Do carbonate skeletons limit the rate of body growth?

The bivalvesSpisula solidissima, the Atlantic surf clam, andArctica islandica, the ocean quahog, from the continental shelf off New Jersey, contain repeating structures in their shells. By analyzing the growing shell margins in living specimens at bi-weekly (or sometimes monthly) intervals throughout two consecutive years, it was possible to define an annual cycle of shell growth increment formation in both species. The shell increments in each species are microstructurally distinct units that form over a period of several months at select seasons of the year. Each species has two alternating shell growth increments, GI I and GI II. GI I (the annual growth line of previous studies) is formed annually in the late summer-fall inS. solidissimaand in the fall-early winter inA. islandica.These periods correspond to the spawning phase of the reproductive cycle in both species. No winter rings were found. The annual increments were used to determine age and growth rate in both Recent and Pleistocene specimens. They may also be useful in determining season of death. Because shell growth increments are formed in synchrony among living populations in these species, mass mortalities may be distinguished in the fossil record. Accurate age and growth rate determinations in fossils are important in many paleobiologic contexts, such as deciding between increased longevity or growth rate in cases of phyletic size increase.

Measurement of the compressive strength and elastic modulus of the skeletal material of three common Caribbean corals suggests that the mechanical properties of coral skeleton are an important factor in the adaptive repertoire of these animals. The strength (stress at fracture) of the specimens tested is 12–81 meganewtons/meter2, with material from branched colonies being generally stronger than material from massive colonies. These values are lower than the strength of most other carbonate skeletal materials, but higher than that of carbonate engineering materials like concrete and limestone. The comparatively low strength of coral skeleton may be the result of architectural properties produced by the requirements of competing adaptive factors, such as polyp phototropism, or it may reflect the low probability that a colony will be broken, and therefore need to be stronger, before it achieves reproductive parity. The skeleton of the three species tested here is strongest when stress is applied parallel to the growth direction of the polyps. Strength varies inversely with skeletal porosity. Decreasing porosity in highly stressed colonies represents a potentially valuable adaptation for enhancing strength. The adaptive value of porosity modification may explain differences in porosity and strength between highly stressed branched growth forms and more moderately stressed massive growth forms. Boring organisms reduce the strength of coral skeleton by increasing its porosity. Only minor amounts of boring can produce strength reductions of up to 50%. Specialized, stress-minimizing branch arrangements help maximize resistance of coral structures to mechanical degradation in situations where colony size is unusually large or hydraulic energy dangerously high.