Eumedonus H. Milne Edwards, 1834

Ng, Diana G. B. Chia Peter K. L., 2000, A revision of Eumedonus H. Milne Edwards, 1834 and Gonatonotus White, 1847 (Crustacea: Decapoda: Brachyura: Eumedonidae), two genera of crabs symbiotic with sea urchins, Journal of Natural History 34, pp. 15-56 : 16-17

Eumedonus H. Milne Edwards, 1834

Eumedonus H. Milne Edwards, 1834: 349 ; Dana, 1853: 1422; Neumann, 1878: 17; Miers, 1879a: 670; Alcock, 1895: 287; Flipse, 1930: 18, 20 [in key]; Gordon, 1934: 66; Monod, 1938: 110; Sakai, 1938: 329 [in key], 347, 348; Miyake, 1939: 83, 87; Buitendijk, 1950: 72; Barnard, 1954: 95, 1955: 18; SereÁne et al., 1958: 137, 139 [in key], 144, 146, 236; Sakai, 1976: 294 [in key], 295, 296 [English text], 178 [Japanese text]; Wu, 1983: 134 [name in Chinese]; Stevcic et al., 1988: 1310, 1317

T ype species. Eumedonus niger H. Milne Edwards, 1834 , by monotypy.

Etymology. The origin of the name `Eumedonus ’ is uncertain. Gender masculine by inference from type species name `niger ’.

Diagnosis. Carapace pentagonal, rostrum long or relatively short; inner supraorbital teeth absent; regions well de®ned; surfaces of carapace, chelipeds and ambulatory legs covered with numerous ¯attened granules which appear squamous in some species or tall in others; dorsal surface of carapace pileiferous to various degrees, usually pubescent between granulated areas with longer setae on the protogastric regions and distal part of rostrum. Antero- and posterolateral margins clearly demarcated by distinct angle or sharp tooth; anterolateral margin entire, without teeth or lobes, distinctly shorter than posterolateral margin. Anterior surface of epistome not low. Antero-external angle of merus of third maxilliped auriculiform in most species. Antennules folding obliquely, ca 45ss from horizontal. Antennal segment 2 short, length to width ratio 0.8±1.3. Male anterior thoracic sternites appearing proportionately narrower compared to other eumedonid genera (except Gonatonotus ). Chelipeds granulose; dorsal margin of palm usually with distinct lamelliform crests in females; carpus with distinct spine on distal inner angle; inner margin of basis-ischium, median, inner and outer proximal margins of merus with strong teeth, structure sometimes appearing lamelliform; chela short, stout, palm 1±2 times length of ®ngers, palm height ca 2±3 times height of ®ngers; ®ngers sometimes carinate, pollex not bent downwards. Anterior margins of ambulatory merus and carpus distinctly cristate, propodus slightly cristate; posterior margin of merus with two parallel crests, dorsal crest stronger than ventral crest in merus 1±3; ventral crest stronger than dorsal crest in merus 4; dactylus of leg 1 not much longer than those on other legs. G1 long, slender.

Sexual dimorphism. High lamelliform crests on dorsal margin of palm in females, low in males.

Remarks. The de®nition of the genus Eumedonus H. Milne Edwards, 1834 , has been unclear for some time. Gordon (1934), Monod (1938), Buitendijk (1950) and SereÁne et al. (1958) have commented on the validity of the genus Gonatonotus (type species G. pentagonus White, 1847 ) and its supposed a nities with Eumedonus . Monod (1938) and Buitendijk (1950) synonymized Gonatonotus under Eumedonus . SereÁne et al. (1958), however, preferred to accept both genera as distinct, albeit with doubt. There is little doubt that Eumedonus and Gonatonotus are very similar, and the apparent`dissimilarities’ are because of the long rostra of the known species of Eumedonus against the very short rostrum of the type species of Gonatonotus , G. pentagonus .

The genus Gonatonotus can, however, always be separated from Eumedonus by one clear characterÐthe dorsal margin of the ambulatory merus is never cristate, and is lined instead with numerous granules (distinctly cristate and almost entire in Eumedonus ). The present rede®nition of Gonatonotus requires the transfer of Eumedonus granulosus MacGilchrist, 1905 , to Gonatonotus . Other than the distinctly longer rostrum and somewhat sharper anterolateral spines which make E. granulosus di erent in appearance compared to G. pentagonus , E. granulosus has slender and non-cristate legs.

In the present study, the genus Eumedonus contains ®ve species, two of which are recognized as new. They are: E. niger H. Milne Edwards, 1834 , E. brevirhynchus n. sp., E. intermedius n. sp., E. vicinus Rathbun, 1918 , and E. zebra Alcock, 1895 .