Brittlestar Beds: Low‐Predation Anachronisms in the British Isles

Abstract

Two epifaunal, suspension—feeding ophiuroids, Ophiothrix fragilis and Ophiocomina nigra, each form dense populations around the British Isles. The ophiuroids carpet the substratum at densities in the hundreds to thousands per square metre. This investigation compared predation on ophiuroids between dense brittlestar beds and subtidal rocky reef communities. Ophiuroids are far less abundant in rocky reefs, and they occupy cryptic microhabitats. The mortality of experimentally tethered ophiuroids was significantly lower in brittlestar beds, where fishes and a portunid crab, Liocarcinus puber, were rare. Starfish, Asterias rubens, ate small numbers of tethered ophiuroids in brittlestar beds and rocky reefs. The low incidence of sublethal damage (regenerating arms) in brittlestar beds reflects a low natural encounter rate with predators. Dense ophiuroid populations declined precipitously in shallow water at the end of the Mesozoic. The near absence of "modern" predators and the consequent low predation levels in living brittlestar bed support the theory of a large increase in consumer pressure beginning in the Cretaceous.