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 (OConnor 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.
Keystone (structuring) species
The presence of burrowing species is essential in providing structure to the biotope.
However, this importance is mainly for the burrowing species themselves.
Importance of habitat for other species
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.
Time for community to reach maturity
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.