Factors controlling spatial distribution of the sea cucumber Psolus chitonoides: Settling and post-settling behavior

Substratum selection experiments were carried out in situ with larvae of the holothurian Psolus chitonoides Clark in the rocky subtidal of the San Juan Islands, Washington. The experiments indicated, in agreement with laboratory findings, that pentactula larvae settle gregariously, either on the adult, or (when the bivium is obscured by epizooites) by clustering around the margin of the adult sole. Late larvae and early juveniles are strongly photonegative. Following metamorphosis, juveniles migrate from the adults into nearby shaded habitats, where they may take up residence indefinitely. Although adults retain the capacity to move, they relocate only when their living conditions become unsuitable; in most circumstances they are effectively sessile. Small-scale spatial pattern of the adults, which was documented quantitatively on subtidal rock walls, boulders and cobbles, correlates well with microtopographical features. The aggregation observed in the field is primarily due to post-settling migration rather than larval substratum choice at settlement.     

Distribution,abundance and survival of juvenile hermatypic corals (Scleractinia) and the importance of life history strategies in the parent coral community

The distribution and abundance of juvenile corals were examined at depths from 3 to 37 m on the reefs of Curaçao and Bonaire (Netherlands Antilles). Juveniles of Agaricia agaricites were most abundant (60.6%), followed by Helioseris cucullata (8.3%). The large massive corals such as Montastrea annularis, M. cavernosa and branched species such as Madracis mirabilis and Acropora palmata had few juveniles. This, combined with species characteristics, shows that these species employ very different life history strategies. In some species the abundance of juveniles over the reef paralleled that of larger colonies, but not for example in Agaricia agaricites. The composition of the coral community was apparently no direct function of juvenile abundance. A change in angle of settlement of A. agaricites juveniles with increasing depth, from vertical to horizontal surfaces, seems to reflect the preferred light intensity. We studied the survival of juvenile corals during a half-year period. One-third remained unharmed, one-third died or disappeared, and one-third was limited in growth by factors such as spatial competition. This was the same for all depths, but factors influencing survival varied with depth.     

Microhabitat-associated variability in survival and growth of subtidal solitary ascidians during the first 21 days after settlement

Newly settled juvenile ascidians were transplanted into shaded and unshaded sites at two subtidal depths to test the hypothesis that photonegative settlement behavior enhances juvenile survival. Silt, filamentous algae and grazing gastropods (Margarites pupillus) were identified as sources of mortality associated primarily with non-cryptic microhabitats. Silt inflicted heavy mortality on all six species tested. Algal overgrowth was important in shallow water, where it increased mortality and depressed growth rates over a 14-d period. Laboratory experiments demonstrated that gastropods can remove juvenile ascidians from the substratum. The sources of post-settlement mortality investigated may help determine distributional patterns of ascidians in the San Juan Islands, Washington, USA, and may also represent selective pressures that maintain negative phototaxis in the behavioral repertoires of ascidian tadpoles.     

Tidal height,rather than habitat selection for conspecifics,controls settlement in mussels

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.     

Population dynamics of the temperate Australian soft coral Capnella gaboensis

Populations of over 100 colonies of Capnella gaboensis Verseveldt, 1977 at each of four study sites in Sydney Harbour were monitored for recruitment, survival, and mortality from October 1982 to November 1984. Larval settlement and survival were observed in the laboratory. Successful in situ larval settlement of C. gaboensis was estimated to be in the order of 0.26%. The predominant mode of recruitment was from larvae. Asexual recruitment resulted only from the fission of large colonies, and not by fragmentation or by stolon development. Mortality of newly settled C. gaboensis in situ during their first year was estimated to be around 99.75%. For colonies more than 1 yr old, mortality was highest amongst juveniles. After colonies reach about 3 yr of age, they have a high expectation of further life, until they reach about 15 yr of age; life expectancy declines each year thereafter. These estimates of life expectancy are based upon the conservative assumption that the largest colonies are about 20 yr old. The likely causes of mortality for colonies in the field are: predation by urchins, ovulids and nudibranchs; abrasion by kelp; and overgrowth by algae and sponges. The causes of mortality for larvae and recruits in the field, however, can be predicted only on the basis of laboratory studies and observations of field conditions.     

Settlement of larval blacklip abalone, Haliotis rubra, in response to green and red macroalgae

Surfaces from the habitat of adult Haliotis rubra were tested as inducers of larval settlement to determine the cues that larvae may respond to in the field. Settlement was high on the green algal species Ulva australis and Ulva compressa (Chlorophyta), the articulated coralline algae Amphiroa anceps and Corallina officinalis, and encrusting coralline algae (Rhodophyta). Biofilmed abiotic surfaces such as rocks, sand and shells did not induce settlement. Ulvella lens was also included as a control. Treatment of U. australis, A. anceps and C. officinalis with antibiotics to reduce bacterial films on the surface did not reduce the settlement response of H. rubra larvae. Similarly, treatment of these species and encrusting coralline algae with germanium dioxide to reduce diatom growth did not significantly reduce larval settlement. These results suggest that macroalgae, particularly green algal species, may play an important role in the recruitment of H. rubra larvae in the field and can be used to induce larval settlement in hatchery culture.     

Spatial and temporal patterns of reproduction,larval settlement,and recruitment of the compound ascidian Aplidium stellatum

Spatial and temporal variation in recruitment of the compound ascidian Aplidium stellatum was examined on a shallow-water limestone outcropping in the Northeastern Gulf of Mexico from 1983 to 1985. Fifty-two percent of the recruits appeared on vertical surfaces, which were rare at this site. In the laboratory, tadpole larvae of A. stellatum consistently settled on vertical vs horizontal surfaces in a 2 to 1 ratio, regardless of the area of vertical surface offered. This settlement response was insufficient to account entirely for the field recruitment pattern, suggesting greater mortality of newly-settled individuals or larvae on horizontal vs vertical surfaces. Fifty-six percent of the variation in recruitment over 21 months could be explained by variation in the percentage of zooids brooding larvae in adult colonies. Over 28 consecutive months, the percentage of zooids brooding larvae was positively correlated with egg counts of the previous month (r ²=0.75), which in turn were weakly correlated with monthly average water temperature (r ²=0.36). These results suggest that larval production contributed substantially to temporal variation in recruitment of A. stellatum. The short larval life of tadpoles of A. stellatum and the relative isolation of the population were probably responsible for the correlation between recruitment and larval production.     

Temporal and spatial settlement patterns of sympatric hermit crabs and the influence of shell resource availability

Larvae of marine organisms often need specific resources or environments at settlement, and their success at settlement might be strongly influenced by the abundance and distribution of such specific resources. The larvae of hermit crabs need small shells to settle, so it is thought that the distribution and abundance of small shells influence the settlement pattern of hermit crabs. To investigate the influence of small shell distribution on the settlement of pagurid hermit crab larvae, we conducted a field experiment at an intertidal rocky shore in Hakodate Bay, Japan. From the line-transect sampling in the field, we found that Pagurus middendorffii settled extensively in the offshore side of the intertidal zone while P. nigrofascia settled in the uppermost area of the intertidal zone. Small shells were most abundant in a narrow shallow trough, slightly offshore from the uppermost area of the intertidal zone. For both species, settler abundance was high where adults were abundant, but settler abundance did not appear to be related to shells abundance. An experiment to clarify settlement patterns showed that larval recruits tended to be similar to those in the line-transect sampling of settlers. Thus shells may not be a primary factor affecting settlement patterns at relatively large scale within the intertidal flat. However, when we analyzed the relationship of settlers and shells separately within each transect, the distribution of settlers was well explained by shell resource availability. Therefore on a smaller scale, shell availability may influence the number of settlers. Settlement periods of P. middendorffii and P. nigrofascia fully overlapped, so their larvae probably were affected by similar transport factors, such as current and tidal movement. Nevertheless they showed different spatial patterns of settlement.Communicated by T. Ikeda, Hakodate     

Inhibitory effects of red algal extracts on larval settlement of the barnacle Balanus improvisus

We examined the chemical antifouling properties of four sublittoral red algae, Chondrus crispus, Delesseria sanguinea, Osmundea ramosissima, and Polyides rotundus, which are all rarely fouled in the field. Two different approaches were used. Firstly, we tested the effects of lipophilic crude extracts on the settlement behaviour of cyprid larvae of the co-existing barnacle Balanus improvisus. Secondly, in a settlement preference experiment, we tested whether B. improvisus cyprid larvae settle on living algae when given a choice between natural algal surfaces and control surfaces. With this procedure, we were able to test both if the algae inhibit recruitment of cyprids, and if this inhibition is a result of chemistry. The settlement of B. improvisus larvae was strongly inhibited at concentrations estimated to be potentially ecologically relevant for all of the tested extracts. However, only C. crispus significantly inhibited settlement in the preference experiment, even though there was also a tendency for settlement inhibition on P. rotundus and O. ramosissima. In contrast, D. sanguinea seemed to stimulate settlement. This contradiction probably resulted from an extraction of metabolites that naturally occur only inside the alga. However, as this study shows, a combination of settlement assays with whole-cell extracts and preference tests of ecologically relevant fouling organisms on natural algal and control surfaces may be a useful procedure to avoid erroneous conclusions regarding natural antifouling roles of compounds based on settlement assays with only whole-cell extracts. Furthermore, this study also shows that production of inhibitory metabolites may explain the low degree of fouling, especially by B. improvisus, on C. crispus.Communicated by L. Hagerman, Helsingør     

Responses of cirripede larvae to light. I. Experiments with white light

The phototactic response of the nauplius larva of Balanus balanoides, B. crenatus and Elminius modestus shows darkadaptation; the response of the cyprid of B. balanoides shows both phototaxis and low photokinesis. The phototactic responses and the orientation of the cyprid to white light at settlement require an intensity of illumination slightly above 10^-5 lux. The ability to select a shaded position by cyprids of R. balanoides requires a higher intensity of 10^-2 to 10^-4 lux; hence a different mechanism may be involved. Barnacle larvae are sufficiently sensitive to be able to respond to light beneath the sea surface, even on cloudy, moonless nights.     

The relationship between allele frequency and tidal height in a mussel hybrid zone: a test of the differential settlement hypothesis

The blue mussels Mytilus edulis L. and M. galloprovincialis Lmk. hybridize in western Europe. Within hybrid populations nuclear alleles specific to M. galloprovincialis increase in frequency with age and size. This relationship changes with tidal height; alleles from M. galloprovincialis occur more frequently high in the intertidal zone, while M. edulis alleles predominate in the low intertidal zone. We tested the hypotheses that larvae with M. galloprovincialis alleles tend to settle higher in the intertidal zone, or that mussels redistribute themselves with respect to tidal height after initial larval settlement. We sampled recently metamorphosed mussels every 2 weeks in a hybrid mussel population at Whitsand Bay in southwest England throughout the summer of 1996. We observed four cohorts of newly settled mussels. There was no evidence of differential settlement of mussels with different genotypes in connection with tidal height, or into shaded versus unshaded microsites. Therefore, we rejected the preferential settlement hypothesis. There was substantial movement of juvenile mussels in the first 4 weeks following initial settlement, but this “secondary settlement” did not result in genetic differentiation with respect to tidal height. Further, significant differences in allele frequencies were found between primary and secondary spat. This allele frequency change was in the opposite direction of that seen in the adult population, suggesting newly settled larvae may be experiencing different selective pressures than adults. We propose that the genetic structure of hybrid mussel populations with respect to tidal height is the consequence of differences in selection intensity. Received: 30 April 1999 / Accepted: 5 May 2000     

Larval period and its influence on post-larval life history: comparison of lecithotrophy and facultative planktotrophy in the aeolid nudibranch Phestilla sibogae

The variable duration of the pelagic phase of metamorphically competent larvae of benthic marine invertebrates is set by an interaction between environmental factors and larval traits that together influence the chance that a larva will encounter and respond to a suitable settlement site. In the Hawaiian aeolid nudibranch Phestilla sibogae Bergh, an extended competent larval phase resulted in a cascade of negative effects on larval and post-larval life-history traits. When raised as fed (i.e., facultatively planktotrophic) larvae, an extended larval period resulted in lower larval survival, slightly lower metamorphic success, and delayed reproduction. When raised as unfed (i.e., lecithotrophic) larvae, an extended larval period resulted in lower larval and post-larval weights, survival, metamorphic success, and reproductive output, and also resulted in a longer juvenile period and delayed reproduction. The chance nature of locating a settlement site generally spreads these negative effects over all larvae of a cohort, and so balances the relative fitness of the genetic lineages within a population.     

Larval depletion by ascidians has little effect on settlement of epifauna

Ascidian densities were manipulated while controlling for negative effects of biodeposition and space preemption to examine the effects of ascidian filter-feeding on larval recruitment in St. Joseph Bay and near Turkey Point, Florida (Northern Gulf of Mexico, Florida, USA). Using three different experimental designs during 1984 and 1985, recruitment near living Styela plicata or Molgula occidentalis was as high as recruitment near ascidian models. Disruption of flow by ascidian bodies had little effect on settlement. Predation rates by ascidians on larvae in six phyla were high in laboratory experiments. The field effects of larval depletion by solitary ascidians are apparently obscured by other factors influencing the abundance of recruiting larvae. Consumption of larvae in the laboratory cannot be used to assume significant inhibitory effects in the field.     

Long-term and trans-life-cycle effects of exposure to ocean acidification in the green sea urchin Strongylocentrotus droebachiensis

Anthropogenic CO₂ emissions are acidifying the world’s oceans. A growing body of evidence demonstrates that ocean acidification can impact survival, growth, development and physiology of marine invertebrates. Here, we tested the impact of long-term (up to 16 months) and trans-life-cycle (adult, embryo/larvae and juvenile) exposure to elevated pCO₂ (1,200 μatm, compared to control 400 μatm) on the green sea urchin Strongylocentrotus droebachiensis. Female fecundity was decreased 4.5-fold when acclimated to elevated pCO₂ for 4 months during reproductive conditioning, while no difference was observed in females acclimated for 16 months. Moreover, adult pre-exposure for 4 months to elevated pCO₂ had a direct negative impact on subsequent larval settlement success. Five to nine times fewer offspring reached the juvenile stage in cultures using gametes collected from adults previously acclimated to high pCO₂ for 4 months. However, no difference in larval survival was observed when adults were pre-exposed for 16 months to elevated pCO₂. pCO₂ had no direct negative impact on juvenile survival except when both larvae and juveniles were raised in elevated pCO₂. These negative effects on settlement success and juvenile survival can be attributed to carry-over effects from adults to larvae and from larvae to juveniles. Our results support the contention that adult sea urchins can acclimate to moderately elevated pCO₂ in a matter of a few months and that carry-over effects can exacerbate the negative impact of ocean acidification on larvae and juveniles.     

Développement larvaire de deux Tellinacea,Scrobicularia plana (Semelidae) et Donax vittatus (Donacidae)

Scrobicularia plana Da Costa and Donax vittatus L. were reared in the laboratory through settlement. Fertilizable eggs were obtained by perfusing the ovary with 5% 0.1 M ammonium hydroxyde in sea water. S. plana eggs have a thick chorion, inside which the early larval stages develop; they hatch as straight-hinge larvae more than 60 h after fertilization. This brood protection is considered to be an adaptation to osmotic pressure changes and pollution in the environment. D. vittatus eggs have a very thin chorion and are unprotected. Further development is planktotrophic and very similar for the two species. Under laboratory conditions, the pediveliger stage is attained 3 weeks after fertilization and settlement occurs 1 week thereafter. S. plana spat stop growing until a suitable substratum is available. Meanwhile they undergo a byssus drifting important for postlarval dispersion. Post larvae resume growth as soon as a small quantity of fine sand is added to the rearing jar. The exhalant siphon is developed first, when the post larvae reach a length of 600 m; the inhalant siphon is formed later, at a length of approximately 900 m. Evolution from the larval hinge to the juvenile hinge stage occurs sooner in S. plana than in D. vittatus. Comparison of laboratory larval development with field development indicates that spawning occurs in June and August for S. plana in North Wales (UK).     

Temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid corals

Acroporid corals are the main reef-building corals that provide three-dimensional habitats for other reef organisms, but are decreasing on many reefs worldwide due to natural and anthropogenic disturbances. In this study, temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid coral species, Acropora muricata and A. valida, were examined through laboratory rearing experiments to better understand the potential for larval dispersal of this important coral group. Many larvae were attached (but not metamorphosed) to settlement tiles on the first examination 3–4 days after spawning (AS). The first permanent larval settlement (i.e. metamorphosed and permanently settled juvenile polyps) occurred at 5–6 days AS, and most larval settlement (85–97% of total) occurred within 9–10 days AS. Larval survivorship decreased substantially to around 50% by the first week of the experiment and to approximately 10% by the second to third week. The rates of larval attachment, settlement, and the initial drop in survivorship of larvae suggest that effective dispersal of some acroporid species may largely be completed within the first few weeks AS.     

Dependence of settlement rate on suitable substrate area

Several recent field studies have found disproportionately high barnacle settlement rates (expressed on a per-area basis) in situations where the amount of suitable substrate is reduced, either due to occupation by other individuals or to physical processes. We call this phenomenon the intensification effect; it is not included in many models of benthic populations, which assume that the per-area settlement rate is a constant, or in field larval-collector studies, where number of larvae caught is assumed to be a function only of larval supply. In this paper we derive a simple Markov chain model that generates the intensification effect. It describes the fate of a settling larva, which may be washed out of the system or may attempt to settle in suitable or unsuitable substrate. If it lands on unsuitable substrate, it returns to the water column to try again. At low values of the washout rate, the per-area settlement rate decreases with increasing substrate area. At high values of the washout rate, per-area settlement rate is constant. We conducted a set of laboratory experiments in March through April 1995 with barnacle larvae (Semibalanus balanoides Linnaeus) to illustrate the predictions of the model. Substrate area was manipulated by varying the number of settling panels available, and the larval loss rate was adjusted by manipulating the residence time of larvae in the experimental units (1.5 h or 12 h). As predicted by the model, in the 12-h treatment settlement per area decreased nonlinearly as the amount of substrate increased, whereas in the 1.5-h treatment no differences were found. These results explain and predict the intensification effect, and suggest that variability in the proportion of suitable substrate may be an important factor in determining variability in settlement rate. Received: 27 May 1997 / Accepted: 10 June 1997     

Role of bacteria in larval settlement and metamorphosis of the polychaete Hydroides elegans

The serpulid polychaete Hydroides elegans Haswell, 1883 is an early colonist of new substrata in Pearl Harbor, Oahu, Hawaii. When metamorphically competent, larvae of H. elegans will settle rapidly upon an acceptably biofilmed surface, but not on a clean surface. In this study we found the ability of larvae to respond selectively to inductive surfaces to be retained for at least 3 wk. Of a series of bacterial strains isolated from Hawaiian marine biofilms, 13 induced larval settlement, 11 gave moderate or mixed results, and 10 others did not stimulate the settlement of H. elegans. The amount of settlement induced by monospecific strains was rarely as great as with natural, multispecies films. Most of the isolated bacteria were motile Gram-negative rods, but Gram-positive strains were also present, and diverse metabolic types were represented in the study. Biofilms killed by treatment with heat, ultraviolet radiation or chemical fixatives were no longer inductive. Soluble, dialyzable, heat-stable bacterial products induced settlement and metamorphosis more slowly. The range of bacteria producing an inductive signal suggests either that there are multiple cues, or that the cue is common to many bacteria. Chemical signals characteristic of early microbial biofilms may indicate freshly available substrata with optimal potential for the growth and survival of H. elegans. Received: 30 January 1998 / Accepted: 12 September 1998     

Larval settlement and metamorphosis of the mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> in response to biofilms

Biofilms were allowed to develop on glass slips immersed 1.0–1.5 m below the sea surface in Tachibana Bay, Nagasaki, Japan, for different periods of time from November 2003 to January 2005. The effects of age, immersion month, dry weight, bacterial and diatom densities of these biofilms on the settlement and metamorphosis of pediveliger larvae of the mussel Mytilus galloprovincialis were investigated in the laboratory. Furthermore, biofilms were subjected to various treatments to investigate the nature of the settlement and metamorphosis cue in the biofilm. Pediveliger larvae of the mussel settled and metamorphosed in response to biofilms. Settlement and metamorphosis to the post-larval stage significantly increased with the biofilm age. In addition, the biofilm activity varied depending on the immersion month (season), e.g., for biofilms with the same age, those immersed between June and August had higher activities than those immersed between November and March. The activity of the biofilm also positively correlated with the dry weight, bacterial and diatom densities. These three quantitative parameters of the biofilm were significantly affected by the film age but were not affected by the immersion month, suggesting that other parameters (e.g., community structures, extracellular products) also affected the inductive activity of the biofilm. The fixative agents (formalin and glutaraldehyde), heat, ethanol, ultraviolet irradiation and antibiotics treatments of the biofilm resulted in significant reduction or loss of its inductive activity. The survival of bacterial cells in the treated films where activities were either reduced or lost also decreased significantly. No settlement and metamorphosis were obtained when larvae were exposed to the conditioned water of the biofilm. Thus, larvae of M. galloprovincialis settled and metamorphosed in response to a cue produced by living bacteria in the biofilm. The cue may be a bacterial extracellular product which was susceptible to the above treatments.     

Horizontal,vertical and seasonal distributions of epiphytic hydrozoa on the algaHalimeda tuna in the Northwestern Mediterranean Sea

The thalli ofHalimeda tuna (Ellis & Solander) grow horizontally from rocky surfaces and present an upper and lower surface which can be colonized by epiphytes. Thalli ofH. tuna were collected twice a month from March 1985 to March 1986 at Tossa de Mar (northeast Spain). Thirty five species of hydroid were recorded living onH. tuna thalli; of these, the 15 most abundant species were classified into three groups on the basis of their horizontal zonation on the thalli: those most abundant on the proximal, medial or distal articles. Those hydroids common on the proximal (oldest) articles were relatively large and were present throughout the year, those common on the medial articles were smaller and often occurred in dense monospecific patches, while those common on the distal (youngest) articles were present for only short periods and were the most opportunistic hydroids found onH. tuna. We hypothesize that this zonation reflects interspecific niche-selection, enabling successful competition for space with other hydroids, algae and bryozonas. Three hydroid species were more abundant on the upper surfaces of the algae than on the lower, probably because of gravitationally enhanced settlement of planula larvae coupled with (unusually) limited hydrorhizal growth on the underside. Three opportunistic species were more common on the lower surfaces of the thalli, possibly reflecting the greater availability of substratum in the shade, where epiphytic algae were less abundant. Many hydroids declined in abundance in summer because of the death of old thalli, the growth of new thalli and of apical articles on existing thalli, and possibly because of interspecific competition with epiphytic algae. The death of old thalli and the growth of new thalli reduced the abundance of epiphytic algae; this may have facilitated the increase in abundance of hydroids recorded in the autumn.