The settlement behaviour of the larvae of Sabellaria alveolata (L.)
Wilson, D.P. (1968) The settlement behaviour of the larvae of Sabellaria alveolata (L.). Journal of the Marine Biological Association of the United Kingdom, 48 (2). pp. 387-435.
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Using larvae reared from artificial fertilizations in the laboratory, often for several months, many experiments have been made on factors promoting settlement of Sabellaria alveolata (L.). When at last able to metamorphose after many weeks of pelagic development, the larvae of this species crawl actively over any solid surfaces with which they happen to make contact, seeking indicative characters distinctive of their normal adult environment. If these are not found at once the larvae are able to postpone metamorphosis for weeks if necessary, remaining in a developmental state able both to swim and to crawl. If delayed too long in reaching a favourable environment some may metamorphose in the absence of normal environmental stimuli, others often become incapable of metamorphosing normally and do so abnormally, or they may die without metamorphosing. Physical factors encouraging settlement include clean stable substrates washed over by water carrying sand grains in suspension. There is some slight preference for shallow cracks or corners, but little preference for rough over smooth surfaces. Bacterial slime films on a surface have little influence on settlement, though larvae may crawl slightly more freely over glass so coated than over clean glass. Purely physical factors have only minor influences on settlement. The strongest stimulus to metamorphose and settle is received by the larva when it makes accidental contact with adult tubes of its own species or with their remains, or with tubes of recently settled young. These latter tubes may be merely primary mucoid tubes secreted by larvae at metamorphosis, or may be first sandy tubes. Many experiments showed that the cement secreted by young or adult worms and used in the construction of their tubes is quickly recognized by larvae making contact with it, even when the cement is present only on loose sand grains from disintegrated tubes. Contact with the cement is essential, for there is no evidence that it is detected from a distance. The metamorphosis-inducing substance of the cement of sandy tubes and of the secreted walls of primary mucoid tubes is insoluble in water and unaffected by drying. In the walls of primary mucoid tubes it is destroyed by cold conc. HCl without destroying the whole of the tube wall. Because it is destroyed by cold acid the substance cannot be identical with that of acorn-barnacle bases to which cyprids respond, though it is probably perceived in much the same way, by a 'tactile chemical sense' detecting a specific molecular configuration of a surface, the surface of the cement. The settlement reactions of Sabellaria larvae indeed parallel closely those of barnacle cyprids, with some differences corresponding with differences of adult habit. Settling larvae of S. alveolata are markedly gregarious, tending to metamorphose on top of, or alongside in contact with, the tubes of previously settled worms. On the sea shore this habit initiates the formation of massive colonies produced by the fusion of very many individual tubes. Larvae of S. alveolata hardly if at all discriminate between tubes of their own species and those of S. spinulosa. Wide variation in the rate of development of larvae growing under identical, or near identical, conditions, as shown in the laboratory cultures, could in nature be of survival value to a species with long pelagic life and restricted adult habitat. It would increase the chances of a proportion of the larvae of each year's spawning eventually reaching the normal adult habitat during a developmental stage able to settle and metamorphose.
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