Wear and tear of mouthparts: a critical problem in stream animals feeding on epilithic algae

Abstract

A large number of stream-dwelling animals feed on epilithic algae and must scrape or brush off food that is firmly attached to rough stones. The mouthparts of these highly specialized alga grazers are equipped with brushes, rakes, gouges, excavators, and rasps. Because they scrub across the rough substratum, these different types of feeding apparatus all become extremely worn down after a short time. The delicate bristles of brushes, the prongs of rakes, and the fine structures of rasps abrade swiftly, and often completely disappear after having been used for removing epilithic algae from stones. The blades of gouges and the scoops of excavators also wear down, but in this case abrasion usually causes only the shortening of these relatively solid tools and does not so strongly impair their effectiveness. Specific adaptations have been evolved to deal with the problem of wear. Either the grazing animals are able to repair their mouthparts frequently, or their feeding apparatus is constructed in such a way that its effectiveness is guaranteed for a long time despite the wear. Typical defects arising from use, and adaptations to wear, were studied comparatively by scanning electron microscopy of different alga grazers: insects of different orders (Ephemeroptera: Rhithrogena, Epeorus, Ameletus, Baetis, Lepeorus; Plecoptera: Brachyptera; Coleoptera: Limnius; Diptera: Liponeura, Oxycera), the isopod Ligia italica, the snail Ancylus fluviatilis, and the tropical fish Garra taeniata. Laboratory experiments with mayfly larvae showed that the mouthparts wear down when they are used to scrape a rough substratum. There was no evidence that moulting intervals were shortened by abrasion of feeding structures in mayfly larvae.