There are few interdependent features within deep faunal-burrowing mud communities and no single species can be considered a keystone species whose activity is essential to community structure. The burrowing species create tunnels in the sediment which themselves provide a habitat for other burrowing or inquilinistic species. Echiuran worms produce long-lasting burrows that provide a habitat for a variety of small polychaetes and bivalves but no obligate species. There are interrelationships. For instance, the shrimp Jaxea nocturna, which often lives in association with the echiuran worm Maximulleria lankesteri (Nickell et al. 1995a) may benefit from the oxygen-rich mud pulled into its burrows by the worm. The Norway lobster Nephrops norvegicus is known to prey on the burrowing shrimp Calocaris macandreae (Smith 1988). However, occurrence together including sharing burrows is mainly by chance. The large sea anemone Pachycerianthus multiplicatus, which occurs solely in these biotopes, creates a habitat for attached species (O’Connor et al. 1977). Some unusual sessile species can be found growing on the mouthparts of burrowing crustaceans and Symbion pandora, which was only recently discovered, appears to belong to a hitherto undescribed phylum (Conway Morris 1995). The rarely recorded deep-water brittlestar Asteronyx loveni occurs in association with Funiculina quadrangularis. The species living in deep mud biotopes are cryptic in nature and not generally subject to predation. Indeed, there is little evidence of predation although tissue from Virgularia mirabilis (most likely the top of the colony) has been found in the stomach of haddock (Marshall & Marshall 1882). The sea slug Armina loveni feeds on Virgularia mirabilis and is known to occur from Norway to western France (Thompson 1988). Birkeland (1974) found that the starfish Crossaster papposus was a common predator on the seapens in Puget Sound and this species also occurs widely in the north-east Atlantic although not commonly in the biotopes included here.

Burrowing activity creates a much more architecturally complex habitat that would be the case for un-exacavated mud. However, the most important feature of excavation is the working of the sediment and the ventilation of the burrows which ensures that sediment is oxygenated to a much greater depth than would be the case in un-burrowed sediment. Such oxygenation should enable the development of a much richer and/or higher biomass community of species living within the sediment and not in contact with the surface.

The major component species in seapen biotopes appear to have plankton stages at some phase in their life cycle so that colonization may be from distant sources.

The presence of burrowing species is essential in providing structure to the biotope. However, this importance is mainly for the burrowing species themselves.

Only a small number of predatory species are likely to utilize this biotope. Nephrops norvegicus is known to be eaten by a variety of bottom-feeding fish including haddock, cod, skate and dogfish. In some areas, up to 80% of cod stomachs are found to contain Nephrops (Howard 1982). Burrowing shrimps and echiuran worms are also found in the stomachs of bottom feeding fish.

Seapen faunal communities appear to persist over long periods at the same location. However, whilst the population density of some species is very stable (for instance, Calocaris macandreae off the Northumberland coast – Buchanan 1974), others vary greatly (for instance Echiurus echiurus in the German Bight – Rachor & Bartel 1981). Virgularia mirablis populations are known to completely withdraw into the sediment, but little is known of the periodicity of this behaviour.

Almost nothing is known about the population dynamics and longevity of seapens in the north- east Atlantic, but data from other species suggest that they are likely to be long-lived and slow growing with patchy and intermittent recruitment. The burrowing decapods which also characterise the biotopes vary in longevity and reproductive strategies with, for instance, the thalassinoid mud-shrimp Calocaris macandreae known to reproduce slowly and live for up to ten years.

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