A new gall crab species (Brachyura, Cryptochiridae) associated with the free-living coral Trachyphyllia geoffroyi (Scleractinia, Merulinidae)

Abstract A new species of gall crab is described from the free-living stony coral Trachyphyllia geoffroyi. Specimens were collected during field work in Lembeh Strait (Indonesia) and off Kudat (Malaysian Borneo). This new species, here named Lithoscaptus semperi sp. n., is the ninth species assigned to the genus. It can be separated from its congeners by not having the internal orbital angle extending beyond the external orbital angle, and by the stout female P2 merus with prominent distomesial projection. In addition, the carapace surface appears smooth, despite having small tubercles on the anterior half, and is without noticeable spines, other than those on the frontal margin. The distinctive carapace pattern in life is a diagnostic character in male specimens.


Introduction
During field work in Indonesia and Malaysia an undescribed gall crab species was encountered living in dwellings in free-living Trachyphyllia geoffroyi (Audouin, 1826) corals. This scleractinian species is usually found on soft substrate of reef bases near coral reefs, where it can occur in large numbers (Fisk 1983, Best andHoeksema 1987). The polyps of T. geoffroyi are fleshy and a large mantle can extend beyond the perimeter of the skeleton.
Trachyphyllia geoffroyi was classified in its own family, Trachyphylliidae Verrill, 1901, but this taxon was recently synonymised with Merulinidae Verrill, 1865(Huang et al. 2014. The sister genera of Trachyphyllia Milne Edwards & Haime, 1849 are Coelastrea Verrill, 1866 andDipsastraea de Blainville, 1830, which include coral species that formerly belonged to Goniastrea Milne Edwards &Haime, 1848 andFavia Milne Edwards, 1857. Corals belonging to these genera are host to cryptochirids of the genus Lithoscaptus A. Milne-Edwards, 1862 Serène 1957, Kropp 1990). Semper (1881) mentioned gall crabs associated with Indo-Pacific and Atlantic "Trachyphyllia", but no formally described gall crab has been recorded living in association with T. geoffroyi. This new gall crab species, here named Lithoscaptus semperi sp. n., is the ninth assigned to the genus.

Methods
Gall crabs were collected in Indonesia (Lembeh Strait, N Sulawesi -February 2012) and Malaysia (off Kudat, N Borneo -September 2012). Corals were searched for gall crabs, taken to the field laboratory and subsequently split with hammer and chisel. The crabs were preserved in 80% ethanol, after being photographed with a digital SLR camera equipped with a macro lens to register colour patterns. All crab specimens are deposited in the Crustacea collection of Naturalis Biodiversity Center in Leiden, the Netherlands (formerly Rijksmuseum van Natuurlijke Historie, collection coded as RMNH.Crus.D).
Drawings were made with a stereomicroscope with camera lucida. Carapace lengths and widths were measured to the nearest 0.1 mm using an eyepiece micrometre, with the crabs positioned on a level surface. Abbreviations used: CL, carapace length; CW, carapace width (at widest point); MXP3, third maxilliped; ovig., ovigerous; P, pereiopod; G, male gonopod. Carapace measurements are given as CL × CW, in mm.  lated, few, scattered setae, fingers slender, mesial surface of fingers smooth, cutting edge entire, tips of fingers crossing. P2 ( Fig. 1G) longer, coarser than P1; ischium without setae; merus stout, plump, smooth with few, small rounded tubercles on distal half of dorsal surface, simple setae on lateral surface, numerous plumose setae on dorsal surface; joint between merus, carpus not extending more than at right angle; carpus smooth with small rounded tubercles on dorsal surface, simple setae on dorsal surface; propodus slightly shorter than carpus, surface smooth with small rounded tubercles on dorsal surface, simple setae on lateral and dorsal surface; dactylus half-length of propodus, smooth, sharp, curved ventrally. P3 (Fig. 1H) ischium without setae; merus length 1.5 times height, rounded, few rounded tubercles on distal half of dorsal surface, simple setae along dorsal, lateral surface; joint between merus, carpus not extending more than at right angle; carpus length 2.5 times height, rounded tubercles on dorsal surface, simple setae on lateral and dorsal surface; propodus length twice height, rounded tubercles on dorsal surface, scattered simple setae; dactylus similar length as propodus, smooth, sharp, slightly curved ventrally. P4 ( Fig. 1I) similar to P3, less coarse; ischium without setae; merus length 1.5 times height, small rounded tubercles close to joint with carpus, carpus length 2.5 times height, rounded tubercles on distal half of dorsal surface, scattered simple setae; propodus half-length carpus, rounded tubercles on distal half of dorsal surface, few scattered simple setae; dactylus similar length as propodus, smooth, sharp, straight. P5 (Fig. 1J) ischium without setae; merus, carpus, propodus, dactylus all of equal length, all with short simple setae; carpus, propodus slender compared to merus; dactylus smooth, sharp, slightly curved ventrally.
Placement in genus. The placement of Lithoscaptus semperi sp. n. in the genus Lithoscaptus is somewhat tentative. The first (partial) molecular reconstruction of relationships within the Cryptochiridae shows that the genus Lithoscaptus is paraphyletic (van der Meij and Reijnen 2014). However, following the diagnosis of Lithoscaptus by Kropp (1990), the new species best fits the genus, except for the absence of a proximal tooth on the cutting edge of P1 dactylus and the presence of a distomesial projection of P2 merus in females. Kropp (1994) noted that his new species, L. prionotus, had the pterygostomial region not fused to the carapace, unlike other species in the genus. It is likely that the characters defining the genus need to be redefined, or that certain species need to be moved to a new genus.
Comparisons. Eight species of Lithoscaptus are currently recognised (Ng et al. 2008: 212, Davie 2015. Lithoscaptus semperi sp. n. can be distinguished from L. nami (Fize & Serène, 1957), L. tri (Fize & Serène, 1956) and L. pardalotus Kropp, 1995 by not having the internal orbital angle extending beyond the external orbital angle. The new species can be separated from L. grandis (Takeda & Tamura, 1983), L. paradoxus A. Milne -Edwards, 1862 andL. prionotus Kropp, 1994 by the smooth appearance of surface of the carapace, despite the small tubercles on the anterior half of the carapace, and the lack of noticeable spines other than the small spines on the frontal carapace margin. Lithoscaptus pacificus (Edmonson, 1933) and L. helleri (Fize & Serène, 1957) lack the stout merus with prominent distomesial projection of P2 (female specimens). The off-white carapace colour and translucent violet colour on P1 and P2 in females, and the distinctive carapace pattern in males differs from patterns found on other Lithoscaptus species.
Distribution. The known distribution of L. semperi sp. n. includes northern Borneo and North Sulawesi. Specimens were collected at water depths between 9 and approximately 30 meters. Its host Trachyphyllia geoffroyi was described from the Gulf of Suez (Egypt), but this species has a wide distribution that includes the Red Sea, East Africa, Seychelles, Maldives, Nicobar Isls., 'East Indies', China Sea, Philippines, Japan, Australia and New Caledonia (Scheer and Pillai 1983). Based on the widespread distribution of T. geoffroyi, a wider distribution range than the two presently recorded locations is expected for L. semperi sp. n.
Coral host. Lithoscaptus semperi sp. n. is so far strictly associated with T. geoffroyi (Fig. 3F). It is the first record of associated fauna for this coral host. Colonies of T. geoffroyi are free-living, have flabello-meandroid colony shapes and fleshy polyps. Cryptochirids have previously been recorded to inhabit free-living corals; crabs of the genus Fungicola are associated with free-living -and attached -mushroom corals (Fungiidae), whereas Troglocarcinus corallicola is associated with a wide range of Atlantic corals, including the free-living coral Manicina areolata (Mussidae) (Fize and Serène 1957, van der Meij 2014. Remarks. Fize and Serène (1957: p. 163) report on Cryptochirus coralliodytes from Trachyphyllia based on a record of Semper (1881: p. 221) who writes: "I found them [C. coralliodytes] in the Philippine Archipelago in cavities in Goniastraea Bournoni [= Goniastrea retiformis (de Lamarck, 1816)], in an undetermined true Astraea, which was unfortunately lost, also in an undescribed Trachyphyllia; finally I received a new form through A. Agassiz from the West Indian seas, which may perhaps form a distinct genus, though it is very nearly allied to the first. It also lives in a Trachyphyllia." The coral genus Trachyphyllia is described from the Red Sea and has a widespread Indo-Pacific distribution; however, it does not occur in the Atlantic Ocean. The most similar Atlantic species would be Manicina areolata (Linnaeus, 1758). Furthermore, on p. 453 (note 103 belonging to p. 221) Semper writes: "This crab, living in Trachyphyllia, a West Indian coral, is extremely like Cryptochirus, and perhaps belongs to the same genus; this can only be determined by future and more exact examination. But the 'cave dwelling' of this West Indian crab is perfectly unlike that of the Eastern species, which is found from the Red Sea as far as the Pacific Ocean; it is not cylindrical, but has one side quite flat, so that its transverse section is almost exactly a half-circle; the underside of the crab rests against the flat side of the cavity." The gall crab Troglocarcinus corallicola Verrill, 1908 has been recorded from a wide range of hosts, including M. areolata Manning 1987, van der Meij 2014). As mentioned by Semper (1881), the dwelling of T. corallicola in M. areolata is shaped like a half-circle (see e.g. Van der Meij 2014: Fig. 1B); therefore, it seems plausible that Semper was referring to the coral M. areolata when he discussed a West Indian Trachyphyllia. Alternatively, Semper could have been referring to the Atlantic genus Colpophyllia because Milne Edwards and Haime (1849), who established Trachyphyllia, compared their new genus with Colpophyllia (see Huang et al. [2014] for a discussion on the genus Trachyphyllia). Like M. areolata, Colpophyllia natans (Houttuyn, 1772) also hosts T. corallicola (see van der Meij 2014). It remains unclear whether Semper found gall crabs in Indo-Pacific corals currently recognized as Trachyphyllia geoffroyi. Semper is not known to have formally described any gall crab species (Ng et al. 2008).
Etymology. Named after the German naturalist Carl Gottfried Semper (1832Semper ( -1893, who was the first to mention gall crabs occurring in Trachyphyllia.