Gesaia

Diagnosis (from Capa & Hutchings, 2019). Operculum completely divided into two elongate lobes and distal disc perpendicular to operculum. Few (4 – 8) pairs of long and conical papillae spirally arranged from nuchal hook to ventral distal end of opercular lobes. Three to five tentacular filaments along margins of buccal cavity. Buccal flaps absent. Pair of fine palps longer than operculum. Median organ at dorsal junction of opercular lobes. Outer paleae arranged in semicircle or spiral, with cylindrical or slightly flattened blades, with frayed thecae and distal ends rolled inward. Inner paleae arranged on dorsal inner margin of opercular lobes, with straight cylindrical and smooth blades. Pair of nuchal hooks without limbation. Neuropodia of segment 1 with two to three pairs of cirri. Thoracic branchiae present. Four parathoracic segments. Parathoracic notopodia with robust lanceolate chaetae and fine capillaries, neuropodia with capillaries only.

Remarks. Kirtley (1994) erected the genus Gesaia with eight species collected from depths 770 – 5790 m (Table 2). The genus differs from other sabellariid genera by having a pair of elongated and well separated elongate opercular lobes and cylindrical outer paleae with distal ends rolled inward, distal disc of opercular lobes positioned perpendicular to longitudinal axis of operculum, few (generally one) pairs of nuchal hooks without limbation, and four parathoracic segments. Kirtley (1994) described seven new species of Gesaia mainly based on the ornamentation of the outer paleae, but did not provide any details of the body. The genus Gesaia has only previously been recorded from Australian waters as a larva collected in plankton samples at Lizard Island (Capa et al. 2012), although Hutchings et al. (2012) indicated that the genus was absent from Australian waters.

Type locality. Off Madeira Archipelago, in 1968 m.

Type species. Phalacrostemma elegans Fauvel, 1911.

Description. Colour pattern. Preserved specimen lacking pigmentation. Body. Larva almost spherical in shape, 650 µm long and 550 µm maximum width excluding chaetae and paleae; with four clearly demarcated segments corresponding to the parathoracic region (chaetigers 3 – 6). Operculum. Operculum completely divided into two free short lobes with distal ends nearly perpendicular to longitudinal body axis (Fig. 1 A – C). Outer row with 8 – 10 paleae on each lobe. Outer paleae circular in cross section at base, tapering distally (Fig. 1 A – C); shaft ornamented for most of its length with thecae as compact rings with smoothly parallel edges (Fig. 1 E). Inner row of paleae with 6 – 8 paleae on each lobe (Fig. 1 A – C), with stout bases (thicker than outer paleae) and either ellipsoid or circular cross section at base (Fig. 1 B, G), distally tapering, hollowed, with truncated tip (Fig. 1 F); smooth for about half of their length, and distally ornamented with thecae as compact rings with irregular edges proximately, becoming parallel towards the tip (Fig. 1 F). Opercular papillae not developed or not observed. One pair of stout nuchal hooks, strongly recurved; margins smooth without limbations (Fig. 1 G). Tentacular filaments small, forming a cluster partially covered by paleae and not clearly arranged in rows (Fig. 1 A, B). Two wide and short palps at dorsal junction of opercular lobes, with ciliation on its surface particularly, long and dense at the base (Fig. 1 D, E). Median organ between palps, with dense ciliation along its dorsal edge (Fig. 1 D, F). Thorax. Chaetigers 1 and 2, and corresponding features not yet defined. Parathorax. Four chaetigers with two types of notochaetae arranged transversely; four large, lanceolate chaetae, slightly concave and with frayed tips (Fig. 1 H – I) interspersed with four small and straight capillaries (Fig. 1 H – I). Incipient conical branchiae present dorsally on notopodia (Fig. 1 H). Parathoracic neurochaetae not formed. Abdomen and cauda. Abdominal chaetigers not observed. Cauda not yet developed (Fig. 1 A – C).

Remarks. The larva studied seems it is at an advanced stage of metamorphosis, after comparison with those from other sabellariids (e. g., Dales 1952; Eckelbarger 1977). Provisional chaetae typical of early larval stages have already disappeared, and the opercular lobes are already well defined and provided with primary paleae. The opercular spines or hooks that appear in advanced larval stage and remain in the adults (e. g., Bhaud 1975 a, b; Mauro 1975; Eckelbarger 1977; Wilson 1977; Bhaud & Fernández-Álamo 2001) are present. Nevertheless, paleae have still not rotated to the dorsal position, and the mouth or building organ are not yet differentiated as observed in ready to settle larvae.

Distribution. Only known from off Lizard Island, close to shore, collected in a plankton tow in 6 – 15 m water depth.

Material examined. AM W. 47551, Queensland, Osprey Island, 14 ° 40 ' S, 145 ° 26 ' E, 6 – 15 m, plankton tow, 7 Oct 1979, larva on SEM stub.

Remarks. This planktonic larva is provided with four parathoracic segments, simple (unbranched) feeding tentacles, nuchal hooks (with strongly bent sharp tips), and without limbations, operculum completely divided in two lobes, features that are characteristic for Gesaia species (Kirtley 1994; Capa et al. 2012; Capa & Hutchings 2015). The paleae morphology, with straight and cylindrical outer and inner paleae, and paleae arrangement, with outer paleae forming a semicircle and the inner paleae in an almost straight line between opercular lobes, is also most similar to benthic stages of members of this genus. However, the thecal ornamentation of the paleae, indicated to be the most important morphological trait for Gesaia species discrimination (Kirtley 1994: 166 – 167) observed in the larva described herein differs from other Gesaia species.