Thenus unimaculatus Burton & Davie 2007, sp. nov.

Thenus unimaculatus sp. nov.

(Fig. 15, 17B, E, 18F–H)

Material examined. Holotype, PMBC-13714, male (71.9 x 86.6), Andaman Sea, Phuket Fish Market, Somchai Bussarawit, 5.10.1997.

Paratypes: PMBC-13715, female (67.7 x 79.2), Phuket Fish Market, Andaman Sea, Somchai Bussarawit, 5.10.1997; QM-W22277, female (69.3 x 85.3), ovig. female (77.8 x 92.1), Phuket Fish Market, Andaman Sea, Somchai Bussarawit, 5.10.1997; QM-W24629, 4 ovig. females (65.4 x 86.2; 67.5 x 86.7; 73.3 x 95.8; 80.5 x 104.4), 2 females (80.7 x 104.9; 64.0 x 82.6), 4 males (61.2 x 79.8; 63.1 x 78.7; 66.3 x 85.6; 66.4 x 82.0), Andaman Sea, Phuket Fish Market, P. Davie & P. Ng, 12.1998; QM-W22098, female (57.0 x 67.8), United Arab Emirates 55°0’E, 26°0’N, 22.1.1995, M. O’Neill; QM-W22099, female (80.8 x 98.7), United Arab Emirates 55°0’E, 26°0’N, 22.1.1995, M. O’Neill; QM-W22096, female (83.4 x 97.5), 100km NW Phuket, Thailand, 9.08.1993, Dr. H. Chansang; QM-W22094, male (83.5 x 98.8), 100km NW Phuket, Thailand, 9.08.1993, Dr. H. Chansang; QM-W22095, male (72.2 x 86.2), 100km NW Phuket, Thailand, 9.08.1993, Dr. H. Chansang; QM-W22097, female (62.5 x 76.1), 100 km NW Phuket, Thailand, 9.8.1993, Dr. H. Chansang; ZRC-1998.1153, male (42.5 x 51.2), Andaman Sea, collected from Phuket Fishing Pier, Thailand, S. Chaitiamvong et al., December 1998; ZRC-1995.977, female (48.6 x 59.4), ovig. female (52.1 x 63.6), off Singapore (purchased from Ponggol Fish Market), P.K.L. Ng, 30.09.1995.

Diagnosis. Purple to black pigmentation blotch on inner face of merus of second and sometimes third legs, usually large but variable in extent and may be reduced to a narrow streak; purple pigmentation occasionally surrounding eye socket on carapace; outer face of propodus of P2 having upper-most longitudinal groove bearing obvious setae over at least proximal half. Merus of third maxilliped with a small spine proximally on inner ventral margin; inner margin of ischium prominently dentate along entire length. No single morphometric ratio has been isolated that will exclusively identify this species, but only T. unimaculatus can have ratios that fall outside the following maximum and minimum values: carapace width (CW1) greater than 1.29 times carapace length (CL); length of propodus of pereiopod 1 (PL1) less than 0.23 times carapace length (CL); length of propodus of pereiopod 2 (PL2) greater than 0.39 times carapace length (CL); width of propodus of pereiopod 1 (PW1) greater than 0.35 times length (PL1). (Table 6: Ratios 2, 7, 8, 9).

Remarks. Because of its lack of regular spotting on the pereiopods this species appears similar to T. indicus. It differs most obviously by the possession of a distinctive blotch of colour on the inner face of the merus of the second and sometimes third legs; the colour of this blotch is variable, present specimens being light to dark purple or indigo.

The mtDNA sequencing results (Table 5) show this species to be most closely related to T. orientalis with only a 2% nucleotide divergence. This level of divergence could be interpreted as within-species variation, except for the fact that both T. orientalis and T. unimaculatus occur sympatrically in the waters off the United Arab Emirates, the morphometric analysis shows them to be discrete, and the allozyme electrophoresis shows a number of fixed differences (see Table 4 and discussion).

Etymology. Name refers to the characteristic colouring.

Distribution. Apparently confined to the Indian Ocean; specimens examined came from Mozambique, United Arab Emirates and south-western Thailand (Fig. 15).