A new species of pea crab of the genus Serenotheres Ahyong & Ng, 2005 (Crustacea, Brachyura, Pinnotheridae) from the date mussel Leiosolenus Carpenter, 1857 (Mollusca, Bivalvia, Mytilidae, Lithophaginae) from the Solomon Islands

Abstract The pea crab genus Serenotheres Ahyong & Ng, 2005 (Pinnotheridae) is currently only represented by one species, Serenotheres besutensis (Serène, 1967). A new species is now assigned to this genus, described from a date mussel Leiosolenus obesus Carpenter, 1857 (Mollusca: Bivalvia: Mytilidae: Lithophaginae) collected in the Solomon Islands. Serenotheres janus sp. n. differs from Serenotheres besutensis in possessing a conspicuously broader carapace, with the lateral margins of the dorsal lamellum distinctly produced and the posterolateral part deeply concave, the dorsal lamellum being highest at the median cleft, the rostrum is relatively more prominent, the surfaces of the anterolateral margin and hepatic region are less prominently pitted and eroded, the ischiomerus of the third maxilliped is relatively more rectangular, and the P2 merus is proportionately longer.


Introduction
revised the species in the Indo-West Pacific pinnotherid genera Durckheimia De Man, 1889 (type species Durckheimia carinipes De Man, 1889) and Xanthasia White, 1846 (type species Xanthasia murigera White, 1846), and established two new genera, namely Serenotheres Ahyong & Ng, 2005, for Durckheimia besutensis Serène, 1967;and Tridacnatheres Ahyong & Ng, 2005, for Xanthasia whitei De Man, 1888. They commented that Serenotheres differed from all pinnotherid genera not only by the unusual carapace which has an additional large plate above its normal carapace surface (a dorsal lamellum) which overhangs the frontal margin, but also by possessing a two-segmented third maxilliped palp (Ahyong and Ng 2005: 121). It is also the only known pinnotherid associated with the rock-boring bivalves of the mytilid subfamily Lithophagidae (see Schmitt et al. 1973).
In this paper, a new species of Serenotheres is described from the Solomon Islands.

Material and methods
The specimen examined is deposited in the U.S. National Museum for Natural History (USNM), Smithsonian Institution, Washington D.C. The following abbreviations are used: MXP3 = third maxilliped; P2-P5 = pereiopods 2-5 (first to fourth ambulatory legs), respectively. Measurements (in millimetres) are of the carapace width and length, respectively. The terminology used essentially follows that in Manning (1993) and Ahyong and Ng (2005).

Molecular data
A mtDNA COI barcode was generated from this individual following standard Sanger sequencing protocols as outlined in Meyer (2003). PCR primers jgLCO1490 and jgHCO2198 (Geller et al. 2013) were used. The resulting sequence is ACC-  Meyer, 9 October, 2014. Diagnosis. Carapace distinctly pentagonal; lateral margins of dorsal carapace lamellum distinctly produced with posterolateral part deeply concave, highest at median cleft with 2 halves sloping gently outwards in direct frontal view; rostrum distinct with surface above antennular fossa prominently concave; surfaces of anterolateral margin and hepatic region less prominently pitted, eroded; MXP3 ischiomerus relatively more rectangular; P2 merus relatively long.
Colour. In life, the species is cream-yellow overall (Fig. 1). Etymology. The species is named after Janus, the ancient two-faced Roman god, alluding to the unusual two parts of the carapace when viewed dorsally. The name is used as a noun in apposition.
Remarks. Serenotheres janus sp. n. can be separated from S. besutensis (Serène, 1967)  The type of S. besutensis (9.0 × 7.0 mm) (cf. Ng and Ahyong 2005) is similar to that of the holotype of S. janus sp. n. (8.9 × 7.9 mm), so the differences observed cannot be explained by size.
The DNA barcode sequence data of S. janus sp. n. indicates a novel lineage among available Pinnotheridae sequences. The closest matches are 86-85% in sequence similarity to a handful of other pinnotherid genera including Zaops, Calyptraeotheres, Austinotheres, and Pinnixa (see Palacios-Theil et al. 2009;Palacios Theil et al. 2016). To date, no other closely related species has been sequenced.
Lithophagine mussels bore into coral rock and until recently, only one species of pinnotherid crab has been reported: S. besutensis from an unidentified species of Lithophaga collected in live coral from an island off the northeast coast of Peninsular Malaysia (Serène 1967). Serenotheres janus sp. n. was collected from inside a large specimen of Leiosolenus obesus (Philippi, 1847) (Fig. 1A). The function of the unusual plates and lamellum is not known.