Cossuridae Day, 1963  

Note: Here we follow recommendation of Zhadan (2015) for cossurid taxonomy, that the most important characteristic is the number of thoracic segments. Thoracic segments are short, usually flattened dorsoventrally and chaetae emerge from the anterior border of the segments; abdominal segments are longer, often bead-like, and chaetae emerge from the middle of them.

Cossura Webster & Benedict, 1887

Cossura sp. 1

Voucher specimen. SEA LION: station 56MFB.

Diagnosis. Small, slender species, all specimens fragmented. Voucher specimen incomplete composed of anterior fragment with about 20 thoracic chaetigers and about 5 damaged abdominal chaetigers (the only example long enough to show transition between thorax and abdomen), about 2.5 mm long and 0.25mm wide. Abdomen and pygidium unknown; few damaged abdominal segments observed, these appear to be inflated, bead-like. Colour in alcohol pale yellow. Prostomium longer than wide, anteriorly bluntly rounded (appear almost slightly truncated). Branchial filament inserted posteriorly on chaetiger 2 (near the border of chaetiger 2and 3). Chaetiger 1 with uniramous parapodia, all the next segments with biramous parapodia. All chaetae hirsute capillaries with smooth shafts; arranged in two rows. Thickened chaetae present in about 20 (=thoracic) segments, tapering into long, thin tip.  Few abdominal segments observed lacking thickened chaetae.

Remarks. Even though this species was very abundant (~400 specimens collected) not a single complete specimen was encountered. Specimens appear to be fragmented at transition between thorax and abdomen, with mid and posterior part of the body (=abdomen), which are diagnostic, not observed. However only few known species possess thickened chaetae in thoracic chaetigers as is the case in Falkland Islands’ specimens. This feature was also described for Cossura heterochaeta (type locality: Argentina), C. rostrata,  C. ginesi and C. hutchingsae. Cossura ginesi differs by the shape of prostomium with anterior extension. C. hutchingsae has inflated pads in thorax and the segmental borders drawn forward dorsally. Cossura heterochaeta and C. rostrata have a conical prostomium, which is rounded in Falkland Islands specimen. Further, there are two types of anterior thickened chaetae—with arista and without it—in C. heterochaeta and Falkland Islands’ specimens have more thoracic segments – about 20, instead of 12–14 in C. heterochaeta. Summary of Cossura species known from the area can be found in table below. Until better preserved specimens become available, we assign Falkland Island specimens to morphospecies Cossura sp. 1.

 Overview of characters of Cossura species known from the area