Systematics of Siphonophores

Abstract

Siphonophores are the most complex of all pelagic medusozoan hydrozoan cnidarians, bearing various types of zooids on a long stem and often termed ``string jellyfish.{''} They are extremely fragile and live almost exclusively in the open ocean. They vary in length from 50 m down to 10-20 mm. Most species bear swimming bells (nectophores) for locomotion, some have a float (pneumatophore), and all have a long stem of iterative units termed cormidia for feeding, reproduction, and also protection and buoyancy. Tentacles from the cormidia bear stinging cells (nematocysts) for prey capture, either in simple groups or lines or in more complex nematocyst batteries on side branches known as tentilla. In life, tentacles and their side branches extend into a three-dimensional net for fishing, into which prey either blunders by accident or, in a few species, is attracted by lures. Such great diversity has led to a complex systematics based on a range of morphological characters, recently enhanced by the first molecular study of the group. From this a new phylogeny has been proposed, for 17 valid families (one semi-benthic) and 177 valid species (some unassigned). Characters of these families are reviewed in two tables and 17 summaries, including diagnostic characters, number and variety of species, and, where appropriate, habitat preferences and relative success in today's seas. Figures and images showing different types of siphonophores, their morphology, stinging organs, and appearance in life accompany the main text.