Life cycle and population dynamics

Burrowing megafauna

Seasonal and diel activity patterns

The sexes are separate in sea pens (ie. each colony of polyps is either male or female). The reproductive biology of the British sea pens has not been studied, but in other species the eggs and sperm are released from the polyps and fertilization takes place externally. In Puget Sound, Washington State, USA, the sea pen Ptilosarcus guerneyi spawns in late March, with up to 200,000 eggs produced per female colony. The free-swimming larvae do not feed and will settle within seven days if a suitable substratum is encountered (Chia & Crawford, 1973). This species may live for up to 15 years, and takes five or six years to reach sexual maturity (Birkeland, 1974). Larval settlement was patchy in space and highly episodic in time, with no recruitment to the studied population taking place in some years. This pattern gave rise to a discontinuous population size structure, made up of overlapping patches of colonies differing in age and size (Birkeland, 1974). Large year-to-year differences in recruitment rates were also seen in the ‘sea pansy’ Renilla kollikeri, a sea pen from the coast of California (Davis & Van Blaricom, 1978).

There have been no studies of population structure or life cycle in any of the British sea pens, but the limited data available from other species would lead one to predict a similar pattern of patchy recruitment, slow growth and long life-span.

In typical decapod crustacean fashion, the fertilised eggs are carried on the abdomen of the female before hatching into planktonic larvae. The mating process is something of a mystery in this group, since in laboratory observations mud-shrimps have been found to be highly aggressive to members of their own species, regardless of sex, and intolerant of intruders into the burrow (Tunberg, 1986; Rowden & Jones, 1994; Dr L.A. Nickell, personal communication). Callianassa subterranea is sexually dimorphic, the major chela of the mature male being massively-enlarged (Rowden & Jones, 1994). However, the situation in this species is complicated by the occurrence of intersexes with both male and female reproductive organs (Dr L.A. Nickell, personal communication), suggesting that at least some individuals may change sex during their lifetime.

In the North Sea, Callianassa subterranea females carrying eggs were found from April to September (Rowden & Jones, 1994). Abundance of planktonic larvae was highest in August, indicating a summer breeding season with about four weeks spent in the plankton before settlement. There was some evidence that large females might have a second reproductive period in late winter. Life-span appeared to be 2 - 3 years. Less is known about reproduction in other species and localities. In the north-eastern Irish Sea, egg-bearing females of C. subterranea, Jaxea nocturna and Upogebia deltaura were found in June (personal observation). In Sweden, U. deltaura females also carried eggs from May to August, with one breeding season per year (Tunberg, 1986).

A rather different life history pattern was found in Calocaris macandreae off the coast of Northumberland (Buchanan, 1963, 1974). Animals were protandrous hermaphrodites (initially male, becoming female later in life) producing eggs in January-February which hatched in September-October. Only about 100 eggs were produced in each batch and the large larvae had no free-swimming phase before settling. In contrast to the short-lived Callianassa subterranea, individual Calocaris macandreae were very long-lived (9 - 10 years) and slow-growing, did not become sexually mature until five years of age, and produced only two or three batches of eggs in their lifetime. Partly as a consequence of this life history pattern, the studied population was very stable in numbers over a 10-year period.

Female Nephrops attain sexual maturity at 2.5 - 3 years of age at a carapace length of 21 - 22 mm (Howard, 1989; Bailey et al., 1986). Males become mature after three years at a carapace length of 25 mm. In Scottish waters the eggs are spawned and fertilized between August and November and carried by the females until the larvae hatch (April-August). The larval stages spend about 50 days in the plankton before settlement. The juveniles appear to preferentially take up residence in existing adult burrows, constructing their own burrows as an extension of these (Tuck et al., 1994).

The economic importance of Nephrops norvegicus means that far more attention has been devoted to its population biology than is the case for any other megafaunal burrower (eg. Bailey et al., 1986; Chapman & Howard, 1988; Tully & Hillis, 1995), and only the briefest outline of this subject can be included here. Local populations may vary considerably in density, individual size composition and growth rate (Tuck et al., 1997a). Individual growth rate may be reduced in high density conditions as a result of increased competition for food. Chapman & Bailey (1987) suggested that high population densities were usually found on coarser muds with a relatively high sand content, whereas lower burrow densities (and animals of larger size) were associated with finer muddy substrata. More recent work suggests that the peak in density occurs on mixed sediments of sand, silt and clay, with lower densities on very coarse or fine substrata (Tuck et al., 1997a), and that the main cause of local variation in population density may be the intensity of juvenile settlement rather than any direct effect of sediment type.

Very little is known about the life cycle of this species (Hughes et al., 1996b). All individuals examined have proved to be female. Related echiuran species show extreme sexual dimorphism, with tiny parasitic males living on or in the female body, but so far these have not been found in M. lankesteri. In Loch Sween, Argyll, females containing eggs can be found almost year-round, with some slight evidence for a winter spawning. The larval form is completely unknown, but the large, yolky eggs suggest that the planktonic stage is brief or absent. Small juvenile worms are also very rarely recorded. The adults appear to be sedentary, never leaving or relocating their burrows (Nickell et al., 1995a). The individual life-span is not known, but a period of at least several years is likely. Diving observations at several sites in Loch Sween over a period of roughly ten years have provided no evidence of any major fluctuations in population size, and it seems likely that M. lankesteri is long-lived, with stable populations and low recuitment rates.

The red band-fish Cepola rubescens, is sexually dimorphic, the males being larger and heavier than the females. Spawning takes place in late summer (Atkinson et al., 1977). Fish spawn just above the bottom, where fertilization takes place. The eggs are pelagic.

On the Scottish west coast, Fries’ goby, Lesueurigobius friesii, lives for up to 11 years (Gibson & Ezzi, 1978). Growth is rapid in the first three years, slow thereafter. the females are larger than the males. Breeding occurs from late May to August. In contrast to Cepola, the eggs are laid on the roof and sides of the burrow, and presumably guarded by one parent.

The snake-blenny Lumpenus lampraetiformis is known to live for up to nine years, but little else is known of its life cycle (Gordon & Duncan, 1979).

Field observational data are available for a few of the principal species in this biotope complex. Hoare & Wilson (1977) found that colonies of Virgularia mirabilis in Holyhead Harbour were not synchronized in their behaviour. Some colonies were retracted into the sediment while others were extended. The sea pens were insensitive to light, but extension was possibly influenced by tidal conditions.

The activity patterns of thalassinidean crustaceans are difficult to identify since most species rarely (Jaxea nocturna) or never (Callianassa subterranea, Calocaris macandreae, Upogebia spp.) leave their burrows. Tunberg (1986) found that Upogebia deltaura remained inactive in the deepest parts of its burrow during the winter. Measurement of the quantities of sediment expelled from burrows can be used as an index of activity rate. Rowden & Jones (1997) found that sediment ejection by Callianassa subterranea from the North Sea was negligible during the period January-April, then increased steadily to a maximum in September before declining again over the autumn and early winter. Sea bed photographs supported this picture of seasonal activity, with a marked contrast between a smooth, inactive sea bed in January, and one covered with numerous ejecta mounds and burrow openings in September.

In shallow water, Nephrops norvegicus usually remain within their burrows by day and emerge at sunset to forage during the night (Chapman & Rice, 1971). The animals return to their burrows around sunrise. In deeper water (~100 m) this activity rhythm is reversed, and the animals are more active by day. Emergence occurs around dawn and dusk at intermediate depths. These patterns suggest that Nephrops is preferentially active at a particular optimum light intensity, and the period of emergence at different depths corresponds to the occurrence of this light level at the sea bed (Chapman et al., 1975). Individual Nephrops do not always return to their original burrow, and fighting for burrows has been observed (Chapman & Rice, 1971). In Loch Sween, burrows were aggregated in groups during the late summer, with the aggregations breaking up into a random distribution during the winter (Tuck et al., 1994). Aggregations may arise from the burrow complexes formed when juvenile Nephrops settle in pre-existing adult systems, then break up as the juveniles gradually extend their own burrows and lose contact with those of the adults.

In the shallow waters of Loch Sween, the echiuran Maxmuelleria lankesteri extends its proboscis to collect surface sediment only at night (Hughes et al., 1993) (a different activity pattern might be expected in deeper water where less light penetrates to the sea bed). The proboscis is only extended for short periods of about ten minutes, and only a small number of extensions (< 10) is made per night. The proboscis is therefore above the sediment surface for only a very small proportion of the total time. The worms are active all year round but seem to show peaks of activity in December and April when the proportion of easily-degradable organic matter at the sediment surface is at its highest (Hughes et al., unpublished data).

Red band-fish feed most actively at dawn and dusk, and enlarge or maintain their burrows by day. At night they remain inactive within their burrows (Atkinson & Pullin, 1996).

Next Section                     References