The feeding mechanism of Yoldia (= Aequiyoldia) eightsi (Courthouy)

The protobranch bivalve mollusc Yoldia eightsi Courthouy is both a deposit feeder (on mud) and a suspension feeder (on diatoms in the ventilatory streams, which are trapped on the ctenidia). The species has a similar anatomy to other Yoldia species, but is a more shallow burrower which adopts a more horizontal shell orientation than the vertically burrowing Yoldia limatula and Yoldia ensifera. Although capable of feeding on the surface layers of mud by extending its palp proboscides outside the partly buried shell, Yoldia eightsi spends most of its time feeding while totally buried. To do this, sediment is taken into the mantle cavity by opening the shell valves, or by foot movements. The sediment is moved by ciliary action to the posterior part of the mantle cavity where it forms a compact, mucus-coated sediment slug. The slug is repeatedly sorted largely by the palp proboscides, fine material being transferred to the mouth via the palps. Sorting appears to be done on a simple size–density basis, with large, dense particles being rejected. After sorting, the inorganic fraction of the slug is expelled through the inhalant siphon (‘pseudofaecal plume’). Expulsions occur every 6–35 min. True faeces (‘faecal plume’) are expelled much more frequently in the expiratory bursts of water from the exhalant siphon. Pseudofaecal output is about 170 times the faecal output (on a dry mass basis), suggesting that Yoldia eightsi ingests 0.6% of processed material.

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