Cebudonus n. gen.

Type species. Cebudonus poppeorum n. gen., n. sp., by present designation.

Diagnosis. Carapace pentagonal; dorsal surface gently convex, covered with numerous small pits (Figs. 1, 2); front with 2 long pseudorostral spines, subparallel, ca. 0.4 times total carapace length, separated by broad U-shaped cleft (Figs. 1, 2, 3 C); anterolateral margin entire, with large, broadly triangular, sharp lateral tooth, projecting laterally, dorsal surface gently convex (Figs. 1, 2, 3 A, 4A); antennae with subrectangular basal article, twice as long as wide (Fig. 3 B); antennules folding at ca. 45° (Fig. 3 B); epistome relatively broad with depressed median part (Fig. 3 A–C); third maxilliped with ischium subrectangular, with deep submedian longitudinal sulcus (Figs. 3 C, 4A), merus subquadrate, anteroexternal angle subauriculiform (Figs. 3 C, 4A); ambulatory legs long, slender (Figs. 1, 2, 4 E); ambulatory merus elongated, slender, surface with margins rounded, not cristate, not distinctly granular, dorsal distal tooth distinct but low (Figs. 1, 2, 4 E); ambulatory propodus, dactylus with ventral margins lined with dense stiff setae, with distinct dactylo-propodal lock, not forming subchelate structure (Figs. 1, 2, 4 D, E); cheliped merus rounded in cross-section; surface covered with numerous small pits (Figs. 1, 2, 3 C); cheliped carpus surface with long, gently curved spine on distal inner angle (Figs. 1, 2, 3 F); chela unarmed, margins not cristate (Fig. 3 F); thoracic sternum relatively narrow transversely, surfaces pitted, sternites 3, 4 completely fused with median depression (Figs. 3 C, 4B); male abdominal locking mechanism present as knob-like tubercle on distal one-third of thoracic sternite 5; male abdomen broadly triangular (Figs. 3 D, 4C); G1 slender, sinuous, distal part elongated (Fig. 4 F, G).

Etymology. The new genus is named after Cebu City in the Visayas, Philippines; in arbitrary combination with the genus name Eumedonus. The gender of the genus is masculine.

Remarks. The unusual combination of carapace, cheliped, ambulatory leg, thoracic sternal and abdominal characters in Cebudonus poppeorum n. gen., n. sp. require the establishment of a new genus. While the carapace shape of Cebudonus n. gen. superficially resembles species associated with echinoids like Eumedonus intermedius Chia & Ng, 2000 (Madagascar), E. vicinus Rathbun, 1918 (Australia), and E. zebra Alcock 1895 (Red Sea to East China Sea), especially with regard to the shape of the laterally directed spines; its front is completely different, with two long pseudorostral spines (Figs. 1, 2) in contrast to Eumedonus, which has a lobiform pseudorostrum that is bifurcated distally to form two teeth (cf. Chia & Ng 2000: figs. 8, 9A, D, 10, 11, 12A, D, N, 13, 14, 15A, D, Q, R). The lateral carapace tooth and the dorsal surface adjacent to it is gently convex dorsally in Cebudonus n. gen. (Fig. 3 A) (the tooth is stout with the dorsal surface distinctly convex in Eumedonus; cf. Chia et al. 1995: fig. 3A); the cheliped merus is relatively long and unarmed (Figs. 1, 2, 3 C) (relatively short with numerous tubercles and granules in Eumedonus; cf. Chia & Ng 2000: figs. 8, 9B, 10, 11, 12B, L, 14, 15B, O); the meri of the ambulatory legs are long, subcylindrical and smooth, without any marginal cristae, and the dorsal distal tooth is relatively low (Figs. 1, 2, 4 E) (short, laterally flattened with distinct marginal cristae and a strong dorsal distal tooth in Eumedonus; cf. Chia & Ng 2000: figs. 8, 9E, G, 10, 11, 12E, K, 13, 14, 15E, K); the anterior male thoracic sternum (sternites 1–4) are proportionately narrow transversely (Figs. 3 C, 4B) (proportionately wider transversely in Eumedonus; cf. Chia & Ng 2000: figs. 9C, 11B, 12C, 14B, 15N); and the male abdomen is relatively wide (Figs. 3 D, 4C) (proportionately narrower in Eumedonus; cf. Chia & Ng 2000: figs. 12J, 14J, 15N).

The unarmed and non-cristate ambulatory meri of Cebudonus n. gen. resembles that of the echinoid symbiont Gonatonotus, but in the new taxon, it is proportionately much longer, more slender and almost smooth (Figs. 1, 2, 4 E) (proportionately shorter and distinctly granular in Gonatonotus; cf. Chia & Ng 2000: figs. 17, 18E, G, 20, 21E, K, 22, 23E, K). The differences previously noted in the form of the front, anterolateral region, cheliped, thoracic sternum and abdomen between Cebudonus n. gen. and Eumedonus also apply for Gonatonotus (cf. Chia & Ng 2000: figs. 17, 18A, D, 19A, 20, 21A, D, M, N, 22, 23A, D [front]; figs. 17, 18B, 20, 21B, 22, 23B [chelipeds]; figs. 17B, 19G, 20B, 21C, 22B, 23C [sternum]; figs. 19H, 21J, 22, 23J [abdomen]).

The form of the front, with the two long pseudorostral spines is similar to that of Tiaramedon and Zebrida, but these genera differ markedly from Cebudonus n. gen. in several other characters. Cebudonus n. gen., can be separated from Tiaramedon by its smooth dorsal surface of the carapace (Figs. 1, 2) (armed with additional gastric and branchial spines in Tiaramedon; cf Chia & Ng 1998: figs. 7A, 8A); the two pseudorostral spines joining medially without any trace of a median plate (Figs. 1, 2) (with a small plate present between the pseudorostral spines in Tiaramedon; cf. Chia & Ng 1998: fig. 8A, B); the chela is unarmed (Figs. 1, 2, 3 F) (with an additional spine on the dorsal distal margin in Tiaramedon; cf. Chia & Ng 1998: figs. 7A, B, 8K); the cheliped carpus has a single long inner spine (Figs. 1, 2, 3 F) (2 large spines in Tiaramedon; cf. Chia & Ng 1998: fig. 7A); the ambulatory merus is long and slender with a short dorsal distal tooth (Figs. 1, 2, 4 E) (short and stout with a long distal dorsal tooth in Tiaramedon; cf. Chia & Ng 1998: figs. 7, 8C); and the male anterior thoracic sternum is transversely narrow (Figs. 3 C, 4B) (transversely broad in Tiaramedon; cf. Chia & Ng 1998: figs. 7B, 8D). The male abdomens of Cebudonus n. gen. and Tiaramedon are nevertheless similar in shape (Figs. 3 D, 4C; cf. Chia & Ng 1998: figs. 7B, 8F). The two genera are also different in terms of host specificity, with Tiaramedon always associated with crinoids (and with young crabs characteristically transversely striped like many other crinoid-associated eumedonines, see Castro et al. 1995; Chia & Ng 1995, 1998) whereas Cebudonus n. gen. with its longitudinal stripes is probably associated with echinoids.

Cebudonus n. gen. can be separated from the echinoid symbiont Zebrida by having two subcylindrical pseudorostral spines (Figs. 1, 2) (distinctly lamelliform in Zebrida; cf. Ng & Chia 2000: figs. 2, 3A, B); the lateral carapace tooth and the dorsal surface adjacent to the tooth is dorsally convex (Fig. 3 B) (the area is distinctly dorsally flattened in Zebrida; cf. Chia et al. 1995: fig. 3B); the chela and merus are unarmed and the carpus has a long spine at the inner angle (Figs. 1, 2, 3 F) (the chela has a large subdistal dorsal tooth, the carpus and merus each having three large lamelliform teeth in Zebrida; cf. Ng & Chia 2000: figs. 2, 3J); the ambulatory merus is long and slender with a short dorsal distal tooth (Figs. 1, 2, 4 E) (short and stout with the margins cristate to dentate and with a large distal dorsal tooth in Zebrida; cf. Ng & Chia 2000: figs. 2, 3C, K, M); the ambulatory propodus and dactylus are normal (Figs. 1, 2, 4 D, E) (forming a subchelate clasping structure in Zebrida; cf. Ng & Chia 2000: figs. 2, 3C, K, M); and the male anterior thoracic sternum is proportionately narrower transversely (Figs. 3 C, 4B) (proportionately wider in Zebrida; cf. Ng & Chia 2000: figs. 2B, 3D). The male abdomens of Cebudonus n. gen. and Zebrida are similar, with both relatively wide transversely (Figs. 3 D, 4C; cf. Ng & Chia 2000: figs. 2B, 3F)

There are also similarities with Zebridonus but Cebudonus n. gen. can be separated by its two long pseudorostral spines (Figs. 1, 2) (with a distinctly dorsoventrally flattened lobiform pseudorostrum that is bifurcated distally to form two teeth in Eumedonus; cf. Chia et al. 1995: figs. 1, 2A, K); the lateral carapace tooth and the dorsal surface adjacent to it is dorsally convex (Fig. 3 B) (the area is distinctly dorsally flattened in Zebridonus; cf. Chia et al. 1995: figs. 1, 2A, 3C); the chela and merus are unarmed, and the carpus has a long spine on the inner angle (Figs. 1, 2, 3 F) (the chela has a distinct distal dorsal tooth, the carpus 2 large lamelliform teeth and the merus several distinct teeth in Zebridonus; cf. Chia et al. 1995: figs. 1, 2 I); the ambulatory merus is long, subcylindrical and smooth, without any marginal cristae and the dorsal distal tooth is relatively low (Figs. 1, 2, 4 E) (short, laterally flattened with distinct marginal cristae and a strong dorsal distal tooth in Zebridonus; cf. Chia et al. 1995: figs. 1, 2B, C); the anterior male thoracic sternum (sternites 1–4) are proportionately narrower transversely (Figs. 3 C, 4B) (proportionately wider transversely in Zebridonus; cf. Chia et al. 1995: fig. 2D); and the male abdomen is proportionately wider (Figs. 3 D, 4C) (proportionately narrower in Zebridonus; cf. Chia et al. 1995: fig. 2F).