ABSTRACT
The adaptative morphology and ecology of spines of the slate-pencil sea-urchin Heterocentrotus mammillatus (L.) was studied in the field in the Gulf of Aqaba (Red Sea). The digitiform spines of this sea-urchin show very clear concentric growth lines produced mainly by differetial deposition of organic material. Observations on marked small and medium-size individuals (D < 60 mm) show that the growth rate per increment of one ring is similar to the annual growth rate (p > 0.5). The study of growth of small individuals (D < 40 mm, age ≥ 4 years), and experiments on regeneration, enable the growth lines to be used as an index for age determination of H. mammillatus. Abrasion, breakage and shedding of whole spines may be easily detected by examining longitudinal sections of spines. This may enable a reasonable assessment of environmental stress on a population of H. mammillatus in a specific habitat. Special emphasis was placed on the understanding of form and function of spines situated on different parts of the test. The oral spines are short, flat and blunt and are used for anchorage and movement. On the other hand, the aboral spines are long, rounded and pointed. Their main function is defence. In situ regeneration experiments showed that truncated spines which grow below the ambitus, become shorter and wider than those of the upper part of the test. Sea urchins living along the reef flat, are more susceptible to mechanical stress by wave action and have thicker spines than those living in calm water, at a depth of 5 m and more.
