Gesaia

Capa, María, Faroni-Perez, Larisse & Hutchings, Pat, 2015, Sabellariidae from Lizard Island, Great Barrier Reef, including a new species of Lygdamis and notes on external morphology of the median organ, Zootaxa 4019 (1), pp. 184-206 : 186-188

Gesaia sp.

( Fig. 1 )

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.

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 1975a, 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.

This Gesaia larva bears a conspicuous organ between the palps, similar to the structure referred to as the dorsal hump in Idanthyrsus Kinberg, 1867 , Sabellaria Lamarck, 1818 and Phragmatopoma Mörch, 1863 species ( Dales 1952; Eckelbarger 1977; Bhaud & Fernández-Álamo 2001). This ‘dorsal hump’ apparently disappears before settlement in species that either lack median organ in the adult stages or it is poorly developed. The position and morphology of the structure herein described show some signs of being an early form of the median organ observed in the benthic stages of other members of the family.

The Gesaia larva described herein bear ornamented paleae with thecae as compact perpendicular rings in both outer and inner opercular rows, different to any other described species from the genus. Some congeners bear poorly expressed thecal margins or smooth outer paleae ( Gesaia lanai Kirtley, 1994 and G. ryani Kirtley, 1994 ) and only a few species (e.g., G. elegans ( Fauvel, 1911) , G. fauchaldi Kirtley, 1994 , and G. fossae Kirtley, 1994 ) are known to have ornamented thecae, but these form rings with irregular edges or oblique and with spikes ( Kirtley 1994), not in regular parallel rings like the Australian larva. The inner paleae described herein have stout bases, similarly to Gesaia hessi Kirtley, 1994 and G. r y a n i. In addition, several circular holes were observed on the surface of inner paleae in G. hessi , possibly attributed to endobiotic activity ( Kirtley 1994), whereas in the described Gesaia larva a unique hollow was observed at the tip. It is well known that sabellariids replace their opercular paleae in later larval stages, and these can differ from the primary larval paleae (e.g., Eckelbarger 1977; Wilson 1977; Lezzi et al. 2015). Therefore, is not clear if the paleae morphology described herein would have remained after settlement and description of a new species based on larvae paleae morphology is avoided.

The paleae show some similarities to those present in members of Phalacrostemma . Nevertheless, members of these two genera are distinguished, among other features, by the number and shape of the nuchal hooks, with multiple pairs and well-developed limbation in Phalacrostemma that is absent in Gesaia species, and the arrangement of outer paleae is a spiral in most Phalacrostemma species, while they are arranged in semicircles in members of Gesaia ( Kirtley 1994; Hutchings et al. 2012; Capa & Hutchings 2014).

This is the first report of Gesaia in Australia, and also the first record of a typically deep-water sabellariid in surface water plankton samples. Members of Gesaia have been reported from all major oceans and from depths ranging from 770 to 5790 m ( Kirtley 1994), but the eight species described to date have each been reported only once or a few times from nearby localities, leading workers to assume that species were allopatric and narrowly distributed. Undoubtedly, the potential for dispersal over long distances is greater when larvae reach shallower waters, due to low speed currents in deep water ( Kelly et al. 1982; Shanks 1986; Bouchet & Waren 1994; Arellano et al. 2014; Pochelon et al. 2014). This suggests a larval ability to survive under a range of physical factors encountered if migrating vertically from the deep to the euphotic zone (e.g., including a broad variation in temperature and pressure). However, before the ability of larvae of Gesaia to disperse long distances can be accepted, substantial investigations on energy content in the oocytes, the velocity of larvae when they migrate upwards and their planktonic larval duration are needed. Interpreting the biogeographic patterns of Gesaia remains a major challenge, and the potential for larval dispersal is important in understanding the spatially fragmented radiation of species distribution in the major oceans.

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

Figures

FIGURE 1. Gesaia sp. SEM images of larva, AM W. 47551. A. Dorsal view; B. Dorsolateral view; C. Antero-ventral view; D. Detail of anterior regions showing median organ and palps, dorsal view; E. Same, ventral view; F. Inner row of paleae; G. Nuchal hooks and shafts of paleae from inner row; H. Parathoracic chaetigers, lateral view; I. Notochaetae of parathoracic chaetiger. Abbreviations: br = branchia, mo = median organ, pa = palp.