Echinometrid sea urchins, their trophic styles and corresponding bioerosion

Abstract

The Echinometridae is a diverse, largely tropical family of echinoids. Several species are active borers in shallow water, at and below the low-tide line, especially in coral reef and beachrock substrates, commonly in high-energy situations. In the Caribbean and Atlantic Ocean, Echinometra lucunter erodes two types of boring. Most commonly, the widely reported elongate grooves are produced. In these traps, drifting fragments of algae are caught on the incoming tide. Drifting algae are also caught actively by ‘chopsticks-like’ manoeuvres of the spines. Some simple gardening is undertaken on the boring walls and floor, where short algal turf and endolithic algae are harvested. Juveniles start by making simple cup-shaped borings. In some cases, however, E. lucunter retains this form of bioerosion to adulthood. Cup-shaped borings must indicate emphasis on alga-catching in highenergy environments and less on the gardening, grazing trophic style.

In the Indo – West Pacific realm, Echinometra mathaei undertakes similar trophic activities to those of E. lucunter, especially in the high-energy environment outside barrier reefs. However, in protected shoreward lagoons, another trophic activity is practiced that has largely been overlooked in echinoids. In this oceanic region, a recently identified food-source is available in back-barrier lagoons, i.e., flocs of organic particle aggregates that, like the marine snow of the open sea, are swept into the lagoon on the rising tide from the reef crest, which is subaerially exposed at low tide. Crowded E. mathaei on beachrock surfaces, each in a cup-shaped boring (Circolites isp.), show alga-catching and within-boring algal turf gardening. But in addition, marine snow is collected on the spines at each rising tide.

Echinostrephus molaris, a small echinoid that practices alga-catching on the outer side of the barrier reef, shows extreme specialisation for particle-aggregate collection in back-barrier sites. E. molaris and E. aciculatus are the only deeply boring echinoids, producing thumb-sized borings (Trypanites isp.). Very attenuated, mucus-coated aboral spines are extended from the mouth of the boring as the nutrient flux of the tidal surge climaxes. Quantities of marine snow are collected from the spines by searching tube-feet.