FAUCHALD, K.: Distribution of scales in polynoid scale-worms (Annelida, Polychaeta)

Polynoids have some of the dorsal cirri replaced by flattened scales (elytrae) more or less covering the body. An overview of the type species of most genera of the family demonstrated a few interesting patterns. Species with less than 12 pairs of elytrae all have less than 30 segments and are found in deep water, either pelagically or associated with hot vents. These taxa appear to belong to several different clades. The first 12 pair of elytrae are always on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23. The next three pairs of elytrae are usually either on segments 25, 27 and 29 or on segments 26, 29 and 32; the latter is by far the most frequent and a very large number of polynoid taxa have 15 pairs of elytrae with this distribution. Most taxa with 15 pairs of elytrae consist of less than 40-45 segments; but a few long-bodied taxa with as many as 100 segments lack elytrae on the posterior half of the body. When more than 15 pairs of elytrae are present, the distribution on segments 30-45 is often irregular and variable, except in taxa with 18 pairs of elytrae in which patterns corresponding either to the 25, 27, 29 or the 26, 29, 32 patterns are present. In long-bodied taxa with 50 or more segments, the distribution stabilizes in most taxa into two patterns with the elytrae present on either every second or every third segment, corresponding to the patterns seen on segments 24-32. In many species the two last pairs of elytrae are on consecutive segments no matter what the previous distribution were. The distribution of the elytrae correlates better with the numbers of segments present than with the length of the specimens. Short-bodied taxa appear to have evolved at least twice within the family, assuming that the long-bodied condition is plesiomorphic.