Exogone sexoculata, a Worm of the Interstitial

Drawings of head and representative chaetae of Exogone sexoculata, from San Martín (2005).
The sort-of-randomly chosen subject of today's post is the marine annelid worm Exogone (Parexogone) sexoculata, a member of the interstitial fauna around the coast of Australia. A previous record of this species from Italy has since been re-identified as the related species E. gambiae (Lanera et al. 1994). Exogone sexoculata is found among sand, mud, algae and dead coral, and in depths of up to 24 m (San Martín 2005). Despite the species name, it actually has four eyes, plus two eyespots that lack the lenses of the true eyes. As this is a fairly standard arrangement for Exogone species, E. sexoculata can't really claim to have one of the most distinguishing of species names.

Unidentified species of Exogone (in its epitokous form, perhaps?) from the Biodiversity Institute of Ontario.
Exogone sexoculata is a member of the family Syllidae, whose sometimes dramatic reproductive habits were discussed in an earlier post. Like other syllids, most Exogone species go through the process known as epitoky, where reproductively mature individuals metamorphose into a more mobile form, with enlarged eyes and parapodia. In the subfamily including Exogone, Exogoninae, the entire animal transforms into an epitoke (rather than epitokes budding off as may happen in other syllids). After releasing its gametes in the water column, the epitoke returns to the substrate and transforms back into the interstitial form. Females of Exogoninae brood their fertilised eggs attached to their body, and juvenile worms hatch out without going through a planktonic larval stage. In Exogone, the brooded eggs are attached on the underside of the female's body, whereas in other genera brooding may be dorsal. Exogone is also distinguished from other exogonine genera by the absence of a covering of papillae, the presence of only a single pair of tentacular cirri, and palps that are fused along all or almost all of their length. Exogone sexoculata is distinguished from other species in the genus by the absence of dorsal cirri on the second chaeta-bearing segment, its long median antenna, and by features of the chaetae (San Martín 2005).

REFERENCES

Lanera, P., P. Sordino & G. San Martín. 1994. Exogone (Parexogone) gambiae, a new species of Exogoninae (Polychaeta, Syllidae) from the Mediterranean Sea. Bolletino di Zoologia 61 (3): 235-240.

San Martín, G. 2005. Exogoninae (Polychaeta: Syllidae) from Australia with the description of a new genus and twenty-two new species. Records of the Australian Museum 57: 39-152.