A new species of Pseudodiaptomus (Crustacea, Copepoda, Calanoida, Pseudodiaptomidae) from the Prasae River Estuary, Gulf of Thailand

Abstract A new species of the calanoid copepod genus Pseudodiaptomus was collected from the Prasae River Estuary, Rayong Province, on the eastern coast of the Gulf of Thailand. This species is definitely assigned to the lobus species group sensu Walter (1986a). The female of the new species differs from other congeners in the elongate genital double-somite with a blunt process ventrally and the second urosomite about 2.54 times as long as wide. The male is also easily distinguished from other congeners by the structure of the right fifth leg. The present new species is a euryhaline species and occurred in brackish waters with salinity ranging from 0.7 to 23.3. Its breeding season may be from June to October, as indicated by the presence of egg-sacs.


Introduction
We have been intensively investigating the taxonomy, biology and ecology of gelatinous and crustacean zooplankters in Thailand since 1997 (Pinkaew et al. 1997(Pinkaew et al. , 2000Pinkaew 2003;Ohtsuka et al. 1999Ohtsuka et al. , 2003Ohtsuka et al. , 2010Ohtsuka et al. , 2012Fukuoka and Pinkaew 2003;Fukuoka et al. 2005;Nishida and Nishikawa 2011;Nishikawa et al. unpublished). Special attention has been paid to copepods, mysids and rhizostome jellyfish, due to their numerical importance in the plankton communities in the coastal and estuarine waters.
Leg 5 (Figs 4D, E, F, G) highly asymmetrical and biramous; intercoxal sclerite and both coxae fused; coxa with fine spinular rows on anterior surface. Right leg (Figs 4D, E) with basis having outer spinular row; endopod rudimentary, represented by knob-like process with fine setule at tip; exopod (Fig. 3F) 3-segmented, first segment protruded into outer process reaching middle of third segment, proximal process with 1 spine and spinular row; second segment expanded midway, each side with spine; third segment curved inward with 3 rows of spinules on anterior surface and middle swelling, distal to which tapering distally. Left leg (Figs 4D, E) with elongated basis having triangular process at midlength; endopod (Fig. 4G) highly developed, bifurcated, inner medial process smoothly curved outward reaching distal tip of second exopod, outer process thickened, foliaceous with 1 subterminal and 4 thin terminal protrusions; exopod 2-segmented, first segment as long as basis, irregularly sinuated along inner margin; second segment triangular with hirsute process proximally and stout serrated protrusion at medio-lateral margin, with 3 processes of unequal length terminally.
Remarks. The present new species can be definitely assigned to the lobus species group sensu Walter (1986aWalter ( , b, 1987 and Walter et al. (2006) in having a combination of the following features: (1) paired egg-sacs; (2) a fusion between the cephalosome and first pedigerous somite; (3) the presence of a large endopod of male left leg 5; (4) the presence of a rudimentary endopod of male right leg 5. Its estuarine habitat in the West Pacific also supports this assignment (see Walter et al. 2002). In this species group two subgroups, forbesi-subgroup and poppei-subgroup, are distinguished and can be readily differentiated by the terminal shape of the endopod of male left leg 5: bifid (poppei-subgroup) or not (forbesi-subgroup) (Walter 1986a). The new species with a bifid tip of the endopod clearly belongs to the poppei-subgroup. The following four species have so far been accommodated: P. poppei Stingelin 1900, P. smithi Wright, 1928, P. tollingerae Sewell, 1919 In the poppei-subgroup the new species is most closely related to P. tollingerae from the Indian waters (Pillai 1976, Reddy andRadhakrishna 1982) in sharing the following features: (1) the genital double-somite of female is relatively elongate; (2) the right endopod of male leg 5 is rudimentary; (3) the terminal exopodal segment of male right leg 5 is swollen midway; (4) the shape of the left endopod of male leg 5 is similar between the two species; (5) the terminal exopodal segment of male left leg 5 bears 3 stout processes terminally. However, the new species can be easily distinguished from P. tollingerae in: (1) the presence of a ventral linguiform process on the genital doublesomite in the female (absent in P. tollingerae); (2) the second exopodal segment of male right leg not so swollen proximally (swollen); (3) the proximal process of the left endopod of male leg 5 smoothly curved outward (abruptly curved at mid-length); (4) the distal process of the left endopod of male leg 5 tapering distally (expanded terminally and divided at tip). In addition, the female of the new species is unique in having a small, inner terminal process on the first exopodal segment of leg 5, which is much more conspicuous in the three known species of the subgroup.
Etymology. The species was named after the type locality "Siam" (an old name of Thailand).

Disscussion
Zoogeography Walter et al. (2002) recognized three types of the distributional patterns in the lobus species group: Type I= wide distribution of the Indo-West Pacific; Type II= confined distribution mainly or restrictedly in the Indian Ocean; Type III= confined distribution mainly or restrictedly in the West Pacific. In the poppei-subgroup of the species group, Pseudodiaptomus tollingerae is assigned to Type I, while P. poppei and P. smithi to Type III (Pillai 1976, Walter 1986a, Walter et al. 2002. Pseudodiaptomus siamensis has so far been recorded only from the type locality or the Gulf of Thailand, and tentatively belongs to Type III. It seems that the poppei-subgroup is highly restricted to estuarine waters of the Indo-Malayan realm. As mentioned above, P. siamensis composes a sister group with P. tollingerae. Pseudodiaptomus poppei from Celebes (Walter 1986a, b) and P. smithi from the Phillipines (Walter 1986b) share synapomorphic characters such as an elongated terminal segment of male right leg 5. Therefore the distributional pattern of these two pairs in the poppei-subgroup implies a speciation around the Huxley's line. A recent molecular analysis of the Indo-West Pacific populations of the giant freshwater prawn Macrobrachium rosenbergii Murphy and Austin (2002), using 16S ribosomal RNA mitochondrial DNA, clearly recognized two clades, each of which is located on either eastern or western side of Huxley's line (Bruyn et al. 2004). Actually these two clades are suggested to represent two distinct species based on great sequence divergences (6.2 % in maximum) (Bruyn et al. 2004). Although exact vicarious events around Huxley's line are still unknown, the scenario might be applied to the speciation of the poppeisubgroup of Pseudodiaptomus occurring in the brackish waters. The important point is that prawn also needs estuarine environments for reproduction (Bruyn et al. 2004).

Ecology
The habitat of the present new species, the Prasae Estuary was euryhaline, where the salinity widely ranged between 0.7 and 23.3 during the present investigation. Dominant copepods that co-occurred with the new species seasonally differed with salinity: Acartia plumosa Scott, 1894, Bestiolina similis Sewell, 1914, Parvocalanus crassirostris Dahl, 1894, Pseudodiaptomus annandalei Sewell, 1919, and Oithona simplex Farran, 1913, were abundant in the wet season (May-October), while B. similis, P. crassirostris, O. simplex, and O. dissimilis Lindberg, 1940 in the dry season (November-April) (Srinui 2007). In the estuary other environmental factors such as water temperature and dissolved oxygen were nearly constant throughout the investigation, 28.1 to 29.5 °C and 4.3 to 5.3 mg/L, respectively.
Although our collections of planktonic copepods were intermittently carried out, some information of the breeding of the new species was obtained. The ovigerous and/ or spermatopore-bearing females of the new species were found during the wet season (June to October). In addition, the density of immature females reached 139 individuals per cubic meter in August 2004, suggesting it was an active breeding season.

Key to species of the poppei-subgroup
Seventy-eight species of Pseudodiaptomus, including the new species P. siamensis, have been recorded from the world (Walter 1986a, Walter et al. 2002, Walter et al. 2006. Walter ( , 1986aWalter ( , 1987 has also recognized seven species groups in Pseudodiaptomus based mainly on sexual dimorphic features. The lobus species group, to which the present new species belongs, has so far accommodated two subgroups and 15 species. The new species is classified into the poppei-subgroup with 4 species. A key to 4 species of the subgroup is provided here.