Introduction
The family Leucosiidae Samouelle (Reference Samouelle1819) comprises three subfamilies: Cryptocneminae, Ebaliinae, and Leucosiinae, encompassing a total of 80 genera and 505 species (Ng et al., Reference Ng, Guinot and Davie2008; DecaNet, 2024). One genus, Bellayra (Trivedi et al., Reference Trivedi, Naderloo, Viswanathan and Mitra2022a), currently remains unassigned to any specific subfamily and includes four species (DecaNet, 2024). The family Leucosiidae comprises tiny crabs that live in the shallow water sediments of tropical and temperate regions (Ng et al., Reference Ng, Guinot and Davie2008). The Indo-West Pacific region is known to have a vast diversity among Leucosiids, new records and new species are frequently discovered (Galil and Ng, Reference Galil and Ng2023; Patel et al., Reference Patel, Naderloo, Trivedi and Mitra2023; Trivedi et al., Reference Trivedi, Naderloo, Viswanathan and Mitra2022a, Reference Trivedi, Mitra and Ng2022b). The crabs in this family have shown some extreme camouflage using their modified carapace which is sometimes seen in the shape of pebble, broken coral, or algal thallus (Ng and Komatsu, Reference Ng and Komatsu2016; Tan and Richer de Forges, Reference Tan and Richer de Forges1993). The genus Leucisca was recently shifted from the subfamily Cryptocneminae to Ebaliinae due to the presence of wide, thick carapace with blunt margins, and fine granulations on the dorsal surface (Poore and Ahyong, Reference Poore and Ahyong2023). The genus comprises four extant species: Leucisca squalina MacLeay (Reference MacLeay and Smith1838), L. rubifera (Müller, Reference Müller1887), L. halimedophila Galil and Innocenti (Reference Galil and Innocenti2019), and L. levigena (George and Clark, Reference George and Clark1976), the first two of which have already been reported in India (Trivedi et al., Reference Trivedi, Trivedi, Vachhrajani and Ng2018). L. halimedophila by Galil and Innocenti (Reference Galil and Innocenti2019) was found off the coast of Kenya in the Indian Ocean. L. levigena was described from the west coast of Australia by George and Clark (Reference George and Clark1976). The specific epithet ‘levigena’ is derived from ‘levis’, meaning smooth, and ‘gena’, meaning cheek, and refers to the smooth pterygostomian region. In the present study, Leucisca levigena (George and Clark, Reference George and Clark1976) is recorded for the first time from India, after several years of its holotype discovery in the Western Australia.
Globally, 80 genera and 505 species have been reported within the family Leucosiidae, with India accounting for 37 genera and 100 species (Trivedi et al., Reference Trivedi, Trivedi, Vachhrajani and Ng2018, Reference Trivedi, Naderloo, Viswanathan and Mitra2022a, Reference Trivedi, Mitra and Ng2022b; Ng et al., PKL et al., Reference PKL, Devi and Kumar2018; Patel et al., Reference Patel, Naderloo, Trivedi and Mitra2023; Sureandiran et al., Reference Sureandiran, Dave, Suyani, Karuppasamy and Vidhya2023; DecaNet, 2024). Specifically, the Andaman and Nicobar Islands are home to 27 genera and 54 reported species (Trivedi et al., Reference Trivedi, Trivedi, Vachhrajani and Ng2018). Only few studies have identified leucosiid crabs within the Halimeda macroalgae. Tan and Richer de Forges (Reference Tan and Richer de Forges1993) discovered Dolos petraeus (Milne-Edwards, Reference Milne-Edwards1874) and Oreotlos etor Tan and Richer de Forges (Reference Tan and Richer de Forges1993), which mimic fragments or dead parts of Halimeda algae. Komatsu and Takeda (Reference Komatsu and Takeda1999) studied Nursia alata Komatsu and Takeda (Reference Komatsu and Takeda1999) and N. mimetica Nobili (Reference Nobili1906), both adapted at mimicking Halimeda fragments. Several Huenia species (Family Epialtidae) are also known to live in association or mimicking Halimeda algae (Barnard, Reference Barnard1950; Griffin and Tranter, Reference Griffin and Tranter1986; Takeda et al., Reference Takeda, Okamoto and Fukuda1976).
Distribution of Leucisca species
Material and methods
Halimeda opuntia (Linnaeus) Lamouroux (Reference Lamouroux1816) macroalgae were gathered from the intertidal regions of South Andaman during low tide (Figure 1) with the aim of identifying the associated macrofauna. Sieving of the macrofauna was conducted using a 500 µm mesh, followed by preservation in a 4% formaldehyde solution in distilled seawater (Taylor and Cole, Reference Taylor and Cole1994). With the aid of a dissecting microscope (Magnus MSZ-TR trinocular version), crabs were differentiated from other macrofauna, and images were taken using a Magcam DC 5MP Camera. Illustrations of various parts of the specimens were created. The map was created with QGIS 3.14, and the images were arranged using Adobe Photoshop CS6. The identification was done using the available literature on the group (George and Clark, Reference George and Clark1976; Poore and Ahyong, Reference Poore and Ahyong2023). The specimens were deposited at the National Zoological Collection of the Zoological Survey of India at the Andaman and Nicobar Regional Centre (Collection number: ZSI/ANRC/M/29228; ZSI/ANRC/M/29229). Abbreviations: CW: carapace width, CL: carapace length, PL: propodus length, PD: propodus depth, IS: ischium length, ML: merus length, CA: carpus length, DL: dactyl length, G1: gonopod 1, G2: gonopod 2, Ab: abdomen width, coll.: collector.
Map showing the distribution of different Leucisca species around the world. Orange dot: Leucisca squalina, red dot: L. levigena, yellow dot: L. rubifera, and green dot: L. halimedophila.
Results and discussion
Systematics
Order Decapoda Latreille (Reference Latreille1802)
Family Leucosiidae Samouelle (Reference Samouelle1819)
Genus Leucisca MacLeay (Reference MacLeay and Smith1838)
Species Leucisca levigena (George and Clark, Reference George and Clark1976) (Figures 2 & 3)
Leucisca levigena (George and Clark, Reference George and Clark1976) (♂). (A) Dorsal view; (B) anteroventral carapace, orbit (OR), antennular fossa (AF), maxillipeds 3 (M3); (C) lateral view; (D) cheliped; (E, F) gonopod 1 & 2.
Leucisca levigena (George and Clark, Reference George and Clark1976). (A) Dorsal view (♂); (B) dorsal view (♀); (C) ventral view (♂); (D) ventral view (♀); (E) maxilliped 3 (♂); (F) cheliped (dorsal view) (♀); (G) front view (♂); (H) hump in the gastric region and the ridge towards anterior region (♂).
Material examined: 1 Male (ZSI/ANRC/M/29228); CW, 4.56 mm; CL, 3.48 mm; coll. N. Muthu Mohammed Naha; 22 November 2019, Carbyn, South Andaman, Andaman & Nicobar Islands, intertidal, on Halimeda opuntia (Lat: 11°37′45″N; Long: 92°45′03″E). 1 Female (ZSI/ANRC/M/29229); CW, 5.29 mm; CL, 3.93 mm; coll. N. Muthu Mohammed Naha; 10 September 2019, Kodiyaghat, South Andaman, Andaman & Nicobar Islands, intertidal, on Halimeda opuntia (Lat: 11°31′23″N; Long: 92°43′31″E)
Diagnosis: Carapace subcircular, wider than long (CL:CW ratio of male 1:1.31 and female is 1:1.35); dorsal surface concave, raised beaded margins; front region narrow, truncate; orbital region with faint sutures (Figure 3G). Anterolateral margins wider than posterior margins; hump on mesogastric region (Figure 3H) with vertical ridge towards epigastric region, beaded. Front region with two diverging ridges, a dip in between; posterolateral and intestinal region margins slightly constricted. Ocular peduncle suborbital (Figure 3G); antennules small in fossae. Ventral surfaces of front, pterygostomial and thoracic regions smooth, punctate.
Third maxillipeds surface smooth, elevated granules along margins of exopod, ischium, and merus.
Cheliped slender, with a trigonal merus featuring prominently granulate margins (male ML 1.67 mm and female ML 1.83 mm). Carpus is globular and granulate, with a carinate upper margin (male CA 0.58 mm and female CA 0.62 mm). Propodus laterally compressed and elongated, with carinate margins and bold granulation, inner surface bears a curving row of pearliform granules extending to the dactylar articulation (male PL 2.34 mm, PD 0.78 mm and female PL 2.51 mm, PD 0.94 mm). Fingers prominent, with rows of granules extending to the tips, which cross over when closed, tips of fingers pointed, with microdont cutting edges (male DL 0.78 mm and female DL 0.91 mm) (Figure 3C, 3D).
Ambulatory legs granulated and laterally compressed, four pairs of equal length. When retracted, legs concealed beneath the extended branchial regions of the carapace in dorsal view. Ischium short (male IS 0.22 mm). Merus unicarinate (male ML 0.86 mm), while the carpus (male CA 0.38 mm) and propodus (male PL 0.35 mm) with bicarinate upper margins. Merus bicarinate, propodus unicarinate on the lower margins, dactylus small and sharp.
Male thoracic sternum punctate, Abdomen arrow shaped, abdominal somites 4–6 fused, telson separate, sub triangular (Ab 1.46 mm) (Figure 4A); female abdomen oval shaped, abdominal somites 5–6 fused, telson separate, sub elliptical, surface perforated (Ab 2.09 mm) (Figure 4B).
Leucisca levigena (George and Clark, Reference George and Clark1976). (A) Abdomen of male; (B) abdomen of female.
Gonopod 1 long, straight, anterior region with a long spine, with an acute tip. Distal and subdistal margins lined with small setae, the distal part is swollen (G1 1 mm) (Figure 2E), Gonopod 2 short with a bulbous tip (G2 0.3 mm) (Figure 2F).
Colour: Alive specimens are creamy yellow coloured (Figures 2 & 3).
Distribution: Western Australia (WAM: C8140 George and Clark, Reference George and Clark1976), South Andaman, India (present study).
Remarks: George and Clark (Reference George and Clark1976) described a female specimen of L. levigena from Western Australia. The present study reported both male and female specimen of L. levigena from South Andaman, India. The specimens examined in the present study agree with the description of female specimen of L. levigena provided by George and Clark (Reference George and Clark1976). However, unlike their findings, where the cheliped merus was described as short with smooth cutting edges, the present study observed that the merus was not short, and the cutting edges bore small teeth. Among the previously reported species, L. levigena exhibits distinct morphological characteristics. Unlike L. rubifera, which has a punctate and granular gastric region with a large round tubercle in the cardiac region, L. levigena features a mesogastric hump and a vertical beaded ridge extending from the mesogastric region towards the epigastric region.
In contrast to L. squalina, which has a moderately convex gastric region and a slightly convex cardiac region with granules, L. levigena lacks a well-defined depression between these regions. Additionally, while L. halimedophila possesses a rounded median ridge running from the front to a conical gastrocardiac protuberance with a cluster of flattened granules, L. levigena lacks such a prominent gastrocardiac feature but has a distinct vertical beaded ridge. The pterygostomial region in L. levigena is smooth, differing from L. squalina, where it is tuberculated. Gonopod morphology (G1) also varies significantly: L. levigena has a long G1 with an anterior spine with acute tip, whereas L. squalina has a stout G1 with a long, slender spiniform apex, and L. halimedophila possesses a stocky G1 with a claw-like tip (Galil and Innocenti, Reference Galil and Innocenti2019; George and Clark, Reference George and Clark1976; MacLeay, Reference MacLeay and Smith1838; Müller, Reference Müller1887).
Specimens were collected from the thallus of the seaweed Halimeda opuntia. The structure of the carapace was camouflaged to blend with the thallus, and the size of the crabs was nearly identical to a piece of the thallus. Additionally, the coloration of the specimens closely matched that of the dried thallus. The occurrence of L. levigena within the H. opuntia thallus may be linked to the high abundance of small invertebrates that inhabit it. Hale (Reference Hale1926) and Schembri (Reference Schembri1981) observed that leucosiid crabs typically feed by capturing small invertebrates from the sediment and through scavenging. Additionally, Halimeda is a key sediment-forming organism on many reefs, providing habitat for a diverse and densely populated fauna (Perry et al., Reference Perry, Morgan and Salter2016), which could further support the presence of these crabs.
Conclusion
The discovery of L. levigena in the intertidal regions of South Andaman is a significant addition to the known distribution of this pebble crab species from Andaman and Nicobar Islands. Originally described from Western Australia in 1976, this finding not only confirms its presence in the Andaman and Nicobar Islands but also provides the first recorded male specimen, previously absent from the initial discovery. This study underscores the importance of continued exploration and documentation of organisms in these regions, revealing similarities between Western Australia and the Andaman Islands. Further research is essential to thoroughly document and understand the diverse organisms associated with these habitats. Ongoing exploration and study will undoubtedly reveal more hidden species and deepen our understanding of the biodiversity and ecological importance of these areas.
Data availability statement
All data analysed during the study are included in this article.
Acknowledgements
We thank Pondicherry University for providing all the necessary facilities to carry out this research work. We gratefully acknowledge Dr Andrew Hosie, Curator of Crustacea and Worms, Western Australian Museum, for sending the pictures of holotype and for providing literature.
Author contributions
Naha NMM initiated the work, field work, specimen collection, identification, drawings, and manuscript making. Padmavati G supervised and reviewed the manuscript.
Funding
This research received no specific grant from any funding agency.
Competing interests
None.
Ethical standards
The authors declare that the work did not involve experiments with vertebrates. The work was carried out within local guidelines without causing damage to the environment.
