ECHINOMETRA LUCUNTER, ECHINOMETRA MATHAEI AND ECHINOSTREPHUS MOLARIS: THREE CLOSELY RELATED BIOERODING ECHINOMETRID ECHINOIDS AND THEIR MARKEDLY DIFFERENT BIOEROSION TRACES

Three members of the “regular” echinoid family Echinometridae are all bioeroders living in shallow, subtropical and tropical waters. Structurally similar, their lifestyles nevertheless show important differences. Echinometra lucunter bores beachrock, particularly in the Caribbean Sea. It erodes grooves, producing networks in the beachrock, creating a habitat in which to browse encrusting algae. These bioerosion structures have not been found fossilized.

Echinometra mathaei bores nearly hemispherical depressions or dishes (ichnogenus Circolites) into beachrock in the Indian Ocean. Populations on the SW coast of Sri Lanka are so crowded that the echinoids are limited in lateral movement and all nutrition must be obtained in situ. Macro-scale suspension feeding or the trapping of floating algal detritus is known to contribute to the nutrition of these echinoids, by catching passing tidbits using spine movements like chopsticks, and retrieval by tube feet.

Echinostrephus molaris in shallow water in Sri Lankan back-reef lagoons bores a tunnel 1-2 cm wide and some 10-12 cm deep, more or less perpendicularly into coral rock (ichnogenus Trypanites). In rough water, the echinoid collects floating pieces of algal detritus. But in calm seas, Es. molaris was seen to suspension-feed at the mouth of its boring during the rising tide. Fine particles were caught on long, mucus-coated aboral spines that protrude out of the boring entrance.

It appears that the three echinoids show different trophic styles. Es. molaris makes huge Trypanites that act as a domichnion, and is apparently a pure suspension feeder. Em. mathaei makes a normal Circolites, which probably functions as both domichnion and agrichnion, the echinoid being a suspension feeder as well as indulging in algal harvesting. The large furrows of Em. lucunter serve as a domichnion and agrichnion and the echinoid is probably an algal farmer and browser; but may well also indulge in suspension trapping during stormy weather.