Sea pens

Burrowing megafauna

Other decapod crustaceans

Sea pens are colonial cnidarians belonging to the Class Anthozoa, which also includes the corals and sea anemones (the Phylum Cnidaria was formerly known as Coelenterata, and the term ‘coelenterates’ is used in much of the older literature). Within the Anthozoa, sea pens form part of the Subclass Octocorallia, distinguished by having polyps with eight pinnate tentacles. The sea pens (Order Pennatulacea) are the only octocoral order adapted for life on soft substrata. Each animal consists of a colony of polyps arising from a central stiffened axis, or rachis. The rachis ends in a basal stalk which anchors the colony in the sea bottom, with the polyp-bearing section held upright above the sediment. The animals are suspension-feeders, living on plankton and organic particles trapped by the polyp tentacles. Three sea pen species are well-known from British coastal waters. A fourth species, Balticina christii, has been recorded from the deeper waters of the North Sea. Illustrations and detailed descriptions of all British species can be found in Manuel (1981) and Hayward & Ryland (1990).

This is a slender sea pen up to 60 cm long. Polyps occur in small clusters of up to 12, arranged in two opposing lateral rows along the rachis. The living colony is white to creamy-yellow in colour, and is able to withdraw itself into the sediment when disturbed. The species is found in sandy or muddy substrata in sheltered inshore waters, or in deeper waters offshore, in depths of 10 - 400 m. Virgularia mirabilis is the most abundant and widespread sea pen in British waters, locally common on all coasts but less frequent in the south. It is often abundant in deep, man-made harbours such as Holyhead Harbour, Anglesey (Hoare & Wilson, 1977), and is also very common in many of the Scottish sea lochs. Howson et al. (1994) recorded the species as present in 83 out of 98 sea lochs listed in their review. Outside British waters, Virgularia mirabilis is widespread around western Europe and the Mediterranean, and occurs throughout the North Atlantic possibly as far as North America.

This species is much stouter and more fleshy than Virgularia, and grows up to 40 cm long (up to 25 cm projecting above the sediment). Fused polyps form large, triangular ‘leaves’ arranged more or less alternately in two opposing lateral rows. The colony is a deep reddish-pink owing to the presence of red sclerites in the tissue. The central axis of the colony is often bent over at the tip like a shepherd’s crook. Both Pennatula and, to a lesser extent, Virgularia, are bioluminescent, emitting light in brilliant flashes or in rhythmic pulses passing along the colony (Nicol, 1958). Pennatula phosphorea occurs in sandy or muddy substrata below about 15 m, probably extending to depths of over 100 m. The species is locally common in the North Sea and around the western British coasts, but appears to be absent from southern Britain. It was recorded as present in 46 of the 98 sea lochs listed by Howson et al. (1994). Other records come from the Mediterranean and North Atlantic, but confusion with other species makes the true geographic range uncertain.

This is the largest of the British sea pens, some colonies extending over 1 m above the sea bed. The polyps are irregularly arranged along the rachis, tending to form oblique rows in places. The flesh is white, yellowish or pale pink, and the central axis has a distinctive quadrangular cross-section. Funiculina quadrangularis typically occurs on soft mud substrata and does not extend into water as shallow as the other two species, being usually recorded deeper than 20 m. The lower depth range extends to over 2000 m. Known distribution in British waters is restricted to the north and west coasts of Ireland and Scotland. Howson et al. (1994) recorded it in only 17 out of 98 listed Scottish sea lochs. Outside British waters, Funiculina quadrangularis occurs in the North Atlantic and Mediterranean, with other records from as far afield as New Zealand and Japan (Manuel, 1981).

Although the fauna of marine sediments has been studied using ship-borne sampling equipment (grabs, dredges, cores) for well over a century, the abundance and diversity of large burrow structures in many soft substrata was not recognized until it became possible to observe the undisturbed sea bed using underwater photography (still and television), essentially from the early 1960s onwards (Barnes, 1963). The increasing use of SCUBA diving as a research technique from the 1970s onwards allowed direct observation of the behaviour of some of the larger burrowing animals (eg. Chapman & Rice, 1971; Atkinson, 1974; Atkinson et al., 1977). The development of the technique of resin-casting (Shinn, 1968; Atkinson & Chapman, 1984) was also an important advance. This allows the three-dimensional preservation of sub-surface burrow structures in the field, aiding the identification of sea floor features and providing information on the ecology of certain burrowing species which rarely or never show themselves at the sediment surface (eg. Pervesler & Dworschak, 1985; Atkinson & Nash, 1990; Nickell et al., 1995a).

Atkinson (1986) extended the use of McIntyre’s (1971) term ‘megafauna’ to cover the larger, deep-burrowing benthic animals that are difficult to sample using conventional ship-borne methods and are most effectively studied in the field using a combination of SCUBA diving and underwater video. ‘Burrowing megafauna’ is therefore a loosely-defined, but useful label for a functional category of benthic animals sharing one major ecological trait - the construction of large, and usually long-lasting burrows. Taxonomically, the burrowing megafauna of British coastal seas includes the species belonging to the decapod crustacean infraorder Thalassinidea (lacking common names in most cases, but sometimes collectively termed ‘mud-shrimps’), a few larger decapods such as the Norway lobster (Nephrops novegicus) and the angular crab (Goneplax rhomboides), several large worms belonging to the Phylum Echiura, and a number of fish species. Some other groups such as the holothurians (sea-cucumbers) and enteropneusts (acorn-worms) have representatives which probably fall within the megafaunal burrower category, but these are little-known in British waters.

Atkinson (1986) and Atkinson & Nash (1985) give useful summaries of the ecology of many of the most prominent megafaunal burrowers in British coastal waters, together with descriptions of the burrows they inhabit. This is still a very active field of research, and much additional information has been gained since the mid-1980s. The current state of knowledge of the species concerned is therefore briefly outlined here.

The British members of this group are illustrated in Hayward & Ryland (1990). All are small animals of broadly shrimp-like appearance and all inhabit complex burrows in subtidal sands or muds.

This species is translucent white in colour and up to 60 mm long. The eyes are tiny, in accord with the burrowing lifestyle. The chelipeds (claws) are asymmetric, with one greatly enlarged. The type of burrow constructed varies depending on the sediment characteristics. In the fine muds of Scottish sea lochs, C. subterranea inhabits a complex lattice of galleries usually at a depth of 30 - 40 cm below the sediment surface, but sometimes extending down to over 80 cm (Atkinson & Nash, 1990; Nickell & Atkinson, 1995). The sub-surface lattice is connected to the surface by one or two vertical shafts, one of which usually opens in the centre of a conical mound of ejected sediment. In the coarser sediments of the southern North Sea, burrows extend less deeply below the surface (9 - 23 cm), and have more surface openings (up to eight) than those from sea lochs (Rowden & Jones, 1995). Callianassa subterranea is a deposit-feeder, ‘mining’ organic material from the sub-surface sediment or possibly feeding on surface detritus trapped in the burrow openings.

Callianassa subterranea is widespread and common around the British coasts, and has been recorded in large numbers in the southern North Sea (Witbaard & Duineveld, 1989; Rowden & Jones, 1994) and the north-eastern Irish Sea (Swift, 1993; Hughes & Atkinson, 1997). The species is also a prominent member of the megafaunal burrowing communities of the Scottish sea lochs (Howson et al., 1994, and personal observations). Total geographic range extends from Norway to the Mediterranean. A second Callianassa species, C. tyrrhena also occurs in southern British waters but there is no detailed information on its behaviour.

This mud-shrimp is more squat in form than Callianassa subterranea, and has relatively small chelipeds. It constructs a system of U-shaped tunnels with distinctive three-way junctions where tunnels connect (Nash et al., 1984). The burrow system is usually built at two interconnected levels in the sediment, with a total depth of up to 21 cm. The openings to the sediment surface also typically occur in groups of three, although this pattern may be obscured by the collapse or blocking of some of the openings. Calocaris macandreae is found in muddy sediments with high silt-clay fractions and does not occur in sandy substrata (Buchanan, 1963). The species occurs throughout the eastern Atlantic and Mediterranean at depths of 30 - 1100 m. In British waters it is best-known from the Northumberland coast (Buchanan, 1963), the Clyde Sea area and Firth of Lorne (Nash et al., 1984), and the Irish Sea (Calderon-Perez, 1981).

In appearance this species resembles a miniature lobster, with very long, slender chelae. Overall body length is 4 - 6 cm, colour a drab greyish-yellow, often obscured by clinging sediment. The animal inhabits a very deep (up to 90 cm) burrow of obliquely-descending tunnels, sometimes with one or more vertical shafts (Pervesler & Dworschak, 1985; Nickell et al., 1995b; Nickell & Atkinson, 1995). Up to 14 surface openings may be present, although at any one time only a few of these may be in use, the rest blocked by plugs of sediment. Active openings are often surrounded by a distinctive ‘collar’ of sediment resembling a pie-crust, formed as the animal ‘bulldozes’ small piles of material out of its burrow. The animal deposit-feeds from the walls of its burrow and may also scavenge organic material from the sediment surface. Jaxea nocturna occurs in muddy sediments at water depths below about 10 m. The species is probably widespread and relatively common in suitable habitats around the British coasts, but is under-recorded owing to its deep-burrowing, cryptic lifestyle. It is certainly present in the Scottish sea lochs (Nickell et al., 1995b), the north-eastern Irish Sea (Swift, 1993) and the English Channel (Marine Biological Association, 1957). Underwater video surveys show that burrows of Jaxea nocturna are abundant in the deeper waters of the Clyde Sea area, especially in Loch Fyne and to the east of Arran (R.J.A. Atkinson, personal communication). Outside British waters, the species occurs south to the Mediterranean and Adriatic Seas (Pervesler & Dworschak, 1985).

Upogebia spp.

Three species of this genus occur in British waters, U. delatura, U. pusilla and U. stellata. All have a similar ecology. Burrows are relatively simple, consisting of one or two connected U- or Y-shaped components penetrating the sediment to depths of up to 25 cm (Dworschak, 1983; Nickell & Atkinson, 1995; Astall et al., 1997b). Shafts decending from the main U-component may penetrate much more deeply into the sediment. Surface openings are usually inconspicuous holes without associated mounds. Upogebia species differ from the mud-shrimps discussed previously in being primarily suspension-feeders, actively pumping water through their burrows and filtering out particulate matter (Dworschak, 1981). They are usually found in sands or muddy sands with mixtures of stones or shell gravel, rather than in the finer muds associated with Calocaris macandreae or Jaxea nocturna. In Stravanan Bay, on the south-west coast of the Isle of Bute, U. deltaura inhabits the sediment underlying an extensive maerl bed (J. Hall-Spencer & R.J.A. Atkinson, unpublished observations), an unusual habitat record for a megafaunal burrower. All three British Upogebia species range from Norway to the Mediterranean. Upogebia pusilla is the least common in British waters. Upogebia deltaura and U. stellata are more widespread, occurring off all coasts, sometimes in mixed populations.

The Norway lobster (sometimes referred to as ‘Dublin Bay Prawn’ or ‘Scampi’) is one of the more familiar megafaunal burrowers. The burrowing habit of this large (total body length may be over 25 cm) crustacean was first confirmed in the field by sea bed photography. Burrows may be very large, with tunnels over a metre in length and up to 10 cm in diameter. Simple burrows consist of a straight or T-shaped tunnel descending at a shallow angle and penetrating the sediment to a depth of 20 - 30 cm (Rice & Chapman, 1971). Large piles of excavated sediment are often seen around the burrow entrances. More complex systems are created when juvenile animals construct burrows leading off from those of the adults (Tuck et al., 1994). A comprehensive analysis of burrow form is given by Marrs et al. (1996). In shallow water, the animals spend most of the day inside their burrows and emerge at night to forage (Chapman & Rice, 1971). Nephrops is carnivorous, feeding on sediment-dwelling polychaetes, bivalves and crustaceans, and also on carrion. The species is found on soft mud substrata all around the British and Irish coasts, from as little as 5 m depth in shallow sea lochs to several hundred metres offshore. Total range extends from Norway and Iceland to the Mediterranean. Nephrops norvegicus is the subject of a major fishery and its ecology and population biology have therefore been intensively studied (eg. Chapman, 1980; Bailey et al., 1986; Tuck et al., 1997a).

This crab has a distinctive rectangular carapace with prominent spines at the front corners. The chelipeds are highly elongated, particularly those of the male. Burrows are quite similar to those of Nephrops norvegicus but are smaller, descending only 10 - 15 cm into the sediment, with 1 - 6 surface openings (Rice & Chapman, 1971; Atkinson, 1974a). Semi-circular ‘runs’ at the burrow entrances, formed as the crab carries out excavated mud and sweeps it aside with its chelipeds, are a useful identifying feature. Goneplax can be found on muddy sands from 8 - 80 m deep on all British coasts. Distribution extends south to the Mediterranean.

The Echiura is a small phylum of marine worms (sometimes referred to as ‘spoon-worms’) distinguished by possession of an unsegmented, sac-like body and, in almost all species, a highly extensible strap-like or ribbon-like proboscis. Most echiurans are burrowing deposit-feeders, using the proboscis to graze detritus from the sediment surface. Only a few species occur in British coastal waters but these can be locally common, and owing to their large size can be an important component of the burrowing megafauna.

This is a large echiuran, with a body length in the contracted state of up to 18 cm. It has the long proboscis characteristic of the phylum and is a vivid dark green colour in life. The animal inhabits a narrow, sinuous burrow in the shape of an elongate U, with a surface opening at each end (Hughes et al., 1996a). One end opens at the apex of a volcano-like mound of ejected sediment up to 30 cm high and 40 cm across (Hughes et al., 1996b). The burrow may extend over 80 cm deep into the sediment (Nickell et al., 1995b), with up to 2 m between the surface openings. Maxmuelleria lankesteri is highly averse to light and, at least in shallow water, extends its proboscis to feed only at night (Hughes et al., 1993). The species is found in fine muds and muddy sands in water depths of 10 - 80 m. Previously thought to be rare and localized, M. lankesteri is now known to be common in the Irish Sea, Clyde Sea, and in many of the Scottish sea lochs (Hughes et al., 1996b). There are also records from south-west Ireland and the English Channel, but so far none from the North Sea. The species is also known from the Kattegat and Skagerrak, and from north-west Spain.

This is a large echiuran worm (body length up to 14 cm) originally described from near Plymouth but now known also from a number of the Scottish sea lochs (Connor, 1990; Howson et al., 1994). The species has been recorded from muddy sand substrata and inhabits a burrow up to 25 cm deep (Connor, 1990). The animal has a long, ribbon-like proboscis with a strongly bifurcated tip, which it uses to pick up detritus particles from the sediment surface. Amalosoma appears to be locally common in some areas but its ecology has not yet been studied in detail.

This echiuran is smaller than Maxmuelleria lankesteri (body length up to 11 cm) and greyish-yellow rather than green in colour. Burrows have not been described in detail, but appear to be U-shaped and perhaps up to 15 cm deep (Reineck et al., 1967). The ecology of Echiurus has not been studied in British waters but it is known to occur off the east coast of Scotland from Peterhead to St Andrews, and in the Firth of Forth (Stephen, 1934). It is also found further south in the North Sea (Rachor & Bartel, 1981), and in the Kattegat and Skaggerak, sometimes in very dense populations.

Many fish species, belonging to a number of different families, have adopted a burrowing lifestyle (Atkinson & Taylor, 1991). In British waters there are three species known to excavate sizeable burrows in shallow coastal sediments.

This is a fish of striking and distinctive appearance, with a slender, elongate body up to 70 cm long, dorsal and anal fins continuous with the tail fin, and large eyes. The body colouration is orange-red with yellowish fins. The fish excavates a large burrow consisting of a wide (up to 20 cm) shaft descending vertically into the sediment, ending in an expanded terminal chamber (Atkinson et al., 1977). A narrow side shaft is often present (Atkinson & Pullin, 1996). Most burrows are 30 - 70 cm deep, but some extend down to over 1 m. The fish feeds on planktonic crustaceans caught during short excursions from the burrow. Cepola occurs on muddy sands from 12 - 200 m water depth off the southern and western British coasts, and ranges south to the Mediterranean and north-west Africa. In British waters it is best-known from around the isle of Lundy and from Irvine Bay off the Ayrshire coast (Atkinson et al., 1977).

A stout-bodied goby up to 13 cm long with a fawn or grey background colouration and conspicuous golden-yellow spots, this species was shown to construct burrows in muddy sediments by Rice & Johnstone (1972). The burrow is a shallow, U-shaped tunnel up to 20 cm long but rarely extending more than 10 cm into the substratum. The burrow openings are narrow and circular, but the tunnel is expanded in the centre and used for the deposition of eggs (Gibson & Ezzi, 1978). The diet consists mainly of small polychaetes. Fries’ goby occurs from the Irish Sea northwards to the Clyde and western Scotland, around the Irish coast, and in the Kattegat and Skagerrak. Known depth range is 10 - 350 m.

The snake-blenny has a very slender, almost eel-like body up to 40 cm in length. Body colouration is pale brown-yellow with irregular darker brown spots. Burrows are shallow, horizontal tunnels of circular cross-section with distinctive, Y-shaped junctions (Nash, 1980; Atkinson et al., 1987). Individual burrow systems may be over 70 cm long, though most are 20 - 35 cm. The species occurs on soft mud bottoms at depths of 20 - 200 m, being more abundant below 50 m. Distribution ranges from the northern and western British coasts, the North Sea, Scandinavia and the Baltic north to Spitzbergen, and from Iceland to north-eastern North America.

The species described above are those which have been most frequently recorded in megafaunal burrowing communities and which have received the most detailed study. However, these studies are biased to some extent by geography, with much of our information coming from a few of the more easily-accessible Scottish sea lochs. The British marine fauna includes a number of other species that can be regarded as megafaunal burrowers, but whose distribution and ecology are still very poorly-known owing to their cryptic lifestyle, occurrence in deep water, or in regions not yet subjected to detailed study. It is likely that future studies will find some of these species to be locally common and ecologically important.

Axius stirhynchus is a thalassinidean crustacean sporadically recorded from the shallow subtidal off southern and western Britain (Marine Biological Association, 1957; Allen, 1967). Little is known of its ecology, but it has been recorded burrowing under stones on sandy and muddy estuarine shores. The related Axius serratus has been recorded burrowing to depths of over 3 m in Nova Scotia (Pemberton & Risk, 1976), the deepest megafaunal burrows known. It is therefore likely that A. stirhynchus has been under-recorded, and may be more common than the few records suggest.

The amphipod crustacean Maera loveni has a northern Atlantic distribution and reaches its southern geographic limit in Scotland, where it occurs on both east and west coasts. In Loch Riddon in the Firth of Clyde, the species constructs a complex burrow system of interconnected U-shaped tunnels. The tunnels are narrowly elliptical in cross-section and reach the sediment surface in a cluster of up to 70 slit-like openings (Atkinson et al., 1982). Uniquely among the species discussed here, the larger burrows appear to be constructed by a colony of animals, but this cooperative social behaviour has not been studied in detail.

Labidoplax digitata, a large, worm-like holothurian (sea-cucumber) up to 30 cm long, is known from western British coasts to a depth of 70 m. The animal burrows in clean or muddy sand and feeds on surface detritus collected with its oral tentacles. Burrow form and ecology have not so far been studied.

The hagfish, Myxine glutinosa occurs on muddy substrata and is known to inhabit burrows with conspicuous, volcano-like mounds (Foss, 1962, 1968). Burrows seem to be a simple U- or J-shape, and to be lined with mucus (Hardisty, 1979). The hagfish is mostly found at depths well beyond the range of air diving and so is difficult to study in the field. Among fish species, another likely burrower is the four-bearded rockling, Enchelyopus cimbrius, which has been observed in association with burrows in the deep waters of the Clyde Sea (R.J.A. Atkinson, personal communication).

In addition to those fish and invertebrates that are specialist or obligate burrowers, there are others that do not normally excavate their own burrows but will opportunistically inhabit those constructed by other species. Of these, the most frequently observed is the black goby, Gobius niger. In Scottish sea lochs, this fish will take up residence in burrows belonging to Maxmuelleria lankesteri and other species, frequently enlarging or modifying the shape of the burrow opening (Nickell et al., 1995a; Marrs et al., 1996). The squat lobster, Munida rugosa, is frequently found inhabiting burrows on the periphery of megafaunally-burrowed muds, where these merge with coarser substrata (C.J. Chapman, personal communication). It is uncertain whether Munida excavate their own burrows or take over those made by other animals.

Much still remains to be discovered about the basic biology of even the best-known species of burrowing megafauna, and the makers of some distinctive burrow types still remain unidentified. Paired sediment mounds, each surrounded by a ring of holes, have been seen using towed underwater video in the Clyde Sea (Tuck & Atkinson, 1995) and elsewhere. These may be the work of enteropneusts (acorn-worms), but this has still to be confirmed. Observations such as these suggest that the list of large burrowing animals in British waters will expand as work continues.

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