Ilyograpsus Barnard, 1955

Komai, Tomoyuki & Wada, Keiji, 2008, A Revision Of The Estuarine Crab Genus Ilyograpsus Barnard, 1955 (Crustacea: Decapoda: Brachyura: Macrophthalmidae), With Descriptions Of A New Genus And One New Species, Raffles Bulletin of Zoology 56 (2), pp. 357-384 : 358-360

Ilyograpsus Barnard, 1955

Ilyograpsus Barnard, 1955: 25 ; Crosnier, 1965: 31; Sawada et al., 2005: 852.

Type species. – Ilyograpsus rhizophorae Barnard, 1955 , by monotypy.

Diagnosis. – Carapace flattened, slightly wider than long, suboctogonal in general outline; dorsal surface smooth or microscopically granular; lateral margin usually with 4 (rarely 5) teeth including external orbital tooth, second tooth usually blunt. Front not constricted basally, proximal width slightly greater than half of front-orbital width. Postfrontal ridges transverse, without long setae. Orbit large; lower orbital margin laterally with 3–5 lobules in males, entirely granular in females, mesial angle not markedly produced; small inner orbital tooth present. Ocular peduncle stout, just reaching or slightly overreaching external orbital tooth; cornea large. Antennules transverse or slightly oblique; basal segment inflated. Interantennular septum narrow. Antennae in orbital hiatus; flagellum long, reaching or overreaching cornea. Central region of epistome distinctly convex. Third maxillipeds ( Fig. 5E ) not completely closing buccal cavern; carpus inserted medially on anterior margin of merus; merus smaller than ischium, with lateral and mesial margins upturned on outer surface, anterolateral angle rounded; merus with sinuous mesial and concave lateral margins, external surface with shallow longitudinal sulcus mesial to midline; no setose ridge on external surface of merus-ischium; exopod slender, reaching anterolateral angle of merus, width about 0.3 times width of ischium; exopodal flagellum well developed. Merus of cheliped not markedly narrowed distally, usually with small subdistal spine on dorsal margin; carpus without prominent tooth or spine on inner surface; fingers spoon-shaped at tips, not crossing when closed. Merus of male cheliped with or without prominent short crest on inner margin; dactylus with or without differentiated tooth on cutting edge. Ambulatory legs slender to moderately stout; meri not markedly tapering distally, each with sharp subdistal spine on anterior margin; propodi each with sharp tooth at each posterodistal angle; dactyli long, weakly curved, slightly compressed laterally, unarmed. Pleon of male ( Fig. 3D ) not occupying entire space between last pair of ambulatory legs, consisting of 7 free somites including telson; no medial constrictions; second somite with angular lateral margins; third somite weakly expanded laterally; third to sixth somites subequal in length, evenly tapering; telson longer than other somites, rounded terminally. Pleon of female ( Fig. 5D ) broadly rounded; fourth segments somite broadest; telson roundly triangular. First gonopod slightly curved, not constricted. Female gonopores some distance from suture between fifth and sixth sternites.

Remarks. – Ilyograpsus Barnard, 1955 , has been generally assigned to the family Grapsidae ( Barnard, 1955; Crosnier, 1965; Takeda & Nunomura, 1976; Yamaguchi et al., 1976; 1987; Sakai, 1983; Yeo et al., 2004), although the subfamilial assignment has been a subject of some disagreement, i.e., Varuninae (e.g., Yeo et al., 2004) or Grapsinae (e.g., Barnard, 1955; Crosnier, 1965). On the other hand, other authors cast doubt on the assignment of the genus to the Grapsidae sensu lato Fukuda (1978) and Flores et al. (2003) showed that the larval morphology of what they regarded as “ Ilyograpsus paludicola ” (these authors probably dealing with two separate species; see Remarks under the account of I. paludicola ) is similar to that of Macrophthalmus , which has been referred to the subfamily Macrophthalminae Dana, 1851 , family Ocypodidae Rafinesque, 1815 . Naruse & Kishino (2006) also highlighted similarities in adult morphology between Ilyograpsus and Macrophthalmus , and they placed Ilyograpsus in the family Macrophthalmidae , following the recent trend in raising of all ocypodid subfamilies to full family status ( Kitaura et al., 2002; Stevcic ˇ ˇ´, 2005; Schubart et al., 2006).

During this study, we also noticed that Ilyograpsus sensu stricto is morphologically very similar to Enigmaplax Davie, 1993 , a monotypic genus (type species E. littoralis Davie, 1993 ) also originally placed in the Macrophthalminae (now Macrophthalmidae ). Shared characters include: (1) the relatively wide orbit which is subequal to the width of the front and which accommodates the large ocular peduncle; (2) the presence of distinct postfrontal ridges; (3) a convex epistomal margin; (4) the third maxilliped with a merus which is smaller than the ischium; (5) the presence of a sharp subdistal spine on the dorsal margin of each ambulatory merus; and (6) the structure of the male pleon and gonopods (cf. Davie, 1993). It should be noted that the last two characters are also found in species of Macrophthalmus (personal observation). As well as Enigmaplax , Ilyograpsus agrees on most points with the hypothesized ancestral Macrophthalmus of Barnes (1967). The main points of differences are that the merus of the third maxilliped is smaller than the ischium as opposed to subequal; and the epistome has a convex central margin, not straight.

Main differences between Ilyograpsus and Enigmaplax are as follow. The general contour of the carapace is suboctogonal in Ilyograpsus , rather than quadrate in Enigmaplax . The lateral margin is armed with four teeth (including the external orbital tooth), rather than three in Enigmaplax . Furthermore, the fingers of the chela are spoon-shaped in Ilyograpsus , whereas they are pointed distally and cross each other in Enigmaplax . Ilyograpsus appears characteristic among macrophthalmids in having four anterolateral teeth, of which the second is always blunt. Most of other macrophthalmid species have two or three anterolateral teeth, except M. philippinensis Serène, 1971 , M. leptophthalmus (H. Milne Edwards, 1852) and M. latreillei ( Desmarest, 1822) , which have four anterolateral teeth ( Barnes, 1967, 1977).

Our study shows that the recently described Ilyograpsus paantu is quite distinctive, and the inclusion of it in the genus complicates the generic diagnosis of Ilyograpsus . In fact, we could not identify any possible synapomorphy between I. paantu and other species of Ilyograpsus when compared with other macrophthalmid genera. Therefore, we propose a new genus to accommodate I. paantu (see Remarks under the account of Apograpsus , new genus). As a result, the family Macrophthalmidae now comprises of five genera, Macrophthalmus Desmarest, 1823 , Australoplax Barnes, 1966 , Enigmaplax Davie, 1993 , Ilyograpsus Barnard, 1955 , and Apograpsus , new genus.

As mentioned before, Ng et al. (2008) recognized two subfamilies within Macrophthamidae, Macrophthaminae and Ilyograsinae, raising the tribes proposed by Stevcicˇ ˇ´(2005). We feel doubt in recognizing such a subfamilial division, because of the close similarities between Ilyograpsus and Enigmaplax , of which the latter is assigned to Macrophthaminae . Our morphological comparison seems to suggest that Ilyograpsus is rather basal within Macrophthamidae. In this study, we do not maintain the subfamilial classification.

Other than the characters relating to the cheliped, pleon and pleonal appendages, the following differences may be related to sexual dimorphism in llyograpsus: the regions of the carapace is more distinctly defined in males than in females; the lower orbital margin is provided with three or four differentiated lobules laterally in males, whereas it is lined with minute to small granules or nearly smooth in females; and the ambulatory legs are relatively longer and more slender in males than in females. Therefore, only comparisons of specimens of the similar sex are effective for species recognition.

Sawada et al. (2005) and Naruse & Kishino (2006) cited several characters for species discrimination (e.g., condition of the dorsal carapace surface, arrangement of the anterolateral teeth of the carapace, ornamentation of the cheliped merus, and structure of the first gonopod). However, our study, using more extensive material, suggests that some of them are not always reliable because of individual or ontogenetic variability. For example, the degree of the dorsal sculpture on the carapace is substantially different between males and females in I. paludicola and I. nodulosus . The dentition of the fingers of the chelipeds is considerably more variable between different sexes and between different growth stages in males (see “Description” of respective species). As mentioned above, the ornamentation of the lower orbital margin is different between males and females. Also the ornamentation of the cheliped merus varies according to sex and different stages of development. We have found that the following characters are particularly useful in discriminating species: general shape of the carapace; development of the delineation of regions and ornamentation on the dorsal surface of the carapace; armature of the cheliped merus; length of the dactylus of the fourth pereopod; and the structure of the first gonopod. The shape of the ambulatory legs is also useful, although the ambulatory legs may become stouter with increase of the body size. In this study, the proportions of the merus and propodus of the fourth pereopod and of the propodus of the fifth pereopod are used as representatives.

Figures

Fig. 5. Ilyograpsus rhizophorae Barnard, 1955. A–E, I, J, ovigerous female (cl 7.2 mm), MNHN-B 12842, Tuléar, Madagascar; F, H, ovigerous female (cl 5.3 mm), MNHN-B 12843, same locality; G, female (cl 7.4 mm), paratype of Ilyograpsus vannini Sawada, Hosogi & Sakai, 2005, RMNH-D 26501, south of Massawa, Gulf of Zula, Ethiopia. A, carapace, ocular peduncles and antennae, dorsal view; B, anterior part of cephalothorax, including left ocular peduncles and antennae, frontal view; C, left lower orbital margin, ventral view; D, pleon, ventral view; E, left third maxilliped, ventral view; F, left cheliped, dorsal view; G, same, merus, outer view; H, same, chela, outer view; I, right fourth pereopod, dorsal view; J, right fifth pereopod, dorsal view. Scale bars: A, D, F = 2 mm; B, C, E, G, I, J = 1 mm; H = 0.5 mm.

Fig. 3. Ilyograpsus rhizophorae Barnard, 1955. Male (cl 6.5 mm), paratype of Ilyograpsus vannini Sawada, Hosogi & Sakai, 2005, RMNH- D 26501, south of Massawa, Gulf of Zula, Ethiopia: A, carapace, ocular peduncles and antennae, dorsal view (setae omitted); B, left lower orbital margin and external orbital tooth, ventral view; C, anterior part of thoracic sternum, ventral view; D, pleon, ventral view; E, left cheliped, dorsal view; F, same, merus, outer view; G, same, chela, outer view; H, left fourth pereopod, dorsal view; I, left fifth pereopod, dorsal view; J, left first gonopod, ventral view; K, same, ventrolateral view; L, same, distal part, dorsal view. Scales: A = 2 mm; B–I = 1 mm; J–L = 0.5 mm.