Hydroid nematophores: morphological, structural, and behavioural variety from old knowledge and new data

There is a rich old literature on nematothecae, which have been described from many species of nine families of thecate hydroids, but detailed knowledge of nematophores is from a very few species. Using both old data and new observations, information was gathered on species belonging to the genera Hydrodendron, Zygophylax, Antennella, Plumularia, Monotheca, Macrorhynchia, Gymnangium, and Thecocarpus. Specimens belonging to five families collected on the coral reefs of Réunion Island (SW Indian Ocean) were observed alive and videotaped to gather information on nematophores. Nematophores are classified and named here as belonging to three types. An amoeboid nematophore of the four plumularid families Kirchenpaueriidae, Halopterididae, Plumulariidae, and Aglaopheniidae usually consists of two parts, which are more separate in aglaophenids than in the others. The cnidostyle (or nematostyle) is fixed, is formed of ectoderm and endoderm, and includes a cluster of large nematocysts at its top. It is inferred to serve in defence because nematocysts inside the nematotheca can be fired when stimulated. The sarcostyle is a mobile amoeboid layer of ectoderm capable of considerable extension; those that are bilobed on top can extend in two directions at a time. The sarcostyle can emit small pseudopods on its edges that phagocytose particles at the surface of the perisarc. Its hypothesized function of cleaning the colony and providing nutrition in the form of small detrital particles and bacteria taken up by phagocytosis remains to be confirmed by experimentation. The cnidostyle-like nematophore of the Campanulinidae, Linolariidae, and Clathrozoidae is a fixed structure formed of a bilayered pedicel ending with a cluster of nematocysts able to fire while still inside the nematotheca, like the cnidostyle part of the amoeboid nematophore. It functions in defence as well. The tentacle-like nematophore of the Haleciidae and Lafoeidae, described here for the first time, is a single very extensible process provided with nematocysts that issue from the nematotheca and formed by a file of endodermal chordal cells surrounded by ectoderm. In halecids, nematocysts are not always gathered on the very top, forming what looks like a strongly capitate tentacle, as was previously thought, but can be more scattered, giving the structure a filiform appearance. This type has active movements, being able to twist and bend in all directions. Its prey capture function has not been demonstrated; a sensory function is more probable. Additional data have to be collected for more species to compare the types with more confidence, but they appear to have different origins for their considerably different morphologies, structures, and behaviour.

Key words: Hydrozoa, Thecatae, morphology, behaviour, nematophore