Settlement and recruitment of benthic marine invertebrates are complex processes, determined by the interaction of biotic and abiotic factors which operate at different temporal and spatial scales. This review analyses the settlement process, attempting to integrate aspects related to different levels of organization (i.e. ecological-physiological-molecular). This is important because many factors that act at any of these levels and at different times can explain by themselves the patterns of settlement andlor recruitment of a large number of species. From an ecological perspective, progress has been made in the identification of causal factors of vanations in larval availability for settlement. Many physical and ethological factors that act during settlement have, however, not received much attention. Likewise, since the great majority of settlement studies have been carried out at restricted spatial scales, fewer works consider different biological and physical factors acting at different scales simultaneously. Settlement patterns are frequently inferred from recruitment. In this sense, a density-independent action of post-settlement mortality has been cons~dered as prerequisite for thls type of inference. This has, however, recently been challenged on the basis that settler-recruit and mortality-settler density relationships change in time. At the physiological-molecular level, different settlement-inducing chemical cues have been identified. Those cues have, however, not yet been characterized to understand better the signal transduction mechanisms involved in larval responses. It is likely that the nervous system is involved. The use of artificial inducers would be useful in studying settlement induction, until more effective natural inducers are isolated and characterized Although few studies have analysed the acquisition of competence, stages of larval development have been related to changes in protein patterns or enzymatic levels of the nervous system. An inopportune exposure of larvae to inducers may delay settlement and may even have a negative impact on growth and subsequent survival of juveniles.

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Settlement of benthic marine invertebrates

Control of larval metamorphosis and recruitment in sympatric agariciid corals

In the laboratory, the swimming larvae of benthic marine invertebrates are generally capable of prolonging their larval period beyond the time that they first become physiologically competent to metamorphose. Larvae seem to differ markedly in their ability to postpone metamorphosis, both interspecifically and intraspecifically. The proportion of this variability that is genetically controlled has not yet been determined; if under genetic control, both pre-competent and competent periods would be subject to selection, although the selective pressures and the physiological or developmental mechanisms through which such pressures might operate remain purely speculative. Limited data strongly suggest that at least a few species delay metamorphosis in the field. The frequency with which they do so, and under what conditions they do so, and for how long, are unexplored questions. Some of these issues can now be explored, with the finding that various inorganic and organic substances can trigger the met...

Delayed metamorphosis by larvae of benthic marine invertebrates: Does it occur? Is there a price to pay?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

On the basis of anatomy and larval behavior, the apical sensory organ (ASO) of gastropod veliger larvae has been implicated as the site of perception of cues for settlement and metamorphosis. Until now, there have been no experimental data to support this hypothesis. In this study, cells in the ASO of veliger larvae of the tropical nudibranch Phestilla sibogae were stained with the styryl vital dye DASPEI and then irradiated with a narrow excitatory light beam on a fluorescence microscope. When its ASO cells were bleached by irradiation for 20 min or longer, an otherwise healthy larva was no longer able to respond to the usual metamorphic cue, a soluble metabolite from a coral prey of the adult nudibranch. The irradiated cells absorbed the dye acridine orange, suggesting that they were dying. When larvae not stained with DASPEI were similarly irradiated, or when stained larvae were irradiated with the light beam focused on other parts of the body, there was no loss of ability to metamorphose. Together these data provide strong support for the hypothesis. Potassium and cesium ions, known to induce metamorphosis in larvae of many marine-invertebrate phyla, continue to induce metamorphosis in larvae that have lost the ability to respond to the coral inducer due to staining and irradiation. These results demonstrate that (1) the ASO-ablated larvae have not lost the ability to metamorphose and (2) the ions do not act only on the metamorphic-signal receptor cells, but at other sites downstream in the metamorphic signal transduction pathway.

The apical sensory organ of a gastropod veliger is a receptor for settlement cues

An increase in the concentration of K@in defined seawater medium induces settlement and metamorphosis in larvae of the marine molluscs Phestilla sibogae, Haliotis rufescens, and Astraea undosa, and in larvae of the marine annelid Phrag matopoma californica. The effect is dose-dependent, optimal at approximately double the normal concentration of K@in seawater, and specific for the K@ion. The ability of K@to directly influence cell membrane potential is proposed as an explanation for its broad effectiveness as a metamorphic inducer for larvae that recruit to different habitats. Depolarization ofexternally accessible, excitable cells thus is suggested to be a mechanism common to the induction ofsettlement and metamorphosis ofa number ofspecies. For Phestilla and Haliotis, the inductive effect ofexcess K@is additive with that of the substratum-derived inducers or analogs. The sensitivity of induction by K@ to external tetraethylammonium (TEA, a K@-channelblocker) reported previously for Haliotis (Baloun and Morse, 1984) is not present in Phestilla or Phragmatopoma. Results presented here indicate that the addition of excess K@may provide a widely useful technique for inducing metamorphosis, and for analyzing the mechanisms which govern this process, in other marine invertebrate larvae.

Douglas A. Markell

Papers

Excess potassium induces larval metamorphosis in four marine invertebrate species