Laonice (Sarsiana) cf. sinica Sikorski & Wu, 1998

Figs 5A–F, 6.

Material examined. MNCN16.01 /19137, 2 specimens, Gulf of Siam, off Thailand, st. 4, 07°32.190′N, 102°47.742′E, 61 m depth, 89.6% of silt and clay (<63 µm) and 10.0% of fine sand (63–250 µm) with shell fragments, 27 July 1998.

Description of specimens from the Gulf of Siam. Two anterior fragments, one very short, longest one 0.8 mm wide with 44 chaetigers (used for numerical characters).

Prostomium T-shaped, with rounded anterior margin, not fused with peristomium by anterolateral angles (Fig. 5A). Caruncle accompanied by nuchal organs, extending to posterior end of chaetiger 12. Occipital antenna quite large, erected, slightly longer than branchiae of chaetiger 2 (Fig. 5A). One pair of bright elongate eyespots along lateral margins of prostomium, in front of occipital antenna.

Branchiae from chaetiger 2, short, one third of length of notopodial postchaetal lamellae of same chaetiger, half longer on chaetiger 4, 1.5 times shorter on chaetiger 5, very gradually lengthening to become nearly as long as notopodial postchaetal lamellae at chaetiger 25, then becoming shorter again, half the size of the last pair of branchiae, on chaetiger 34 (Fig. 5A–B).

Notopodial postchaetal lamellae leaf-like, narrowing into sharply acute tips on first three chaetigers (Fig. 5A), slightly rounded from chaetiger 4 (Fig. 5A), becoming acute again at posterior branchiate chaetigers (Fig. 5B). Notopodial postchaetal lamellae gradually shortening on four last branchiate chaetigers, without erect terminal peak on postbranchiate chaetigers (Fig. 5B). Neuropodial postchaetal lamellae as leaf-like triangles with dorsolateral tips; tips acute on four anteriormost chaetigers (Fig. 5A) and rounded from chaetiger 5. Neuropodial postchaetal lamellae gradually becoming shorter from posterior branchiate chaetigers, and then much shorter from chaetiger 42 onwards.

Dorsal transverse crests first appearing on chaetiger 31, as square tongue-shaped membranes on middle of dorsal surface of chaetigers 31–32 (Fig. 5B). Continuous crests connecting notopodial postchaetal lamellae from last branchial chaetiger to at least the 14th post-branchial chaetiger, first two with strongly elevated middle region (Fig. 5B). Ventral borders between chaetigers bearing dorsal transverse crests with two parallel transverse short strips (Fig. 5C).

Lateral inter-neuropodial pouches from chaetiger 7 (right side) and 8 (left side), until last chaetiger present.

Capillaries of anterior chaetigers arranged in two vertical rows. Neuropodial hooks from chaetiger 37, up to five per neuropodium, bidentate in lateral view (Fig. 5D), tridentate in frontal view (Fig. 5E). Notopodial hooks not seen. Sabre chaetae from chaetiger 31, one per fascicle.

Pygidium not seen, missing.

Pigmentation absent.

Methyl Green staining. Strong on inner upper parts of notopodial postchaetal lamellae from chaetiger 5–9 (Fig. 5F).

Distribution. Gulf of Siam (Pacific Ocean) (Fig. 6).

Remarks. The examined specimens were consistent with the original description of L. sinica in nearly all characters, except in nuchal organs length, which reach chaetiger 12 in Siamese specimens instead of 8–11 as in Chinese specimens. Moreover, the Chinese specimens did not show a distinct Methyl Green staining pattern. The populations from the Bohai and Yellow seas and from the Gulf of Siam may represent opposite clines at the extremes of the species distribution area, but assessing conspecificity will not only require molecular methods, but also finding additional populations between the two extremes. Therefore, we are proposing a tentative attribution of the Siamese specimens to the species, as L. (S.) cf. sinica.