Exogoninae Langerhans, 1879
Genus Exogone Örsted, 1845
Exogone cf. heterosetosa McIntosh, 1885
Vouchers. Sea Lion: station 7MFC.
Diagnosis. Voucher is a complete specimen; very small, slender species; 2.8mm long, 0.25mm wide for 33 chaetigers. Integument smooth; colour in alcohol pale yellow.
Prostomium about twice as wide as long bearing three antennae and two palps. Palps about as long as prostomium; fused throughout their length. Antennae smooth, arising nearly at same level, positioned anterior to eyes; antennae spindle-shaped with median antenna slightly longer than lateral ones. Two pairs of large red eyes present, positioned close together, anterior pair large, cup-shaped, posterior pair smaller, spherical. Pharyngeal tube extending through 3 chaetigers; proventricle extending through chaetigers 3-6. First segment achaetous laterally with a pair of smooth, very short, rounded tentacular cirri.
Parapodia uniramous, short but distinct. Dorsal cirri missing on chaetiger 2, otherwise present, smooth ovoid, dorsal cirri; ventral cirri smaller than dorsal cirri, ovoid, inserted near parapodial base.
Chaetae of several different types, up to 6 per fascicle in mid segments, arranged in the following way (dorso-ventrally): type 1 one, simple, bidentate upper (dorsal) chaeta, long, distally expanded with distinct serration; type 2 one modified compound falciger, with expanded serrated joint, distally bidentate with very slender secondary tooth, short blade with margin distinctly serrated; type 3 three compound falciger, smaller and less developed than type 2, with small, serrated joint and short, distally bidentate blade with slender secondary tooth, margin serrated; type 4 one, simple, bidentate lower (ventral) chaeta, long, slender distally tapering into blunt, smooth tip; both simple chaetae lacking in anteriormost chaetigers. Pygidium damaged.
Remarks. This species is very similar to Exogone heterosetosa McIntosh, 1885 described from the Southern Ocean, off Marion Island and commonly recorded from the geographical area of interest here (see Hartman, 1966 for details), but it is clearly beset with many taxonomic problems. McIntosh's (1885) description is relatively brief and appears to be based on a specimen with damaged median antenna, which has led to subsequent misinterpretation of this species. Several other species were eventually synonymised with E. heterosetosa, E. turqueti Gravier, 1906; E. heterochaeta Augener, 1913; E. clavator Ehlers, 1913; E. anomalochaeta Benham, 1921 but it is not clear if this synonymy has always been justified. In our opinion the drawings of chaetae (see image below) by McIntosh (1885) of holotype of E. heterosetosa , by Benham (1927) of E. anomalochaeta Benham, 1921 and those of E. clavator by Ehlers (1913) represent different species, with different developments of modified compound chaeta and neither of which correspond to the Falkland specimens. Ehlers species is in fact very similar to Exogone heterosetoides australis Hartmann-Schröder & Rosenfeldt, 1988 (as it possesses longer median antenna and long, robust spiniger-like modified compound chaeta), which has also been recoded from Falkland Islands in this study (see separate entry).
In recent times, San Martin (2005) provided description of E. heterosetosa (which he considered identical to E. heterosetoides australis) based on Australian material and relying on non-type material of Haswell (1920) commented: The Australian specimens agree with Haswells (1920a) description of E. heterosetosa; the original description did not specify the length of the median antenna, because the state of the preparation renders its presence doubtful (McIntosh, 1885); subsequent descriptions of the species, however, show a long median antenna. The original description by Hartmann-Schröder and Rosenfeldt, 1988 stated that median antenna is 3 times longer than lateral ones. In our opinion the species with long antenna belongs to E. heterosetoides australis rather than E. heterosetosa and the two are distinct species. Confusion cannot be clarified without a full revision of both species, but it is very likely that several species were in fact lumped under each name and both species were in turn mistaken for each other. It appears that the development of modified compound chaeta (ranging from short falciger-like to long or extremely long spiniger-like blade) and at which segment these appear in the body is of taxonomic importance, but has not been paid much attention to in the past.
In summary, Falkland Island specimens assigned to Exogone cf. heterosetosa possess short median antenna and modified compound chaeta essentially falciger-like, with short blade. Similar species encountered in this study with long median antenna and modified compound chaetae clearly spiniger-like with long blade is assigned to Exogone heterosetoides australis .
Fig. 1 Comparative images of E. heterosetosa, McIntosh 1885 and species considered its junior synonym (modified compound chaetae marked by arrow).
E. heterosetosa from McIntosh, 1885; E. clavator , from Ehlers (1913); E. anomalochaeta from Benham (1927)
