On a new commensal species of Aliaporcellana from the western Pacific (Crustacea, Decapoda, Porcellanidae)

Abstract Aliaporcellanaspongicolasp. n. from the Philippines and Indonesia is described. The new species has been frequently photographed by divers because of its striking coloration, but has not been described yet. Aliaporcellanaspongicolasp. n. is in fact a widespread commensal of barrel sponges of the genus Xestospongia and other sponges. Morphological characters and ecological information of all described species of Aliaporcellana, and of other porcellanids associated with sponges and soft corals, suggest that all members of the genus are commensals, and that similar morphological adaptations to dwelling on these hosts have evolved independently in different evolutionary lines within Porcellanidae.


Introduction
The porcellanid genus Aliaporcellana was established by Nakasone and Miyake (1969) for a group of Indo-West Pacific species previously assigned to Porcellana Lamarck and to one of three natural groups within Polyonyx Stimpson, designated by Johnson (1958) as the P. denticulatus Paul'son 1875, group. A diagnostic character considered by Nakasone and Miyake (1969) to raise Aliaporcellana is the dactylus of all walking legs bearing two or more distinctively well-developed fixed spines. Aliaporcellana contained nine species until Haig (1978) restricted the genus to the species of the Polyonyx denticulatus group, which now includes the type A. suluensis (Dana 1852), A. pygmaea (de Man 1902) and A. telestophila (Johnson 1958), and the species described by Nakasone and Miyake (1969), A. kikuchii. A fifth species, A. taiwanensis, was subsequently described by Dong et al. (2011).
Here we describe a new sponge-dwelling species of Aliaporcellana from material collected in the Philippines and Indonesia. Despite having been frequently photographed by divers because of its striking coloration and relatively large size, the species has not been described. With the exception of A. telestophila , commensalism has never been reported for other congeners. We highlight the characters distinguishing the new species from its congeners, and discuss the morphological traits, present in all Aliaporcellana species and other porcellanids associated with sponges, which we interpret as adaptations to living on these hosts.

Material and methods
We found the new species in material collected in the Philippines by G. Paulay [Florida Museum of Natural History, Gainesville, U.S.A. (UF)] and in Indonesia by C.H.J.M. Fransen [Naturalis Leiden, The Netherlands (RMNH)]. The holotype is deposited in the National Museum of Natural History, Philippines (NMCR). Color photographs of the holotype and of the live crab in the field were provided by G. Paulay, and were included in the description. Measurements of carapace length and width (in mm) of type individuals follow collection information. Description. Carapace rounded (Figures 1, 2), considerably variable in form and in length-width ratio; larger females with carapace broader than long (ratio < 1), smaller individuals with carapace relatively longer than broad (ratio > 1); dorsal surface convex, glossy, with faint, transverse striae on branchial and intestinal regions; cervical grooves gently depressed. Front (Figures 1, 2) broad, slightly produced beyond eyes, weakly trilobate, somewhat deflexed; frontal lobe visible in dorsal view, grooved, overreaching lateral ones. Distal margin of entire front lined with row of rounded, upwardly directed small spines (Figure 3a), the largest on supraocular edges. Outer orbital angles ( Figure  2) forming acute, bifid tooth followed by hepatic spine of similar size. Epibranchial margin rounded, produced outwards, marked with epibranchial spine; cervical groove faintly marked. Mesial branchial margins crested, with row of 5 or 6 strong, anteriorly, upwardly directed spines of increasing size posteriorly. Sidewalls entire.

Family Porcellanidae
Eyes moderately large (Figures 1, 2, 3a), retracted, ocular peduncles short. First movable segment of antennal peduncle (Figures 2, 3b) with strong, anteriorly curved distal spine, second with smaller, anterodistal, acute protuberance, third one globular. Basal segment of antennular peduncle (Figure 3c) with anterior surface transversely Chelae moderately different in size and form (Figures 2, 4a-c); merus short, dorsal surface faintly rugose, inner margin with strongly projecting, sub-rectangular projection, fringed distally with cockscomb-shaped row of teeth, other large spines on proximal and distal edge of outer margin, one on distal margin; ventral side with two large spines on distal margin. Carpus 1.5 times as long as wide, dorsal surface evenly convex, similarly structured as carapace, with some faint transversal plications; inner margin with 3-5 low or sharply hooked teeth, decreasing in size distally, distal edge rounded. Outer margin with a row of six or seven acute, upwardly directed spines, the last one forming distal edge. Palm slender, surface rounded, similarly structured as carpus, with faint, transverse striae. Smaller chela with outer margin bearing row of approximately ten sharp spines on proximal half, with scattered, long, simple setae; fingers reaching up to half length of chela, dactylus moderately twisted, opening vertically, cutting edges denticulate, without teeth, both fingers with narrow fringe of fine, plumose setae in proximal 2/3 of length. Larger chela somewhat stouter, outer margin with row of spines less developed or disappearing in large specimens, with scattered, long, simple setae, fingers relatively shorter as in smaller chela; dactylus moderately twisted, opening vertically, cutting edges in pollex and dactylus with broad, shallow tooth, gape naked.
Walking legs (Figures 2, 3f, g) stout, merus with some transversal striae, with scattered, long, simple setae, increasing in number towards dactylus; carpus in first and second leg ending dorsodistally in two minute spines, propodus ventrally with 1 movable spine in addition to terminal triplet; dactylus terminating in bifurcate, curved claw.
Coloration. The background color of carapace and extremities is bright orange (hexadecimal color #e86700), overlain with a reticulate bright blue (hexadecimal color #000de8) pattern (Figures 1, 5). A broad, black band crosses the carapace transversely at the level of the hepatic region; it is fringed on both sides by a small, blue line and a broad, orange band. A similar band extends along the outer border of the chelipeds from the carpus to the tip of the pollex. In a number of individuals the blue color prevails over the orange, and the entire crab appears blue.
Ecology. Aliaporcellana currently consists of six species. Of all species, A. spongicola sp. n. is by far the most strikingly colorful, and has, therefore, become popular among underwater photographers and marine aquarists. Aliaporcellana spongicola sp. n. dwells on large barrel sponges of the genus Xestospongia Laubenfels [family Petrosiidae; e.g., X. testudinaria (Lamarck 1815)] and on other types of sponges, like the "large, grey foliose sponge", on which the crabs from Sulawesi included in this study, were found.
The porcellanid lies in the sponge's folds, where it is most protected from predators ( Figure 5).
Distribution. The type specimens come from the central Philippines and northern Sulawesi, Indonesia.
Etymology. The name spongicola (from the Latin word spongia, meaning sponge, and the Latin suffix cola, meaning dwelling) refers to the sponge-dwelling habit of the new species.
Remarks. Aliaporcellana spongicola sp. n. is considerably variable in the shape of carapace and the degree of spination on body and extremities. As in other porcellanid species, the spines are more defined in smaller specimens. The new species is distinguished from A. pygmaea and A. kikuchii by the lack of acute spines on the dactylus of the smaller cheliped (Osawa 2007;Dong et al. 2011), and by its smoother surface of carapace and chelipeds (Lewinsohn 1969;Nakasone and Miyake 1969;Werding and Hiller 2007;Osawa and Chan 2010). Aliaporcellana spongicola sp. n. can be distinguished from A. suluensis, A. telestophila and A. taiwanensis by its regularly denticulated front (Figures 2, 3a), which is smooth in the other species, and by the basis of the antennular peduncle, which is crowned with a ring of spines ( Figure 3c) and is at most granulate or faintly serrate in the compared species (see Lewinsohn 1969;Werding and Hiller 2007;Dong et al. 2011 for A. suluensis;Ng and Goh 1969 for A. telestophila;Dong et al. 2011 for A. taiwanensis).

Discussion
With the description of Aliaporcellana spongicola sp. n., the genus now comprises six species.
Up to now, A. telestophila is the only species of the genus reported to live as commensal (Johnson 1958;Ng and Goh 1996). Johnson (1958) described this species based on his own collections and observations, highlighting that A. telestophila was found "strictly [in] commensalism with the octocoral Telesto". However, Ng and Goh (1996) doubted the identification of the octocoral host and referred to it as Solenocaulon Gray (family Anthothelidae Broch), instead. Ng and Goh (1996) and Goh et al. (1999) described the porcellanid as dweller inside of the hollow branches of the octocoral, communicating with the outer medium through the openings of these branches. The species lives in male-female pairs; sometimes two pairs are found in one host colony.
Our own observations of the morphology and ecology of A. suluensis collected from sponges in Saudi Arabia, and of all other Aliaporcellana species, led us to conclude that perhaps all species of the genus are commensals. We base our conclusions on the well-developed, fixed spines on the dactylus of the walking legs, a character present in all Aliaporcellana species (see Figures 3f-g) and other porcellanid commensals that inhabit sponges (e.g., Pachycheles ackleianus A. Milne-Edwards, 1880, Polyonyx hendersoni Southwell, 1909and P. splendidus Sankolli, 1963see Haig 1960;Hiller et al. 2010). This morphological trait is probably an adaptation to moving on the surface of this type of host. We hypothesize that all members of the genus Aliaporcellana are commensal of sponges or octocorals, and that this morphological trait has evolved independently in different evolutionary lines within Porcellanidae. Aliaporcellana spongicola sp. n. probably lives in male-female pairs, as A. telestophila does on the octocoral Solenocaulon (Ng and Goh 1996;Goh et al. 1999).
The association between crab and sponge may be easily overlooked because sponges are often attached to each other and to rocks, and are damaged when the rocks are lifted. More collection data of other Aliaporcellana species are needed to confirm the commensal status of the genus.