Holotype: UF43955, male, Line Islands, Palmyra Atoll, N side of Atoll, outer reef slope, from dead Pocillopora cf. verrucosa head, 10.6 m.

Paratypes: UF43956, 1 male, same collection data as holotype; UF10692, 1 male (with bopyrid), 1 ovigerous female, Kiribati, Line Islands, Tabuaeran Atoll, SSW side of Atoll, outer reef slope, under rock, 10-23 m; UF10693, 2 females (1 ovigerous), Kiribati, Line Islands, Tabuaeran Atoll, outer reef slope, from Halimeda sample, 10-23 m; UF10711, 1 female, Line Islands, Tabuaeran Atoll, W side, S of Main Reef Pass, outer reef slope, from dead Pocillopora cf. verrucosa head, 10-15 m; UF15894, 1 male, (photographed specimen, Fig. 3), French Polynesia, Society Islands, Moorea, Haapiti, just NW of Matauvau Pass, outer reef slope, 15-23 m.

Figure 1. 

Petrolisthes paulayi sp. n. Male, holotype, dorsal view, UF43955, Line Islands, Palmyra Atoll. Right third walking leg supplemented. Scale bar: 2 mm.

Figure 2. 

Petrolisthes paulayi sp. n. Male, holotype, UF43955, Line Islands, Palmyra Atoll. a Carapace front, dorsal view b right, lateral margin of carapace showing epibranchial and mesobranchial spines, dorsal view c third thoracic sternite, ventral view d last abdominal segment, telson and uropods, external view. Scale bar: 1 mm.

Figure 3. 

Petrolisthes paulayi sp. n., UF15894, Society Islands, Moorea. Photographed by the Moorea Biocode Marine Invertebrate Team (catalogue number dMBC08_03896, Florida Museum of Natural History).

UF10588, 12 specimens, Line Islands, Palmyra Atoll, Line, N side of Atoll, outer reef slope, dead Pocillopora cf. verrucosa head, 10.6 m; UF41926, 1 male, Kiribati, Line Islands, Starbuck, Starbuck Island, dead Pocillopora, 12 m; UF41916, 1 female, Kiribati, Line Islands, Starbuck, Starbuck Island, 12 m; UF40590, 2 males, 1 ovigerous female, Kiribati, Line Islands, Starbuck, Starbuck Island, 7 m; UF41980, 4 males, 4 females (3 ovigerous), Kiribati, Line Islands, Vostok, Vostok Island, dead Pocillopora, 10 m.

Male holotype: carapace length 4.5 mm; carapace width 4.2 mm.

Largest male paratype: carapace length 4.8 mm; carapace width 4.5 mm.

Largest female paratype: carapace length 4.5 mm; carapace width 4.3 mm.

Carapace (Figs 1, 3) slightly longer than broad. Front (Fig. 2a) trilobate, markedly produced beyond eyes, median lobe broadest, rounded, overreaching the slender, forwardly produced lateral lobes; frontal borders finely serrated. Orbits deeply rounded, inner margin armed with a small supraorbital spine, followed mostly by a second tubercle-like elevation that is sometimes armed with a spine; infraorbital angles forming an acute edge. Lateral margins (Fig. 2b) with a prominent epibranchial spine on epibranchial edge, and followed by a second smaller spine; two strong spines on mesobranchial margin. Carapace with few transverse, piliferous striations on protogastric ridge and on gastric region; epibranchial region rugose.

Third thoracic sternite (Fig. 2c) anteriorly trilobate, median lobe broad, lateral lobes slender, exceeding median lobe.

Telson (Fig. 2d) with seven plates.

First movable segment of antenna with an anterodistal slender projection bearing a narrow tooth. Basal segments of antennular peduncle bearing acute, irregular spines on anterior margin.

Chelipeds (Figs 1, 3) subequal, robust, dorsal surface somewhat convex, covered with interrupted scale-like ridges, granulated on ventral side. Merus with a large, serrate-edged tooth on anterior margin; carpus about 2.5 times as long as broad, with 4 rounded, serrated teeth on anterior margin, the proximal ones spine-tipped in some specimens; anterodistal edge with an additional blunt tooth; posterior margin with scale-like granules forming a row of 5 to 6 curved, upright spines, distally increasing in size. Manus moderately broad, posterior margin serrated but without spines, fringed with an irregular set of stiff and feathered setae; gape of fingers without distinct setation.

Walking legs (WL; Figs 1, 3) relatively robust, irregularly covered with scattered, simple and feathered setae of different size; merus of WL1 without spur-like spine on ventral, mid-distal margin; merus of WL1 and 2 with continuous transverse ridges; merus of WL3 without such ridges; merus of all WL with row of spines on anterior margin, distributed as follows: WL1 and 3: 5, WL2: 4-5. Merus of WL1 and WL2 with posterodistal spine; Dorsodistal edge of carpus in WL1 produced into a sharp spine. Propodus of all WL with 1 or 2 movable spines in addition to the terminal spine-triplet on posterior margin. Dactylus of all WL with 4 movable spines on posterior margin.

Coloration (Fig. 3). The carapace has a yellow-orange background with a white, reticulate pattern in the posterior half. The white markings on the gastric region fuse into an irregular, transversal stripe towards the epibranchial edges. The inverted figure of a butterfly is depicted by a broad, white, curved band connecting the hepatic margins on both sides, two semicircular white spots mesial to the orbits, and another backwardly curved white band behind the front. The reticulate pattern continues to the anterior part of the abdomen and the proximal parts of the walking legs, covering half to most of the merus. This pattern is then replaced by a dark purple band that increases in size, and is followed by a narrower white band bordering the articulation with the purple-colored carpus. Dactylus is also purple. White bands on both ends of the propodus give the walking legs a uniform, ring-like aspect. The chelipeds exhibit a similar ground color that becomes darker distally, with white marks forming irregular rows of round spots on carpus and manus.

The specimens examined were collected in depths between 7 and 23 m, on the outer reef slope, from Halimeda and dead Pocillopora. Further collections will probably confirm that P. paulayi sp. n. inhabits other exposed coral environments of the tropical western Pacific.

The new species is known only from the Line and Society Islands in the Central Pacific.

The new species is named after Gustav Paulay for supporting this and other studies on Porcellanidae, and for entrusting us with the porcellanid collection of the Florida Museum of Natural History.

P. paulayi sp. n. can be be easily distinguished from other Petrolisthes species of the Indo-West Pacific by its unique color pattern, and by the combination of the following characters on the carapace: two mesobranchial spines, two epibranchial spines and a conspicuous trilobate front. The later character is known only in P. elegans Haig, 1981, which lacks mesobranchial spines, and only bears one epibranchial spine.

UF12962, 1 ovigerous female, Mascaréne Islands, La Réunion Island, Saint-Leu, Sec Jaune, rocky slope, basalt blocks, fore reef, under rocks, 10-19 m; UF13075, 1 male, La Réunion Island, Boucan Canot, Paine au Sucre, 10-15 m; UF33079, 1 male, 1 ovigerous female, Red Sea, Saudi Arabia, Thuwal, Al-Fahal reef, 1-37 m; UF36734, 1 specimen (identified from photograph), Red Sea, Saudi Arabia, offshore of Farasan Banks, Shib Radib, fore reef wall and barrier reef flat, 7-9 m; UF15472, 1 male, French Polynesia, Society Islands, Moorea Island, mid N coast, off Sheraton Hotel, outer reef slope, from within rubble; UF15474, 1 female (ovigerous), same data as UF15472.

Coloration. Fresh specimens from the Indian Ocean and the Central Pacific are white or beige on the anterior part of carapace and chelipeds (Fig. 4). The chelipeds may be entirely white, but usually show vivid red spots towards the fingertips. A row of small, purple spots may border the outer edge of carpus and manus. The front of the carapace may be fringed with some irregular, purple-brown spots, while a larger purple spot delimits the infra-ocular edge. The metabranchial regions bear a large, semi-lunar red blotch that extends towards the basal parts of the walking legs. The walking legs show a ring-like pattern. The surface of legs can be dark-purple or red with white marks, one of them on the distal margin of the merus, one at half distance of the propodus, and another near the articulation with the dactylus.

Figure 4. 

Petrolisthes aegyptiacus Werding & Hiller 2007, female, Red Sea, Egypt, Dahab. Scale bar: 3 mm.

The red marks of the posterior part of the carapace extend to the lateral part of the segments of the abdomen that are visible from dorsal view. The median part of the first two or three segments of the abdomen is beige, interrupted by irregular red spots. The remaining posterior segments of the abdomen are entirely purplish. The whitish and reddish color, typical of P. aegyptiacus, suggests a camouflaging strategy, as the color of the substrate inhabited by the species is usually spotted with red Foraminifera (Homotrema Hickson, 1911).

Distribution. Previously only known from the Red Sea and the Mascaréne Islands in the southwestern Indian Ocean, and the Society and Line Islands in the Central Pacific Ocean (for the occurrence in the Pacific, see discussion below).