A Phylogenetic Analysis of Higher Taxa in the Phylum Sipuncula

Abstract

-Twelve taxonomic characters of the genera in the phylum of marine worms, Sipuncula, are examined and polarities assigned to the various states. A Hypothetical Ancestral Sipunculan is proposed and probable evolutionary pathways are presented. The PHYLIP Camin-Sokal and the PAUP Wagner parsimony programs are used, as well as Dice's similarity coefficient and UPGMA cluster analysis. These provided both cladistic and a hybrid phenetic/ cladistic analyses. Only 5 of the 12 characters are clearly not homoplastic. While monophyly is not demonstrated in all cases, the 17 genera are now grouped into six families, four orders, and two classes as a result of using evolutionary, phenetic, and phylogenetic methodologies. The selective removal of homoplastic characters within the two classes did not significantly alter the composition of the orders or families. The evident congruence among these methods affirms the value of a synthetic approach. [Sipuncula; higher taxa; evolution; phylogenetic; similarity coefficient; cluster analysis.] Systematically, the marine worms comprising the phylum Sipuncula (peanut worms) have had a comnplex history. They have been ranked as a family, order, or class, and have been allied in various combinations with priapulids, echiurans, or annelids. In reviewing the history of sipunculan classification, Hyman (1959) regarded the group as being worthy of separate phylum rank, an idea first forwarded by Sedgwick (1898). The currently accepted spelling of the phylum was proposed by Stephen (1964). As a result of its confused classification, intermediate taxa (orders and classes) have not been developed for Sipuncula. Pickford (1947) suggested the genera should be arranged into four groups, while Akesson (1958) recommended an arrangement of the genera into three groups based on different characters than those used by Pickford. However, neither author forwarded taxon names or ranks for these groupings, and the void was only partially filled when Stephen and Edmonds (1972) erected the four families that are widely used to date. These families were employed by Murina (1975) in her consideration of the evolutionary relationships between sipunculan genera. This present work is an attempt to apply some of the extant phylogenetic methodology and logic to a phylum of poorlyknown, soft-bodied marine invertebrates for which there is no fossil record, an inadequate outgroup comparison on which to root character polarities, and only a modest number of useful characters. This proposed phylogeny is a testable model and can be improved as additional information becomes available. We herein interpret the evolutionary significance of the various taxonomically useful characters in the context of erecting new families, orders, and classes. The ideas outlined below were proposed in a preliminary fashion by Cutler (1980). Since that time efforts have been made to more adequately characterize certain genera and subgenera, such as: Siphonosoma (Cutler and Cutler, 1982); Antillesoma (Cutler and Cutler, 1983); Golfingia (Golfingiella) and (Siphonoides) (Cutler et al., 1983); Thysanocardia (Gibbs et al., 1983); Sipunculus and Xenosiphon (Cutler and Cutler, 1985); and Phascolion and Onchnesoma (Cutler and Cutler, unpubl. manuscript). Consideration of morphological characters useful at the generic level, their plesiomorphic /apomorphic states, and the