Evolution of the Chondrophora: Evidence from Behavioural Studies on Velella

Abstract

The chondrophore Velella shows concert behaviour closely resembling and presumably homologous to that described for Porpita, Pelagohydra, and for the sessile tubularian hydroids Tubularia and Corymorpha. The movements consist of orally directed tentacular flexions, accompanied by retraction of the blastostyles, along with muscular contractions in the mantle and in the crest surmounting the sail. Concerts may consist of single contractions or sequences of contractions, the latter sometimes summing to produce a sustained declination of the tentacles or contraction in the other parts. Concerts are repeated periodically at intervals of between 1 and 3 min.Electrical potentials (concert pulses) are recorded from all parts of the body surface at each concert contraction. The potentials are through-conducted at 30–35 cm/sec in parts of the aboral surface. Feeding changes the concert pattern, introducing a type of event called a feeding pulse. Peristalsis in the proboscis, as in Tubularia, is exhibited without electrical concomitants.The observations support the view of Velella as a large individual hydroid in no way comparable to a siphonophore. It is regarded as a floating tubularian hydranth with inverted stem. Velella and Porpita (family Velellidae) are assigned to the superfamily Tubularoidea. The freshwater hydra shows a type of behaviour resembling modified tubularian concerts, and its possible affinity with the tubularoid group is discussed.