The nature and construction of skeletal spines inPocillopora damicornis (Linnaeus)

Extreme variability in the size, shape and spacing of skeletal spines ofPocillopora damicornis has been demonstrated both within single colonies and also between colonies from different environments. Preliminary studies indicated that the majority of spines from branch tips at the apex of the colony display a ‘fasciculate’ growth surface in contrast to partly fasciculate or ‘smooth’ growth surfaces exhibited by spines from branch tips at the base of the colony. No significant differences in the height and width of costal spines from apical and basal branch tips within a single colony were observed, although spines from colonies exposed to strong wave action tended to be significantly shorter and narrower than those from more sheltered environments. Both costal and coenosteal spines from wave-exposed colonies displayed branching and divided extremities while those from sheltered environments consisted of simple cones. Spines develop as an outgrowing of the calicoblastic ectoderm which secretes the skeleton. Growing costal and coenosteal spines are enveloped by a layer of calicoblastic ectoderm which penetrates through mesogloea, aboral gastroderm, coelenteron, oral gastroderm, mesogloea and finally oral ectoderm. Spines within the corallite are surrounded by calicoblastic ectoderm, mesogloea and aboral gastroderm only. A scheme for the growth of the spines is discussed.