Hemichordate skeletal growth: shared patterns in Rhabdopleura and graptoloids
Article first published online: 9 OCT 2007
Volume 27, Issue 4, pages 317–324, December 1994
How to Cite
RIGBY, S. (1994), Hemichordate skeletal growth: shared patterns in Rhabdopleura and graptoloids. Lethaia, 27: 317–324. doi: 10.1111/j.1502-3931.1994.tb01581.x
- Issue published online: 9 OCT 2007
- Article first published online: 9 OCT 2007
- revised 28th September, 1994.
Rhabdopleura shows several features of skeleton growth that are also seen in graptoloids. The similarities between the growth patterns, in terms of the plan towards which the zooids aimed and of their response to environmental disturbance, are profound. Both demonstrate a high degree of genetic control, not only on the gross morphology of the tube or theca but also on the pattern of increments by which this must be achieved. The execution of this ‘blueprint’ is facilitated by spatial awareness in the zooids of both groups. The main variable left to the graptoloid zooid was the number of increments used in building a theca. Variations in the number of increments probably reflect differences in the productivity of the environment and hence the amount of spare energy in the colony budget. An important new observation is that mortality is common amongst zooids in both Rhabdopleura and graptoloids, with new animals taking over tube or thecal building from where the previous zooid left off. This is identifiable in the increment patterns of tubes and thecae. Several generations of zooids can inhabit a rhabdopleuran tube and can be demonstrated to have inhabited a graptolite theca. This means that innate senescence was not a major cause of death for graptolite colonies. It also means that all thecae might have been continuously occupied and that the colony could have survived significantly bleak environmental conditions by large-scale zooid mortality followed by regeneration. □Graptolite, ecology, hemichordate, pterobranch, RHABDOPLEURA, growth.