Corophiine amphipods of the genera Chelicorophium and Paracorophium from the lower Gulf of Thailand (Crustacea, Amphipoda, Corophiidae, Corophiinae)

Abstract Two species of corophiine amphipods from Songkhla Lake, in the lower Gulf of Thailand, are described and illustrated. Chelicorophium madrasensis (Nayar, 1950), found in the mangrove forest, has not previously been observed in Thai waters. Paracorophium angsupanichae sp. n. is characterized by its chelate male gnathopod 2, obtuse palm with subrectangular distomedial elevation, and urosomites 1-3 free. This is the first record of the genus Chelicorophium and Paracorophium in Thai waters. All specimens are deposited in the Princess Maha Chakri Sirindhorn Natural History Museum, Prince of Songkla University, Thailand and the Museum für Naturkunde, Berlin.


Introduction
The subfamily Corophiinae was established by Bousfield and Hoover in 1997 and is defined by its gnathopods 1 and 2, together forming a sieving structure with dense sieving setae on the posterior margins of the carpus and ischium. Corophiinae are world wide distributed, most of them living in brackish or freshwater (Myers 2009). In Thai waters, only Monocorophium acherusicum (Costa, 1853) has been recorded in Songkhla Lake, the largest natural lagoon located in Southern Thailand (Angsupanich and Kuwabara 1995). This study focuses on the hitherto poorly known gammarid amphipods in the Gulf of Thailand. The first is a new corophiine species of Paracorophium angsupanichae, which was found in Songkhla Lake. The second is Chelicorophium madrasensis (Nayar, 1950), which has not been previously recorded in Thai waters. Figures and descriptions of both species are provided.
Etymology. The species is named in honor of Professor Dr. Saowapa Angsupanich of Prince of Songkla University, Thailand, who contributed to the study of the ecosystem in Songkhla Lake.
Remarks. Paracorophium angsupanichae sp. n. is characterized by male gnathopod 2 chelate and shares this character with Paracorophium chelatum in the Palau Islands, in the southern Pacific Ocean, east of the Philippines. Both amphipods inhabit freshwater environments above tidal influence. However, P. chelatum has the following characters (Karaman 1979): 1) its maxilliped lacks the distinctive rows of long setae on the basal segment (present in P. angsupanichae), 2) its male gnathopod 2 palm is smooth (palm is obtuse with subrectangular distomedial elevation bearing 4 robust setae), and 3) its urosomites 1-2 are fused (urosomites 1-3 are free in P. angsupanichae).
Female. No sexual differences. Remarks. Nayar (1950) described Chelicorophium madrasensis from the Madras Coast, India, which is characterized by antenna 2 article 4 inner surface with 2 proximal spines and epimeron 1 smooth and naked. The specimens from the current study are similar to those of Nayar, but the telson is truncated, whereas it is pointed in C. madrasensis.
Distribution. Indian Ocean and Songkhla Lake (current study).