and the large, conspicuous macrofauna such as bivalve molluscs or large worms. These are all resident animals although their larvae may have originated elsewhere. Beaches also receive visitors such as shorebirds (p. 144) and turtles (p. 151).
The meiofauna, being too small to move the grains, generally comprises elongate creatures able to wriggle between the grains. Among the most common animals are nematode worms of which hundreds of thousands may occupy a single square metre of beach. These worms are an important food source for larger animals. A new species of Collembola (p. 48) was recently found in the sandy beach near Tangkoko-Batuangus Reserve, Minahasa (Greenslade and Deharveng 1986). The worms and most other members of the meiofauna are most common in fine sediments since coarse sands dry out too quickly and very fine sands are too easily deoxy-genated. The meiofaunal infauna is less rich in sediments comprising a mixture of grain sizes, probably because the weak waves which result in such sediments are incompatible with the animals in some way.
Figure 2.25. Polychaete worm burrowing into sediment. The anterior segments act alternately as a terminal anchor while the longitudinal muscles contract and the middle segments are pulled in (left), and as a penetration anchor as the circular muscles are contracted and the segments of the head are pushed further in (right).
After Trueman 1975
The ability to burrow is crucial to beach animals in order to avoid excessive wave action, surface predators, high temperatures and desiccation. Soft-bodied animals such as bivalve molluscs and polychaete worms pump their way through sediments (fig. 2.25).
The beach ecosystems are unusual in that the common plant-herbivore-predator structure of food chains is absent. The only 'plants' available are diatoms and bacteria, and predation on or in the sediment is difficult. Predators are found primarily among the epifauna-birds, certain mudskippers, polychaete worms, and snails. Thus the majority of animals either filter plankton from the seawater (suspension feeders) or suck organic deposits and micro-organisms off the sediment surface or sort out edible particles after ingesting sediment (deposit feeders), although the distinction between these is not always clear. The amount of organic material on or in the sediment is generally greater in the finer sediments for these contain higher concentrations of organic carbon and nitrogen (protein in bacteria) (p. 113), and so deposit feeders flourish here. Suspension feeders are more common lower down the beach because they are covered by water for longer and so able to feed for longer.
All these animals have an affect on the environment within which they live. Burrows increase the depth to which oxygen can penetrate and the digging of them brings lower sediments to the surface; suspension feeders deposit faecal pellets on the sediment surface which become a food source for deposit feeders; the action of deposit feeders can resuspend deposits in the sea water. These suspended particles can, however, clog up the filters of suspension feeders so deposit feeders often predominate in very fine sediments while suspension feeders predominate in coarser sediments (Brafield 1978).
Animals living in finer sediments may, when the tide is out, experience oxygen deficiency. Those with burrows opening into surface pools will have few problems, and nor will those that can utilize atmospheric oxygen. Most, however, have to rely on oxygen in the pore spaces which is at low concentrations at the best of times (p. 113). Some animals reduce their metabolic rate below normal levels at low tide, some create currents of water past their gills or body by waving cilia or fine hairs, whereas others move towards the surface as soon as oxygen levels fall below a threshold. Others are able to withstand low oxygen concentration because of the structure of their respiratory pigments, such as haemoglobin, in their body fluids which have exceptional affinity for oxygen. Some, such as fiddler crabs Uca, have no problems at low tide and are able to respire anaerobically for short periods when the sea is covering the sediment and the crabs are in their burrows.
The sediment on a shallow sloping beach can be extremely soft, comprising about 75% very fine sand with the remainder being even finer particles. The area below the mean low water level of neap tides is covered by every tide of the year and never left exposed for many hours. The fauna here is marine with certain crab species, bivalve and gastropod (snail) molluscs12 such as Telescopium telescopium (fig. 2.26), and two or three species of polychaete worms predominating. A variety of mudskippers13— an unusual group of fish capable of living out of water for a short periods—occur commonly along the water's edge and in burrows in the mud (MacNae 1968).
Underwater, mudskippers breathe just like other fishes, but in air they obtain oxygen by holding water and air in their gill chambers. This water to be renewed about every 5-6 minutes (Burhanuddin 1980). The oxygen they obtain in this way is supplemented by gas exchange through their skin and fins (Stebbins and Kalk 1961). On land, mudskippers move in a variety of ways: they themselves forward on their pectoral fins which move in synchrony with each other (i.e., not true walking) leaving characteristic tracks in the mud, they skip over the mud by flicking their tails, and they climb on vegetation using their pectoral and pelvic fins.
Figure 2.26. Telescopium telescopium.
Although they look very similar, the different species of mudskipper have very different diets; some such as Boleophthalmus boddarti take mud into their mouths, retain algal material, and blow out the rest; some are omnivorous, eating small crustaceans as well as some plant material; and others such as Periophthalmus hoelreuteri are voracious carnivores feeding on crabs, insects, snails and even other mudskippers (Burhanuddin and Martosewojo 1978; Mcintosh 1979; Martosewojo et al. 1982).
Most mudskippers occupy deep burrows and some species have burrows with turretted tops (fig. 2.47). All their activities centre on these burrows which are entered briefly throughout the day to moisten the skin and to replace water in the gill pouches. They are usually evenly spaced on the mud thereby reducing the likelihood of conflicts (Stebbins and Kalk 1961), but fights within and between and species are avoided by complex behavioural mechanisms (MacNae 1968). Males dig the burrows and make courtship displays to nearby females by jumping, and by erecting their dorsal fin in the hope of being chosen as a mate. After pairing, eggs are laid on the sides of the burrow which is then defended by the male. Territorial defence is rare outside the breeding season (Nursall 1981; Hutomo and Naamin 1982).
The other main representatives of the epifauna are fiddler crabs Uca which can be observed by waiting patiently on the beach. Population densities of the often colourful crabs can reach 50 per m2. Male fiddler crabs have one huge claw which is useless for feeding because it cannot reach the mouth, and they therefore have to pick organic deposits off the sediment surface at half the rate of the females which have two normal-sized claws. Presumably the males have to eat for twice as long. The large claw is used for warning away males and for attracting females.
Males compete with other males for females rather than for breeding sites because food resources are so rich and potential sites so abundant. Mating occurs near the burrow defended by the female, but the male-female association is very brief. Preliminary observations suggest that large and small, resident and non-resident males all have the same likelihood of breeding, but smaller males produce fewer viable larvae. Since food resources are not limiting and do not have to be defended, promiscuity is possible (Christy and Salmon 1984).
On coarser-grained beaches near the high water level, burrows about 8 cm across can be found with small piles of sand around them. Their occupants are adult, beige-coloured sand crabs Ocypode (fig. 2.27) which are rarely seen during the day. It is extremely difficult to dig these crabs out of their burrows or to catch them as they run across the sand. Young Ocypode are very numerous, and can be seen on the sand surface both by day and night. Ocypode feed mainly on organic material in the sand, but are sometimes predatory on small crustaceans.
On
