Lebenszyklen von Nudibranchiern aus dem Nordost-Atlantik und ihre Beziehung zur Ökologie von Aufwuchsorganismen

Abstract

1. The most abundant nudibranchs in southern New England are small aeolids and sacoglossans. These gastropods are characteristically sub-annual species with asynchronous growth, continuous egg production and recruitment. Growth of individuals is notably rapid for molluscs, and is probably related to the possession of cerata, which appear to be an adaptation permitting increased rates of assimilation and metabolism. These characteristics represent adaptations to transient food sources which appear early in the microsuccession of fouling communities. Larger species, almost entirely dorids, have slower growth, restricted periods of egg production, synchronous growth cycles, and lower metabolic rates. These feed on more stable, longlived food sources characteristic of later successional stages. 2. The cerata represent convergent adaptations in four major taxa (Sacoglossa, Dendronotacea, Arminacea, and Aeolidacea) permitting an increase in growth and nonhomeostatic respiration. This is accomplished by an increase in both respiratory and digestive surface area. 3. The major component of recruitment of nudibranch populations is due to allochthonously-produced larvae. This may be of great value in repopulation of areas similar to southern New England, where temperature instability may preclude survival of a population for more than a few months. 4. High thermal sensitivity is characteristic of most nudibranchs of the western Atlantic, as indicated by high Q10 values. This sensitivity does not always cause mortality in natural populations, but is related to the rapid changes in population activity, and may represent a further adaptation to prey-species' life cycles. The interaction of high thermal sensitivity and a wide environmental temperature range, however, does limit the zoogeographic ranges and accounts for the low species diversity of the west Atlantic littoral fauna. Thermal sensitivity also explains the scarcity of intertidal species in southern New England, and accounts for the vernal disappearance of a few species. 5. Most species have type 1 (planktotrophic) development, which is of value in dispersal to and exploitation of new fouling growths. Some evidence is given that the proportion of type 1 development is higher in the western Atlantic than in the eastern Atlantic, which may relate to a greater instability of food species. 6. The widely-reported sudden appearances of populations are due to arrival of large numbers of larvae, followed by rapid growth to visible size. A critical temperature may stimulate settling and metamorphosis. 7. Sudden disappearances of adult populations are due to several causes. Small exploitist species normally overgraze food supplies following peak recruitment periods. This leads to destruction of the microhabitat and reduction of absolute population size, with apparent disappearance of individuals. Annual species normally die following periods of egg production, probably due to physiological weakening caused by extreme gonad output.