Reproductive adaptations

The difficulties of survival on the shore have their effects on all phases of life, including reproductive processes and larval and juvenile stages. The majority of benthic organisms start life as floating or swimming forms in the plankton, and may become widely dispersed in the water before they settle on the sea bottom. Shore creatures face special risks of great losses of pelagic eggs and larvae during this phase if they drift far from the shore and settle outside the zone in which survival is possible. For some inhabitants of the shore the chances of successful settlement in suitable areas are enhanced by certain aspects of the behaviour of their larvae. Some produce larvae which are at first strongly attracted by light, and presumably rise close to the sea surface during the day. Wind direction is often landward during the day time, driving the surface water towards the coast, and so it is likely that positive phototaxis improves the chances of pelagic larvae returning to the shore. At night, offshore winds tend to predominate, and surface water is moved away from the shore with replacement water coming in shorewards along the bottom. Under these circumstances, descent of larvae to deeper levels away from the surface must then have a similar effect of keeping them concentrated along the shoreline.

It has been mentioned earlier that the larvae of many benthic species can discriminate between substrates and can for a time delay settlement until favourable conditions are encountered (see Section 6.2.2) (Rainbow, 1984). Some tend to settle gregariously, often in response to the presence of successfully metamorphosed members of the species, for example barnacles, the reef-building worm Sabellaria, serpulid worms, mussels and some bryozoa. Settlement in shallow water may also be favoured by larval response to wave action; for example, cyprids of Semibalanus balanoides are reported to settle more readily under fluctuating water pressure. When settlement occurs sublittorally, or at lower levels of the shore than are occupied by the adults, for example Melaraphe neritoides (see Section 8.7.5), the responses of the juveniles to various environmental stimuli may cause migration upshore towards the appropriate zone.

ln many shore animals the planktonic phase is abbreviated or omitted, and this simplifies the problem of finding the correct shore level. For instance, the lugworm Arenicola marina, one of the most successful littoral worms burrowing in muddy sand, has only a brief pelagic period. In late autumn or early winter the gametes are shed from the burrow onto the surface of the sand during low spring tides, and here fertilization occurs. The fertilized eggs may be dispersed upshore to a limited extent by the rising tide, but being heavier than water and slightly sticky, they tend to adhere to the surface of the sand. They hatch after about 4-5 days and the larvae, although capable of swimming, seem from the outset to burrow into the deposit wherever the substrate is suitable.

Direct development

Other shore animals completely eliminate pelagic stages by developing directly from egg to miniature adult form. These eggs are well charged with yolk to enable the young to hatch in an advanced state. Littorina obtusata and L. mariae deposit eggs in gelatinous masses on the surface of seaweed. The young occasionally emerge as advanced veliger larvae, but probably more often do not hatch until after the velum has been resorbed and then appear as tiny crawling winkles. The dogwhelk, Nucella lapillus, lays vase-shaped egg capsules which often occur in large numbers stuck to the underside of stones or sheltered rock surfaces. Each capsule contains several hundred eggs, but eventually only a dozen or so whelks crawl out of each capsule, the rest of the eggs serving as food for the first few to hatch.

Parental protection

Eggs laid on the seashore are exposed to all the vicissitudes of this environment, and certain shore creatures contrive in various ways to give their eggs some protection. For example, several species of inshore fish guard their eggs, such as the butterfish (Pholis gunnellus) and the shanny (Lipophrys pholis). They lay sticky masses of large eggs from which advanced young are born, and one of the parent fish, usually the male, remains close to the eggs until they hatch, courageously protecting them against marauders. Female pipefish lay their eggs in a pouch on the male's belly. Here they remain until they hatch as miniature adults. Amphipods and isopods, often very numerous on the shore, also retain their eggs within a brood-pouch from which fully formed young emerge. In Littorina saxatilis, L. rudis and L. neglecta the eggs remain in the mantle cavity until they hatch as minute winkles. The tiny bivalve Lasaea rubra, often abundant in rock crevices and empty barnacle shells, incubates its eggs and young within the gills until they are sufficiently developed to crawl out and maintain themselves near the parent. The viviparous blenny, Zoarces viviparus, gives birth to well-developed young about 4 cm in length.

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