the male individuals of R. pallidus exhibits a sexual dimorphism characterized by an inflated telson carinae and tubercles, whereas the growth of the telson width resembles in both sexes [38].

Morphological characteristics were in agreement with the description provided by Manning [18] and Lewinsohn and Manning [16]. It is worth noting that, R. pallidus can sometimes be confused with R. desmaresti [12]. Nevertheless, such features that allow its recognition may occur in the colour and morphology of the body. Specifically, R. pallidus is characterized by the long antennular peduncle, lateral process of the fifth thoracic somite (that is considered laterally sharp), lateral carinae of the fourth abdominal somite (that is considered armed posteriorly) and a series of spinules on the inner margin of the basal prolongation of the uropod [16,25]. Although the colour pattern of both species closely resembled, the R. Pallidus shows a pigment that appears much less well marked, with a noticeably less pigment that is laterally placed on the abdomen, between the intermediate and lateral carina [18].

The specimens of the current study were caught in a fishing ground locally known as 'Gigibau'. This is an area of about 500 km² with a depth range of between 100 and 170 m and 10 nautical miles (SW) from Mazara del Vallo, which equally characterized by two different biocenoses namely: coastal detritus and coastal terrigenous mud [42,43]. Concerning the habitat and geographic distribution, some previous studies have reported the occurrence of R. pallidus at different substrates, notwithstanding that the muddy areas of the deep continental shelf and upper slope seem to be the preferred habitat of this species (Table 1). The R. pallidus, similarly to the congeneric species R. desmaresti [44], might build burrows. According to Laban and Lindeboom [45], such burrows would have to avoid the catch of animals by trawls that penetrate into the sediment, and much less the depth. However, in the present study, R. pallidus specimens were caught using a trawl net in a deep water rose shrimp (Parapenaeus longirostris Lucas, 1846) fishing ground [46,47,48,49,50]. In particular, the present study asserts the occurrence of R. pallidus in the Strait of Sicily. The last record to our best knowledge about the R. pallidus specific to the Strait of Sicily appears after twenty-eight years [29].

Relevant information concerning the biology of R. pallidus appears scanty. For instance, the information about the size at maturity, and growth appears not available in the scientific literature, to our best knowledge. In particular, the females of R. pallidus might depict a similar resembling reproductive period to Squilla mantis especially during the fall and winter seasons [51]. Besides, peak abundance of R. pallidus has been reported during the autumn and winter months in the Adriatic Sea [52]. Given that, the R. pallidus specimens of this study were caught in the December month, such a catch might be linked to the feeding activity beyond the burrows prior to egg incubation [51,52].

Overall, the current note has provided an update regarding the spatial distribution of R. pallidus in the Mediterranean Sea. Furthermore, in order to provide a complete picture for anyone aiming to study the R. pallidus, as much relevant biological information that is available in the scientific literature has to be collected, and that is what we have attempted to do here. Despite this, much more is yet to be learned about the distribution and biology of R. pallidus. Not only that there is no commercial interest for this R. pallidus, some stomatopods to live in specific biotopes [53] - both reasons may be why the records of this species are scanty. However, as suggested by Falsone et al. [53] and Sardo et al. [55], there is need for an adequate sampling methodology, which might help overcome such a challenge. Lastly, to improve the knowledge about the spatial distribution and biological features of uncommon or threatened species, an ad-hoc research project [56,57] and a closer collaboration with fishers [58,59,60] should be encouraged.