Pista Malmgren, 1866
Pista sp. 2
Voucher. TOROA: station 1MFA ad VINSON WEST: station 5 MFA.
Diagnosis. Voucher (TOROA: station 1MFA) incomplete specimen in good state of preservation, 20mm long and 1.8mm wide with 17 thoracic chaetigers and at least 35 abdominal uncinigers (posteriorly incomplete). Body cylindrical with thorax enlarged and abdomen tapering; anteriorly segments crowded, segments progressively longer from segment 9.
Head not clearly visible, other than its basal part (=lower lip) forming a thickened ring clearly visible dorsally; eyes absent.
Segment 1 dorsally low, ventrally better developed, but still short with one small triangular lobe at each side of lower lip. Segment 2 forming small protruding crests ventrally (visible ventrally between segments 1 and 3) ventrolaterally, segment 2 with one pair of thick rounded lobes aligned slightly dorsally to lobes on segment 1. Segments 3 with laterally placed enlarged, round lobes, lobes on segment 3 larger than those on segments 2 and 4; dorsally segment 3 forming a low rim with two very small triangular dorsal lobes (on which 2nd pair of branchiae is attached). Segments 4 and 5 thickened laterally otherwise similar to subsequent segments.
Two pairs of branchiae on segments 2–3; first pair present, shifted dorsally to second pair, second pair inserted at dorsal end of dorsolateral lobes; when branchiae present plumose (when missing, a scar can be detected), with pair on segment 2 larger than on segment 3; the ratio of stem to crown length about 1:1. Pygidium unknown.
Seventeen pairs of notopodia, starting from segment 4 and extending until segment 20 (the last thoracic segment/unciniger); first pair of notopodia about same size as following pairs, notopodia of segments 4–7 inserted progressively more laterally, than vertically aligned. Notochaetae narrowly bi-limbate, distally smooth, arranged in two rows.
Neuropodia starting from chaetiger 2 (segment 5); in thorax as low rectangular ridges slightly raised from surface of body until segment on which notopodia terminate; in abdomen as short rectangular pinnules situated laterally on body. In thorax uncini arranged in a single row in chaetigers 2–7 (segments 5 – 10) and in intercalated double rows from chaetiger 8 (segment 11) until the end of thorax; thoracic uncini with elongated handle originating from heel, with rounded rostral tooth situated at mid-length between base of main fang and tip of prow, distally rounded prow and crest with 3–4 rows of secondary teeth. Abdominal uncini from chaetiger 18 (segment 21) short-handled with rostral tooth situated at mid-length between base of main fang and tip of prow, distally rounded prow and crest with up to 4 rows of secondary teeth.
Tube: smooth, grey mud (TOROA: station 1MFA); muddy tube covered densely with coarse black particles and sparsely embedded with larger stones.
Remarks. Many reports of Pista species with 2 pairs of branchiae from the region were previously (likely erroneously) assigned to P. cristata (Müller, 1776) or Pista corrientis McIntosh, 1885. There are several species found in the Falkland Islands material, which are likely new to science and are here assigned morphospecies name only. Pista is a large genus, reported worldwide and comparisons with these are not provided here as further work by specialists will be needed on this group.
This genus is characterized by the presence of lobes on anterior segments, long-handled uncini occurring for variable number of segments on the region with biramous parapodia, and 17 pairs of notopodia with broadly winged notochaetae, starting from segment 4. Santos et al. (2010) also proposed that morphology of lobes in this group, especially with regard to the shape of lobes on segment 1, is important for distinguishing species. Those characters were used here to separate different morphospecies of Pista (see comparative table).
Although the tubes of the two voucher specimens vary, no other morphological characters were observed, for now these are considered the same species.
