Two new species of Urothoe (Crustacea, Amphipoda, Gammaridea) from the East Johor Islands Archipelago, Malaysia

Abstract Two new species of urothoid amphipods from Pulau Sibu and Pulau Tinggi, Johor are described and illustrated. The specimens of Urothoe sibuensis new species were collected by vertical haul plankton net and is distinctively different from other existing Urothoe species by these combination of special characters; similar gnathopods 1–2 with short and stout propodus expanded into poorly defined palms; large eyes and epimeron 3 smooth. Urothoe tinggiensis new species as collected using an airlift suction sampler at seagrass area is characterized by its different gnathopodal configuration with setose dactylus of 5th pereopod; eyes minute; carpus is wider than merus in the 5th pereopod; subquadrate coxa 4; merus and carpus of pereopods 6–7 are linear.


Introduction
Th e genus Urothoe Dana, 1852 has been known to be cosmopolitan and is one of the most diverse genus of the family Urothoidae Bousfi eld, 1978. Th ey are easily recognized by their small sized body and furnished with various fossorial adaptations in the form of extremely setose appendages (Barnard and Drummond 1982). Th ey are also known to be very diverse in shallow habitats and act as an important component of the burrowing fauna of intertidal and shallow subtidal areas but scarce in the deep sea (Barnard and Karaman1991).
Up till now, only one species, Urothoe gelasina Imbach, 1969 was recorded in Malaysian waters by Azman (2007), which was previously recorded in the Bay of Nhatrang, Vietnam. Taxonomic studies on Amphipoda in this region particularly in Malaysian waters is still sparse, nevertheless there is an increasing attempt to improve the knowledge and several contributions have been recently published such as Müller (1993), Morino (1996, 2006), Othman and Azman (2007), Azman (2007), and Lim et al. (2010). In this paper we describe two new species from this region: Urothoe sibuensis new species and Urothoe tinggiensis new species.

Material and methods
Th e amphipods in this study were obtained from Pulau Tinggi for Urothoe tinggiensis and Pulau Sibu for Urothoe sibuensis; both from the waters of Johor, southeast coast of Peninsular Malaysia (Figure 1). Airlift suction sampler and plankton net of 100 μm mesh size were used in the collection of the specimens. All materials are lodged in the Universiti Kebangsaan Malaysia Muzium Zoologi (UKMMZ). Th e following abbreviations are used on the plates: A, antenna; G, gnathopod; HD, head; LL, lower lip; MD, mandible; MX, maxilla; MP, maxilliped; P, pereopod; PL, pleopod; EP, epimeron; T, telson; U; uropod; UR, urosome; UL, upper lip; ♂, male; ♀, female.
Remarks. Urothoe sibuensis is very closely related to the 'elegans' group, defi ned by J.L. Barnard (1962), which included twenty other Urothoe species. Th is group of species is characterized by having similar gnathopods 1 -2 with short and stout propodus expanded into poorly defi ned palms. Despite of that, detectable distinctions of morphological characters are found between the present species and the members of this group. As such, U. sibuensis diff ers in not having defi ning robust setae in propodus of gnathopods 1 -2 (defi ning robust setae present in U. gelasina, U. gelasina ambigua, U. poucheti, U. varvarini), the antenna 2 signifi cantly longer than antenna 1 (antenna subequal in length in U. bairdii, U. brevicornis, U. hesperiae, U. intermedia, U. marina), epimeron 3 smooth (epimeron 3 having distinct tooth in U. chosani, U. dentata, U. denticulata, U. marionis), presence of eyes (absence of eyes in U. abbreviata, U. latifrons, U.vemae), merus and carpus of pereopod 5 are both subequal in width (merus is wider than carpus of pereopod 5 in U. pulchella, U. spinidigitus), epimeron 2 having several plumose setae (epimeron 2 naked in U. elegans) and a less pronounced cephalic lobe (lateroventral cephalic corner of U. corsica produced and upturned).
Th e present species is especially close to U. spinidigitus Walker, 1904. It diff ers from U. spinidigitus by the absence of rostrum in the present species (small rostrum present in U. spinidigitus) and the absence of defi ning palmar spines in both gnatho-pods. Th e dactylus of pereopod 5 in Walker's species bears 4 short and 4 long robust setae whereas U. sibuensis possesses a nodulate dactylus in its pereopod 5. Th e subequal merus and carpus of pereopod 5 in the present species clearly distinguish it from U. spinidigitus that has carpus twice as broad as merus.
Distribution. Malaysia. Johor, Pulau Sibu. Head lateroventral cephalic lobes rounded and very pronounced, rostrum absent. Eyes minute, circular. Antenna 1 peduncular articles 1 -2 provided with setae along the ventral margin; accessory fl agellum triarticulate; main fl agellum distinctly shorter than peduncle, about as long as peduncle article 3, 5 articulate. Antenna 2 subequal to antenna 1; fl agellum biarticulae; fi rst article with several robust setae and long setae all over; second article with several robust setae and long setae. Upper lip semicircular. Lower lip inner plate large, shoulders broad, mandibular process prominently winged     outward. Maxilla 1 inner plate with pinnate seta apically; outer plate with two setae, one pinnate and a seta; palp densely fi lled with robust setae apically. Maxilla 2 inner and outer plates furnished with pinnate and simple setae. Mandible molar rounded and well-developed; incisor smooth; palp strong; fi rst article shorter than second article; second and third article subequal in length; third article with several robust setae distally. Maxilliped inner plate elongate with about 2 blunt robust setae; outer plate suboval with several spines and setae; palp 4-articulate, second article wide and densely setose on inner margin, third article subtrianglular with several long setae, fourth article subtriangular and small.
Pereon Gnathopod 1 coxa narrow, subrectangular; basis elongate with several long setae along the posterior margin; carpus broad furnished with several setae at posterior margin, about as broad as long; propodus wide with several long setae situated anterodistally; dactylus normal. Gnathopod 2 coxa narrow, subrectangular; basis elongate with several long setae along the posterior margin; carpus suboval in shape; propodus wide provided with several long setae at anterodistal angle; dactylus falcate, slightly extending palm when closed.
Remarks. According to the proposed classifi cation of the 'falcata' group by Barnard (1962), members of this group possess dissimilar gnathopods; gnathopod 1 simple, elongated and slender propodus, bearing no palm; gnathopod 2 has a suboval or slightly expanding propodus with a distinct, rounded palm. With these characters our species is clearly attributed to the 'falcata' group. However, there are perceivable diff er-ences that can be ruled out between the new species and all the members of the group. As such, the setose dactylus of pereopod 5 in U. tinggiensis is readily distinguished from the non-spinose dactylus of pereopod 5 in U. pinnata and U. tumorosa. Diff ering from U. platydactyla in the expanding and elliptical shape of the pereopod 5 dactylus. Both the pereopod 6 and 7 are somewhat diff erent from U. platypoda; the merus and carpus in U. tinggiensis is considerably linear and elongated while the merus and carpus of both pereopod 6 and 7 in U. platypoda are lobed posteriorly. Th e enormously produced coxa 4 in U. coxalis is contrastingly distinctive from the normal subquadrate coxa 4 of U. tinggiensis. Th e new species appears to have close affi nities to U. orientalis due to the wide carpus of pereopod 5 but clearly distinguished from the latter in the shape of propodus of gnathopod 1 and the absence of robust setae on the outer ramus of uropods 1 and 2. Urothoe tinggiensis is also distinguished from U. oniscoides by the presence of eyes. Th e new species has some remarkable similarities with U. cuspis Imbach, 1969 from Bay of Nhatrang, Vietnam, but seem to be diff erent from the latter in the acuminate cuspidate coxa 2.