Magrofaunal utilization of plant remains in the deep sea

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Abstract
Plant remains of terrestrial, shallow-water, or epipelagic origin (wood, leaves, fruits, seagrass, and Sargassum) are abundant in many areas of the deep sea. Collections of plant material from the Caribbean area (> 1200 m) have been examined. Fragments of wood and blades and rhizomes of seagrass (Thalassia testudinum) are particularly abundant in the Puerto Rico and Cayman Trenches into which they are transported by turbidity flows. The study has revealed a rich fauna which is listed together with previous, scattered records of association between deep-sea animals and plant remains. About 50, mainly sessile, organisms utilize seagrass and wood as a substrate. Most common are byssus-producing mytilid bivalves. Polychaeta is the dominating group seeking shelter under bark and in cavities in wood (more than 25 species), and 10 species of isopods were found in hollow rhizomes (about 50 specimens of one species). Several animal groups utilize plant remains as food. The opportunistic boring bivalves (Xylophaga) are for the first time recorded attacking seagrass rhizomes. Cocculinid gastropods (at least 9 species) are so abundant that it may be justified to regard them as another example of opportunistic species in the deep sea. Chitons and limpets are probably also herbivorous, and so are several species of polychaetes. (mainly Nereidae), a remarkable echiuran worm, several species of amphipods and isopods, more than 10 echinoids (although mainly from the upper bathyal zone) and perhaps a few ophiuroids and asteroids. It is suggested that the main nutriments for the herbivores, as in shallow-water areas, are provided by bacteria and fungi and possibly by their grazers. The discrepancy of data on growth rate and substrate conversion of bacteria in the deep sea may be attributable to several factors, e.g., the availability of energy, the stimulating effect of macrofaunal consumers, and the apparent age or residence time of deep-water masses. The extremely rapid settlement of Xylophaga larvae indicates an even faster deterioration by bacteria or fungi or both. The macrofaunal herbivores represent the final link in the decomposition food chain. They are available as prey for predators and as dead remains for scavengers, and their fecal pellets can be utilized by deposit feeders.