KEOUGH, MICHAEL
Acadia University, Wolfville, N.S., B4P 2R6

Chaetae form the basis for polychaete taxonomy but their ultrastructure and development is poorly known. Amongst the polydorid polychaetes, for example, the fifth chaetiger is highly modified and displays chaetae that are unique to that segment. The diversity in the fifth chaetiger makes it key to polydorid taxonomy, and suggests that it is the most rapidly evolving morphological trait in this group. Our objective is to analyse the morphology of the modified spines of the fifth chaetiger in Boccardia proboscidea (Witty's Beach, Vancouver Island, B.C.) and Polydora cornuta (Conrad's Beach, Lawrencetown, N.S.). The fifth chaetiger of Boccardia proboscidea has falcate and bristle-topped major spines, whilst the same segment has falcate and feathery, companion chaetae in Polydora cornuta. We examined these spines with scanning electron microscopy and two µm resin sections. Boccardia proboscidea and Polydora cornuta have chaetal sacs with similar morphologies. Chaetal sacs are epidermal invaginations consisting of three types of cells: epidermal cells, follicle cells and chaetoblasts. Epidermal cells surround the distal sac and possibly form the stem cell population. Follicle cells make up the majority of the sac and surround each spine. They secrete chitin onto the lateral surface of each spine and possibly provide hydrostatic support. Chaetoblasts are basal cells that secrete chitin extracellularly and shape it via microvilli on their apical surface, thus producing the unique shape of each spine. We found the structure of the chaetal sacs to be similar in these two species, although the resulting spine structure is highly divergent. These similarities suggest that the chaetae are formed in a similar fashion and that differences in chaetal morphology arise from variation among chaetoblasts in shaping of microvillar canals within each spine.

SUPERVISOR: Dr. Glenys Gibson