Worms by number

Abstract

This paper investigates alternation patterns in length, shape and orientation of dorsal cirri (fleshy segmental appendages) of phyllodocidans, a large group of polychaete worms (Annelida). We document the alternation patterns in several families of Phyllodocida (Syllidae, Hesionidae, Sigalionidae, Polynoidae, Aphroditidae and Acoetidae) and identify the simple mathematical rule bases that describe the progression of these sequences. Two fundamentally different binary alternation patterns were found on the first four segments: 1011 for nereidiform families and 1010 for aphroditiform families. The alternation pattern in all aphroditiform families matches a simple one-dimensional cellular automaton and that for Syllidae (nereidiform) matches the Fibonacci string sequence. Hesionidae (nereidiform) showed the greatest variation in alternation patterns, but all corresponded to various known substitution rules. Comparison of binary patterns of the first 22 segments using a distance measure supports the current ideas on phylogeny within Phyllodocida. These results suggest that gene(s) involved in post-larval segmental growth employ a switching sequence that corresponds to simple mathematical substitution rules.

Figure 1

Scanning electron micrograph of an undescribed species of Gyptis (Hesionidae) from Papua New Guinea, showing parapodia with alternating DC on median segments; dc, dorsal cirrus; ne, neuropodium; no, notopodium.

Figure 2

(a) Macro photograph of Trypanosyllis zebra (Syllidae) from England, showing shorter and horizontally directed DC alternating with longer and more upturned ones. The arrows point to two median segments with shorter DC and the asterisks are situated at the bases of longer DC. The whole animal is 5 cm long. (b) Macro photograph of Bylgides elegans (Polynoidae) from Sweden, showing alternating elytrae (scales) and DC (the elongated appendages that emerge below the elytrae but above the chaetae). The arrows point to DC on two segments, and the asterisks are situated on two elytrae. The whole animal is 3 cm long.

Figure 3

Evolutionary changes in DC alternation patterns on the first four segments in Nereidiformia and Aphroditiformia under (a) loss hypothesis and (b) convergence hypothesis. Sequence for Chrysopetalidae is uncertain (see text). Root of the tree is conjectural. Black square, synapomorphy; open squares, loss; crosses, convergence.