Macrophthalmus (Macrophthalmus) pentaodon, Mendoza & Naruse, 2009

Mendoza, Jose Christopher E. & Naruse, Tohru, 2009, A New Species Of Macrophthalmus Desmarest, 1823 (Crustacea: Decapoda: Brachyura: Macrophthalmidae) From Bohol Island, The Philippines, Raffles Bulletin of Zoology 57 (2), pp. 475-480 : 476-478

Macrophthalmus (Macrophthalmus) pentaodon , new species

Fig. 1

Material Examined. – Holotype: female (5.0 by 3.5 mm) ( NMCR 27012 ), Stn. S 21, reef slope with silt, 4–12 m, 9°41.7'N 123°50.9'E, Manga , Bohol Island, Philippines, coll. by PANGLAO 2004 Marine Biodiversity Project, 20 Jun.2004.

Description of female holotype. – Carapace ( Fig. 1a ) rectangular, regions not well-defined; dorsal surface naked, relatively smooth, with short transverse ridge on metabranchial region; greatest width across external orbital teeth, 1.4 times as wide as long. Front 0.2 times as wide as carapace, with shallow median furrow, not constricted at bases of ocular peduncles; frontal margin thinly rimmed. Upper orbital border steeply divergent posteriorly, with wide convexity on inner third, smooth, sub-cristate; lower orbital border sub-cristate, terminating just anterior to anterior margin of chelipedal coxa, outer half sparsely lined with very low granules. Lateral margins of carapace subparallel, armed with 5 anterolateral teeth (including external orbital tooth), followed posteriorly by small tubercles; external orbital tooth narrow, acute, exceeding all other subsequent teeth; tips of subsequent teeth more rounded, bases wider, third largest, fifth smallest; third tooth separated from fourth by wide V-shaped incision. Posterior margin of epistome ( Fig. 1b ) convex medially, without endostomial ridge.

Ocular peduncle ( Fig. 1a, b ) long and stout, margins slightly sinuous, exceeding exernal orbital tooth by 11.5% of its total length and half length of cornea in frontal view.

Third maxillipeds separated by median hiatus, not completely covering buccal cavern when closed; median length of ischium about 1.5 times that of merus; merus and ischium with setose inner margins; distal external margin of merus rounded. Exopod narrow ( Fig. 1c ).

Chelipeds ( Fig. 1d ) symmetrical, weak. Palm of chela smooth on inner surface, outer surface sparsely granulated, upper margin with a single row of minute granules, lower margin with row of granules adjacent to it, extending as a distinct ridge onto fixed finger, proximal part on palm granular; fingers nearly flat, dactylus about 1.2 times longer than palm, slightly curved, cutting edge toothless, fixed finger slightly deflexed, cutting edge with a few minute teeth on the middle region.

Second to fifth pereiopods ( Fig. 1a ) relatively long; meri of second to fifth pereiopods with subdistal tooth, third and fourth pereiopods with wider meri; dactyli with subrhomboidal cross-section, rather compressed laterally, dactylus of fourth pereiopod about 0.8 times as long as propodus, sparsely setose on flexor surface.

Abdomen ( Fig. 1e ) wide, fully expanded, lateral margins convex; telson 2.3 times as wide as long, proximal margin straight, slightly narrower than and appearing embedded into distal margin of sixth segment; sixth segment 2.2 times wider than telson, distal margin concave; fifth segment 1.1 times wider than third; fourth segment widest; first segment with a transverse ridge dividing it into upper and lower halves.

Genital opening situated just posterior to distal margin of thoracic sternite 6; operculum sub-ovate, directed medially.

Comparative Material. – Macrophthalmus (Macrophthalmus) philippinensis Serène, 1971 , holotype male (10.0 by 5.6 mm) (ZRC.1969.12.12.1), paratype male (10.0 by 5.6 mm) (ZRC. 1969.12.12.2), Busuanga , Palawan, Philippines, coll. by Norton and Dayrit, 10–30 May.1933.

Macrophthalmus (Macrophthalmus) ryukyuanus Naruse & Kosuge, 2008 , holotype male (9.3 by 5.3 mm) ( ZRC 2007.0524 ), paratype male (CL 3.5 mm) (RUMF-ZC-538) off mouth of Urauchi River , Iriomote Island, Ryukyu Islands, Japan (24˚24.98'N, 123˚46.04'E), 19 m depth, coll. T. Kosuge and K. Higa, dredge, 16 Aug.2005.

Macrophthalmus (Tasmanoplax) latifrons Haswell, 1882 , 1 male (27.0 by 16.7 mm), 1 ovig. female (23.1 by 15.0 mm) (ZRC 1965.7.20.43–44), Oxford, Tasmania, coll. by Anonymous , 1941.

Ilyoplax dentata Ward, 1933 , 1 male (5.5 by 4.0 mm) ( ZRC 1995.959 ), estuarine muddy bay, 10.44°S, 142.33°E, far north of Queensland, Australia, coll. by J. Short, 26 Oct.1990.

Ilyoplax gangetica ( Kemp, 1919) , 1 male (7.0 by 4.5 mm) ( ZRC 2001.2339 ), Stn. KR (S)1, mud, Ranong, Thailand, coll. by P. Clark, 11 Nov.2001 .

Ilyoplax obliqua Tweedie, 1935 , 1 male (4.6 by 3.3 mm), 1 ovig. female (4.7 by 3.2 mm) ( ZRC 1987.227 – 228 ), Lim Chu Kang, Singapore, coll. by P.K.L. Ng, 19 Mar.1987.

Ilyoplax orientalis (De Man, 1888) , 1 male (5.8 by 4.3 mm) ( ZRC 1987.231 – 239 ), Lim Chu Kang, Singapore, coll. by P.K.L. Ng & S. Harminto, 3 Sep.1986 .

Tmethypocoelis ceratophora ( Koelbel, 1897) , 1 male (6.4 by 3.8 mm) ( ZRC 1993.622 – 625 ), Horikawa reef flat, Tamagusuku Village , Okinawa, Japan, coll. by P.K.L. Ng, Apr.1992 .

Habitat and Distribution. – The sole specimen of Macrophthalmus (Macrophthalmus) pentaodon , new species, was found in muddy to silty bottoms at depths ranging from 4 to 12 m. This species, collected by vacuum suction of the bottom sediment, is reported, thus far, only from the type locality, Bohol island, Philippines .

Etymology. – The specific epithet, pentaodon , is an arbitrary combination from the Greek words penta (n., five) and odon (n., tooth), referring to the five anterolateral teeth on the carapace of this species. Used as a noun in apposition.

Remarks. – Macrophthalmus (Macrophthalmus) pentaodon , new species, is morphologically unique in the Macrophthalmidae in its posteriorly divergent upper orbital borders. The new species clearly belongs to Macrophthalminae , typically possessing the features of the subfamily such as elongated eyestalks, a rectangular carapace, antennules that fold transversely or obliquely, a narrow inter-antennulary septum, and external maxillipeds that do not completely close the buccal cavern ( Barnes, 1967; Sakai, 1976; Davie, 2002). It can easily be distinguished from Dotillidae Stimpson, 1858 , and superficially similar genera such as Ilyoplax Stimpson, 1858 and Tmethypocoelis Koelbel, 1897 , by having a more rectangular carapace ( Fig. 1a ) (vs. more quadrate in dotillids); having rectangular merus of the third maxilliped about half the size of the ischium ( Fig. 1c ) (vs. merus oval, larger or as large as ischium in dotillids); having no tympani on the merus of the walking legs (vs. present in some species of Ilyoplax ); and having a distinct and acute subdistal spine on the anterior margin of the merus of the walking legs (vs. no subdistal spine) ( Fig. 1a ). (cf. Kemp, 1919; Serène & Lundoer, 1974; Davie, 1990). Members of Camptandriidae Stimpson, 1858 have various carapace shapes. Since the most discriminative character of the family is the strongly recurved G1 of the male, it is sometimes difficult to identify the familial place of female specimens. However, Macrophthalmus pentaodon has relatively long ocular peduncles and a narrow front, and in this regard, this new species can be easily distinguished from Camptandriidae .

Within Macrophthalminae , M. pentaodon clearly falls under the genus Macrophthalmus primarily due to condition of the third maxilliped, wherein the merus is wider than long and is much smaller than the ischium, which is longer than wide; the long ocular peduncles; and the anteriorly divergent lateral carapace margins (cf. Barnes, 1967). It can easily be distinguished from Australoplax Barnes, 1966 , under which there is only one species, A. tridentata (A. Milne-Edwards, 1873) by 1) the more rectangular carapace outline (vs. more quadrate in A. tridentata ) ( Fig. 1a ); 2) the straight frontal margin (vs. obtusely convex) ( Fig. 1a, b ); 3) having 5 triangular anterolateral teeth (vs. having 3 quadrangular anterolateral teeth) ( Fig. 1a ); 4) having long eyes that go beyond the external orbital tooth (vs. short eyes that do not go beyond external orbital tooth) ( Fig. 1a ); 5) the merus of the third maxilliped which is half as large as the ischium (vs. merus and ischium subequal) ( Fig. 1c ); 6) having the exopod of the third maxilliped reaching up to the anterior margin of the merus (vs. exopod shorter, reaching only to mid-level of merus) ( Fig. 1c ); and 7) the relatively longer and more slender walking legs (vs. shorter and stouter) ( Fig. 1a ) (cf. Barnes, 1966: pl. XXIV; Barnes, 1967: 241–244, fig.14, pl. IV). Macrophthalmus pentaodon can also be easily distinguished from the monotypic genus Enigmaplax Davie, 1993 , which contains E. littoralis Davie, 1993 , by the following characters: 1) the more rectangular carapace outline, with the lateral margins divergent anteriorly (vs. more squarish, lateral margins subparallel to convergent in E. littoralis ) ( Fig. 1a ); 2) the relatively narrower front (vs. very wide front in E. littoralis ) ( Fig. 1a, b ); 3) the steeply oblique upper orbital border (vs. horizontal in E. littoralis ); 4) the relatively longer ocular peduncles, which go just beyond the tip of the external orbital tooth (vs. short and stout, not reaching or just reaching the tip of the external orbital tooth) ( Fig. 1a ); 5) the convex central portion of the epistome posterior margin (vs. concave) ( Fig. 1b ); 6) and the rectangular aspect of the third maxilliped, without any external or internal extensions in either merus or ischium (vs. the sigmoidal aspect of the third maxilliped, due to an anterolateral extension in the merus and a posteromesial extension in the ischium) ( Fig. 1c ) (cf. Davie, 1993: 5–9, figs. 1, 2).

At the subgeneric level, we believe that M. pentaodon belongs in the subgenus Macrophthalmus (see Barnes, 1967; Komai et al., 1995) wherein the posteromedian margin of the epistome is protuberant, not concave, and the ocular peduncles are more than 0.5 times as long as the carapace ( Figs. 1a, b ). It differs from the subgenus Tasmanoplax Barnes, 1967 , the only other subgenus with a protuberance in the epistome’s posteromedian margin, in that the ischium of the third maxilliped is distinctly larger than the merus ( Fig. 1c ), whereas in Tasmanoplax they are subequal (cf. Barnes, 1967). Macrophthalmus pentaodon differs from all other species in the subgenus Macrophthalmus in having five anterolateral teeth, the greatest number for this genus, thus far. By contrast, other species of Macrophthalmus (Macrophthalmus) have either two or three (rarely four) anterolateral teeth, including the external orbital angle (cf. Barnes, 1967; 1970; 1971; 1977). Species in the subgenus Macrophthalmus tend to have relatively thicker ocular peduncles as juveniles, this was also observed in M. pentaodon although the female holotype is clearly adult as it already possesses a fully expanded abdomen ( Fig. 1e ). In most other species the adults have thin ocular peduncles together with bulbuous corneas. The relatively thick ocular peduncle and small cornea ( Fig. 1a, b ) of M. pentaodon can be also considered as a unique character of this small species.

Macrophthalmus pentaodon , new species, is perhaps most similar to M. ryukyuanus Naruse & Kosuge, 2008 , in being relatively small in size and in having the ocular peduncle only weakly exceeding beyond the external orbital tooth. However, M. pentaodon differs from M. ryukyuanus in the following features: (1) absence of a constriction on the front across the bases of the ocular peduncles ( Fig. 1a ) (vs. present in M. ryukyuanus ), (2) steeply oblique upper orbital border ( Fig. 1a ) (vs. moderately oblique in M. ryukyuanus ), (3) five teeth on the anterolateral margin of the carapace ( Fig. 1a ) (vs. three teeth on the anterolateral margin of the carapace in M. ryukyuanus ), and (4) acicular dactylus of the fourth ambulatory leg (vs. lanceolate dactylus of the fourth ambulatory leg in M. ryukyuanus ) (cf. Naruse & Kosuge, 2008). In addition to this, M. pentaodon differs from M. ryukyuanus in the relative length of the ocular peduncle and in body size. The extent to which the ocular peduncles exceed the length of the orbit in M. pentaodon is 11.5% of its total length (holotype, CW 5.0 mm), whereas it is 21% in M. ryukyuanus (holotype, CW 9.3 mm). Macrophthalmus pentaodon is also superficially similar to M. philippinensis due to their small size and the general carapace outline. However, it can be distinguished from M. philippinensis by 1) the 5 anterolateral teeth on the carapace (vs. 3 anterolateral teeth in M. philippinensis ); 2) the absence of a constriction on the front across the bases of the ocular peduncles (vs. present); and 3) the shorter, thicker ocular peduncles with small corneas (vs. longer ocular peduncles, half their length going beyond the tip of the external orbital angle, with larger, bulbous corneas) (cf. Serène, 1971).

Figures

Fig. 1. Holotype of Macrophthalmus (Macrophthalmus) pentaodon, new species, NMCR 27012 (5.0 by 3.5 mm). a, habitus; b, frontal view showing eye, front, antenna, antennules & epistome; c, right third maxilliped; d, left chela, external view; e, telson and abdominal segments. Scales, 1 mm.