Feeding biology of the sea-starAstropecten irregularisPennant

Abstract

As other species of Astropecten, A. irregularis rests buried in the substrate between periods of feeding activity. These usually occur at twilight hours. The sea-star can sense and localize prey buried underneath it with great precision. Prey animals are quickly pushed through the mouth opening and into the stomach by the (sucker-less) tube feet surrounding the mouth. Astropecten cannot dig deep enough to seize deep-sitting prey such as e. g. members of the Tellinidae and non-juvenile Mya spp. It feeds mainly on small animals, but may occasionally swallow very large prey. Digestion of prey takes place in pockets of the stomach wall and requires that lobes of the stomach can be inserted and applied in direct contact with the digestible tissue. Protected prey organisms such as bivalves with tightly appended valves cannot be digested before they succumb and start to gape permanently due to a lack of oxygen. Since oxygen requirements differ, prey are retained in the stomach for varying periods of time. Whereas Spisula subtruncala, for example, is digested and the empty shells ejected well within 24 hours, specimens of (non-juvenile) Venus gallina and Corbula gibba may be retained for 2-3 weeks or more. Certain prey as e. g. non-juvenile Natica spp., V. gallina of all sizes and C. gibba more than 2-3 mm long may often be ejected alive after a shorter or longer stay in the sea-star stomach. Astropecten is a selective feeder, mainly engulfing bivalves, among which it exhibits a distinct preference for species with a low resistance to anaerobic conditions. This behaviour and a pronounced tendency to feed primarily on juvenile specimens ensure that the digestible matter of swallowed prey in most cases may be quickly assimilated. Metabolic rate may seemingly be used as an index for attractiveness of bivalve prey, but the real stimulus is probably one or several metabolic end products. Food uptake is significantly reduced in the absence of “first class” food species. If only highly undesirable food species are present, the sea-star may almost stop feeding and loose weight. A medium sized Astropecten may pick up and digest more than 400 S. subtrullcata spat within 24 hours and theoretically destroy some 30,000 spat annually. Among other bivalves taken in considerable numbers are Mya truncata and Montacuta ferruginosa. Newly settled. Echinocardium cordatum are also heavily preyed upon, at least when attractive molluscan species are absent or scarse. In Danish waters fee.ding practically stops below 40 C. Above this temperature the feeding rate depends highly on the density and types of potential prey spesies present. In the 0resund, for example, feeding rates are usually low at the most favourable temperature level because such attractive prey as O-group S. subtruncata are exterminated while the tempereture still is low. This results in undernourishment and stops reproduction. At 11-16°C, small specimens fed on attractive prey may daily consume an amount of food equal to about 18% of their own living weight, whereas adult specimens consume considerably less, i. e. about 2%. Data from the literature indicates that the majority, if not all, of the Astropecten spp. probably are selective feeders which mainly feed on molluscs.