Laonice (Laonice) siamica, Sikorski & Pavlova & Martin & Gil, 2023

Sikorski, Andrey, Pavlova, Lyudmila, Martin, Daniel & Gil, João, 2023, New sublittoral species of Laonice (Annelida: Spionidae) from southern Asian coasts, Zootaxa 5277 (3), pp. 490-508 : 494-496

Laonice (Laonice) siamica sp. nov.

Figs 2A–I , 6 .

LSID: urn:lsid:zoobank.org:act:545BCAEE-01D8-454F-960D-86B54F9025DA

Holotype. MNCN16.01 /19128, Gulf of Siam, off Thailand, st. 5, 07°36.018′N, 102°50.543′E, 64 m depth, 92.2% of silt and clay (<63 µm) and 7.6% of fine sand (63–250 µm) with shell fragments, 28 July 1998.

Description. Holotype 0.7 mm wide, 20 mm long for 104 chaetigers (one anterior fragment of 60 chaetigers and one posterior with 44).

Prostomium fused with peristomium at fronto-lateral margin ( Fig. 2A, C ), anteriorly rounded, extending posteriorly to beginning of chaetiger 41 as a low narrow caruncle, accompanied by nuchal organs as lateral ciliary bands. Occipital antenna thin, erect, half length of notopodial postchaetal lamellae of chaetiger 1. One pair of small, distinct, faded brownish eyespots in front of attachment of occipital antenna.

Branchiae from chaetiger 2; first pair half as long as notopodial postchaetal lamellae, then increasing their length but still shorter until chaetiger 4 ( Fig. 2A, E ), of equal length on chaetiger 5, and twice longer after chaetiger 10 ( Fig. 2F ); then branchiae gradually shortening from chaetiger 38 ( Fig. 2B, G ), being absent after chaetiger 50 (last branchiate chaetiger).

Notopodial postchaetal lamellae leaflike, almost triangular, with narrow acute tips through all body ( Fig. 2A–C, E–G ), but particularly pronounced on the four anteriormost chaetigers ( Fig. 2A, E ), gradually shortening to half the length of anterior ones on last branchiate chaetigers, becoming narrow triangles on posteriormost chaetigers ( Fig. 2G ). Anterior postbranchiate chaetigers with notopodial postchaetal lamella extending slightly dorsally, not reaching middle dorsum ( Fig. 2B ). Neuropodial postchaetal lamellae almost trapezoidal anteriorly ( Fig. 2E ), then triangular, with acute tips through all body ( Fig. 2F–G ).

Dorsal transverse crests absent.

Lateral inter-neuropodial pouches from chaetiger 5–6, irregularly present from chaetiger 50 to posteriormost chaetigers.

Capillaries of anterior chaetigers arranged in two vertical rows. Neuropodial hooks from chaetiger 39, up to 6–8 per fascicle, bidentate in lateral view, tridentate in frontal view, with one pair of small apical teeth side by side above main fang ( Fig. 2H–I ). Notopodial hooks absent. Sabre chaetae from chaetiger 14, one per fascicle.

Pygidium with ten lobes forming a ring around the anus: four bilaterally symmetrical pairs bearing one thin threadlike anal cirrus, and one ventral pair closely set, pointed and lacking cirri ( Fig. 2D ).

Pigmentation absent.

Methyl Green staining. As oval spots on upper parts of notopodial postchaetal lamellae of chaetigers 4–8 ( Fig. 2C ).

Type locality. Gulf of Siam, off Thailand (07°36.018′N, 102°50.543′E), 64 m depth .

Etymology. The specific epithet siamica derives from the toponymic name of the inlet where the type was collected, the Gulf of Siam.

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

Remarks. Laonice siamica sp. nov. can be referred to L. ( Laonice ) as defined by Sikorski et al. (2017), as it has the prostomium completely fused with the peristomium. It is closely related to the L. cirrata complex [ L. brevicornis (with both L. aperata and L. petersenae included), L. quadridentata , L. cricketae and, probably, L. bassensis , L. shamrockensis , L. asaccata , L. pinnulata and L. plumisetosa ]. However, the gradual reduction of the posterior branchiae is quite unique of L. siamica sp. nov. While measuring only 0.7 mm wide, the holotype of L. siamica sp. nov. has nuchal organs extending to chaetiger 41 and up to 50 pairs of branchiae, which only occurs in specimens of L. cirrata measuring> 2.2 mm wide. Nuchal organs of L. shamrockensis , L. asaccata , L. pinnulata and L. plumisetosa are much shorter and do not extend over chaetiger 7 (41 in L. siamica sp. nov.). In L. brevicornis and L. cricketae nuchal organs are shorter as well: up to chaetiger 26 in L. brevicornis (with last branchiae on chaetiger 36) and up to chaetiger 20 in L. cricketae (with last branchiae on chaetiger 39 and lateral inter-neuropodial pouches only from chaetiger 40, instead of chaetiger 7 in L. siamica sp. nov.). Nuchal organs, as in the case of the species described above, are also remarkably shorter in L. quadridentata (up to chaetiger 18) and L. bassensis (up to chaetiger 7).

Figures

FIGURE 2. Laonice siamica sp. nov. A: anterior region (9 chaetigers), dorsal view. B: end of branchiate region (chaetigers 46–53), dorsal view. C: staining pattern with Methyl Green, anterior region (9 chaetigers), dorsal view. D: pygidium, rear view. E–G: parapodia of chaetigers 3, 25 and 49 (chaetiger anterior to last branchiate chaetiger), frontal view. H: neuropodial hooded hook, chaetiger 49, lateral view. I: neuropodial hooded hook, chaetiger 49, frontal view (scheme). Scale, mm:A–B, 1; C–D, 0.7; E–G, 0.5; H–I, 0.05. Material: holotype, Gulf of Siam, st. 5 (MNCN16.01/19128).

FIGURE 6. Location of the different species of Laonice discussed in the study: Persian Gulf (dot), Andaman Sea (square), Gulf of Siam (star), and Yellow Sea (triangle).