Effect of excretory-secretory products of some fouling organisms on settling and metamorphosis of the larvae of <Emphasis Type="Italic">Styela rustica</Emphasis> (Ascidiae)

The effect of the excretory-secretory products of some fouling animals on the settling and metamorphosis of larvae of the solitary ascidian Styela rustica was assessed. The substances secreted by the sponge Halichondria panicea stimulated settling of larvae, but concurrently blocked their metamorphosis. The excretory-secretory products of the mussel Mytilus edulis and the ascidian Molgula citrine did not affect settling of the S. rustica larvae but impeded their subsequent development. Water conditioned by the bivalve Hiatella arctica, stimulated settling and, apparently, metamorphosis of the larvae of S. rustica. The chemical substances produced by adult individuals of S. rustica facilitated settling of conspecific larvae but slightly delayed their metamorphosis.     

Observations on the settling of Simulium damnosum larvae on artificial substrates,in the Ivory Coast

The settling rates of the larvae of the blackfly Simulium damnosum Theobald, were monitored at a Simulium breeding site on the Bandama River in the Ivory Coast. Significantly greater numbers of larvae were found to settle during the night than during the day, and there was a significant increase in the settling rate immediately following the onset of darkness.The drifting larval population contained significantly greater numbers of smaller larvae than the general population.     

Development of the sand dollar <Emphasis Type="Italic">Scaphechinus mirabilis</Emphasis>

Under laboratory conditions, the development of larvae of the sand dollar Scaphechinus mirabilis (Agassiz) took 28.5–29 days from fertilization to settling and the end of metamorphosis at a temperature 20°C and salinity 32.2–32.6‰ The cleavage divisions were completed in 12 hours after fertilization (AF) by the release of free swimming ciliary blastula from the egg membrane. The larvae attained pluteus I stage with one pair of arms at an age of 40 hours. In 4.5–5 days the pluteus II stage with three pairs of arms, and in 9 days the pluteus III stage with four pairs of arms were formed. On the 20–21st day AF the larvae developed a vestibule, in which the adult skeleton rudiments, spicules, plates and pedicellariae were formed on the 26–27th day AF. The size of the larvae at an age of 22.5 days was 1064.3 ± 44.7 μm. The settling of larvae was recorded on the 28–29th day of development. Most of the larvae completed their metamorphosis in 4.5–5 hours after settling.     

Synchronous spawning in a recently established population of the zebra mussel,Dreissena polymorpha,in western Lake Erie,USA

Seasonal patterns of reproduction in a newly established population of Dreissena polymorpha are described for a site in the western basin of Lake Erie. Reproduction was monitored by histological examination of gonads, analysis of shell length-dry weight relationships, and following abundance of planktonic and settling larvae. Patterns of planktonic larval abundance and settling showed a distinct bimodal pattern in July and August, but conflicted with histological and length-dry weight data that showed spawning was a brief, highly synchronous event occurring in late August. Differences between histological data and abundance of larvae in the plankton can be explained as resulting from drift of larvae from disjunct populations of D. polymorpha which spawned earlier, into the study area. Veliger larvae were present in the plankton at low densities throughout the warm months before and after periods of peak abundances. The presence of these larvae can be explained by a combination of asynchronous spawning among local populations and postponed settlement by planktonic larvae.     

Settlement behavior of Chthamalus anisopoma larvae largely determines the adult distribution

Summary In the northern Gulf of California the adult distribution of the intertidal barnacle species, Chthamalus anisopoma, on exposed shores is approximately between 0.0 and 2.0 m above mean low water (MLW). The species is typically absent in protected (from wave splash) areas. In this study, I examined a series of alternative hypotheses relating to the factors that could be responsible for limiting the distribution. Post-settlement factors appear to be unimportant because settlement was largely restricted to areas within the adult distribution. Two processes could account for the high correlation between settlement and adult distributions. First, hydrodynamic factors could restrict deposition of larvae to sites that coincidently were in areas in which individuals could survive to maturity. Second, larvae may choose to settle only on sites where they can survive to maturity. Of the two, the later was supported as settlement could be induced on surfaces outside the adult distribution using transplanted adult conspecifics as cues. Thus, competent larvae were present outside the adult distribution of Chthamalus zone but did not settle under normal conditions. Also, there was no evidence that pre-emption of space by other sessile species, by itself, restricted the distribution of Chthamalus. Settlement within the existing adult distribution may be an evolutionary response to increased mortality for individuals settling outside the adult distribution compared to those settling within it.     

Temporal settlement patterns of larvae of the broadcast spawning reef coral Favites chinensis and the broadcast spawning and brooding reef coral Goniastrea aspera from Okinawa, Japan

The faviid corals, Favites chinensis and Goniastrea aspera are widely distributed in the Indo-Pacific region. Both corals are hermaphroditic broadcast spawners, but G. aspera is also known to brood planula larvae in Okinawa. This study investigated the temporal settlement patterns of planula larvae of the scleractinian corals F. chinensis and G. aspera that developed from spawned gametes, and planula release and settlement of brooded larvae of G. aspera from Okinawa, Japan. Some of the broadcast-spawned larvae of F. chinensis and G. aspera had very short pre-competency periods of 1–2 and 2–3 days after spawning, and relatively long maximum settlement-competency periods of 56–63 and 63–70 days after spawning, respectively. These pre-competency periods are among the shortest reported for larvae of broadcast spawning coral species, and appear to be negatively correlated with seawater temperature. F. chinensis larvae tended to settle rapidly with 34–39% of larvae settling in the first week after spawning, while broadcast-spawned G. aspera larvae had a slower settlement pattern with 11–15% of larvae settling in the first week after spawning. Brooded larvae of G. aspera settled more rapidly, with settlement rates of 27–31% within the first 24 h and 45–65% within the first week after the start of the experiment. The production of planula larvae with rapid settlement capabilities may enable F. chinensis and G. aspera to establish and maintain populations in shallow reef sites at Okinawa. The release of the brooded planulae for up to 2 months may explain why G. aspera is locally more dominant on shallow reefs in Okinawa than F. chinensis. On a broader scale, the longer settlement competency periods of some of the broadcast-spawned larvae of these species increase their potential for longer-distance dispersal and may partly explain the wide biogeographic distribution of these species in the Indo-Pacific region.     

Adaptive responses of embryos and larvae of the heart-shaped sea urchin <Emphasis Type="Italic">Echinocardium cordatum</Emphasis> to temperature and salinity changes

The combined effects of temperature (8, 12, 14, 17, 20, 22 and 25°C) and a salinity decrease from 36 to 12‰ on the development of the sea urchin Echinocardium cordatum (Pennant) were studied. Embryonic development proved to be the process most vulnerable to a salinity decrease. It was completed successfully at 8–20°C within a narrow salinity range of 36–28‰ Larvae at the most resistant stage, the blastula, survived at 12–22°C and a salinity of 36–18‰. Larvae at the most sensitive stage, pluteus I with the first pair of arms, died even in a favorable environment, a temperature of 17–20°C and a salinity of 34–28‰. That may be related to qualitative alterations during skeleton formation and to transition to phytoplankton feeding. The resistance of larvae to variations in environmental factors gradually increased in the pluteus II and III stages; however, it significantly decreased before the settling of the larvae. Larvae that were 37 days old survived at a temperature of 14–20°C within a salinity range of 36–22‰ and at 22 and 25°C, they survived at a salinity of 36–24‰; however, all the larvae became abnormal at 25°C. The larvae settled earlier on sand inhabited by adult individuals of E. cordatum than on sand from other locations, and they settled faster at 20–25°C, than at 14 and 17°C. The juveniles, if lacking an opportunity to burrow in the sand, died within 14 days after settling.     

In situ estimation of ephyrae liberated from polyps of Aurelia aurita using settling plates in Tokyo Bay, Japan

The continuous changes in the number of newly established polyps of Aurelia aurita (L.) on settling plates under natural conditions were observed from August 1998 to September 1999 in Tokyo Bay, Japan. A sharp decline in survivorship of newly settled polyps was observed within the first few days, however, survivorship of polyps settled in October increased by budding up to 399% after two months. The number of discs in each strobila varied from 1 to 6, however, most of the strobilae formed single discs. The percentage ratios of the total number of ephyrae to the initial number of polyps on settling plates were generally lower than 10%, but the highest ratio of 594.4% was estimated for the polyps settled in October. It is considered that most of the liberated ephyrae originate from the polyps settled in October in Tokyo Bay. This study suggests that the occurrence of ripe medusae with planula larvae throughout the year contributes to the success of settlement and growth of the polyp stage in Tokyo Bay.     

Characteristics of Settling Coral Reef Fish Are Related to Recruitment Timing and Success

Many marine populations exhibit high variability in the recruitment of young into the population. While environmental cycles and oceanography explain some patterns of replenishment, the role of other growth-related processes in influencing settlement and recruitment is less clear. Examination of a 65-mo. time series of recruitment of a common coral reef fish, Stegastes partitus, to the reefs of the upper Florida Keys revealed that during peak recruitment months, settlement stage larvae arriving during dark lunar phases grew faster as larvae and were larger at settlement compared to those settling during the light lunar phases. However, the strength and direction of early trait-mediated selective mortality also varied by settlement lunar phase such that the early life history traits of 2–4 week old recruit survivors that settled across the lunar cycle converged to more similar values. Similarly, within peak settlement periods, early life history traits of settling larvae and selective mortality of recruits varied by the magnitude of the settlement event: larvae settling in larger events had longer PLDs and consequently were larger at settlement than those settling in smaller pulses. Traits also varied by recruitment habitat: recruits surviving in live coral habitat (vs rubble) or areas with higher densities of adult conspecifics were those that were larger at settlement. Reef habitats, especially those with high densities of territorial conspecifics, are more challenging habitats for young fish to occupy and small settlers (due to lower larval growth and/or shorter PLDs) to these habitats have a lower chance of survival than they do in rubble habitats. Settling reef fish are not all equal and the time and location of settlement influences the likelihood that individuals will survive to contribute to the population.     

Larval settlement of the bivalvesMya arenaria andM. uzenensis on scallop collectors in Vostok Bay, Sea of Japan

This paper discusses data on the horizontal and vertical distribution of the spat ofMya arenaria andM. uzenensis on collectors for the cultivation of the scallopMizuhopecten yessoensis in Vostok Bay. The maximum settlement of the mollusks on scallop collectors occurred in the innermost part of the bay. The spat density ofM. arenaria on collectors was 2.6 times that ofM. uzenensis. M. arenaria larvae preferred settling on the upper portions of collectors, while the distribution ofM. uzenensis was random. The time of collector placement in the sea influenced spat numbers.     

The importance of flow and settlement cues to larvae of the abalone, Haliotis rufescens Swainson

In flow tank experiments, I tested the relative importance of active and passive processes to larvae settling on manufactured casts that were hydrodynamically rough at a small scale (mm to <1 cm). I predefined two distinct regions of small-scale flow that I used to manipulate larval settlement behaviour of the red abalone Haliotis rufescens Swainson. The larvae show a stringent settlement response associated with coralline red algae. Haliotis rufescens larvae settled preferentially to an inducer regardless of the flow conditions, as expected. However, the ability of H. rufescens larvae to show this stringent behaviour was altered by changing the small-scale flow. When the free-stream velocity was low, the larvae responded to a settlement cue regardless of the small-scale hydrodynamics. When free-stream velocity was higher, the larvae acted increasingly as passive particles in their deposition, but settled only in response to an inducer. The results were consistent in two flow tanks, across 2 years and between different batches of larvae.     

Xenopus laevis tadpole limb regeneration in vivo and in vitro: thyroxine directly promotes blastemal cell proliferation and morphogenesis

Regeneration in hindlimbs of Xenopus laevis larvae which were amputated at stage 53 and 55 through the tarsalia region is promoted by thyroxine (T4), while propyl-thiouracil (PTU) inhibits regeneration when compared to controls. In this paper, by in vivo and in vitro experiments, we demonstrate that the promoting effect of T4 on the regenerative processes of larval X. laevis hindlimbs is a direct effect of this hormone on the blastemal cells. By contrast, the inhibitory effect of PTU on the regenerative process is not due to a direct effect on blastemal cells or to a general toxic effect on the treated larvae, but is related to hypothyroidism induced by the drug. We find that: (i) an increase in blastemal cell proliferation is observed not only in blastemata of T4-treated larvae, but also in blastemata cultured in vitro in a medium supplemented with T4; (ii) the renegerative process is accelerated not only in larvae reared in T4 but also in larvae submitted to a combined treatment of T4 and PTU; (iii) inhibition of cell proliferation is observed in blastemata of PTU-reared larvae but not in blastemata cultured in vitro in a medium supplemented with PTU. Experiments on thyroidless larvae (which were submitted to transplantation of hindlimbs from larvae at stages 53 and 55 followed by amputation of their own right hindlimb and the transplanted limbs) have shown that without thyroid hormone the regenerative process is arrested at cone stage and the promoting effect of T4 treatment is dependent on limb stage and amputation level.     

Life History of the Barnacle Balanus Amphitrite Darwin and Its Role in Fouling Communities of Peter the Great Bay, Sea of Japan

The life history of the barnacle Balanus amphitrite Darwin and its role in fouling communities of Golden Horn Bay (Peter the Great Bay), which is subjected to thermal pollution, were studied. The warm-water B. amphitrite occurs as a common minor species on operational vessels and waterworks in Peter the Great Bay, where it was brought by ocean-going ships operating on Russia–Japan lines. Even in the conditions of the higher temperature regime of Golden Horn Bay, the reproductive season of B. amphitrite is confined to the summer and autumn months. The adult individuals brought by ships in summer produce 2–3 generations of larvae. The development of larvae and their settling on the substrate occurs from August to October within a broad temperature range from 22.5 up to 12°C. Even in the low temperatures of Golden Horn Bay the larvae attain a greater size than those in tropical and subtropical waters. The juveniles have time to reach maturity and to produce their own progeny, but most often they perish with winter drop in the water temperature. It was shown that in Peter the Great Bay there is dependent population of B. amphitrite inhabiting the anthropogenic substrates only in the warm season: water works, idle vessels, and operational offshore vessels. The water temperature is the limiting factor of successful acclimation of that species.     

Substratum preferences of settling larvae of marine fishes reared in the laboratory

Various species of marine fish larvae were reared in the laboratory to allow observation of the substratum preferences of newly settling fish. The range of preferences for settling larvae of intertidal species corresponded to the adult niche breadth. The preferred substratum was always an element of the adult habitat, although not necessarily the same substratum preferred by the adults. Experiments with artificial substrata indicated that settlement preferences are based on tactile cues and light transmission. Depending upon the species, other factors such as current speed or salinity can also influence settlement.     

Effects of structural habitat on drift distance and benthic settlement of the caddisfly,Ceratopsyche sparna

We conducted two experiments in flow-through, artificial streams to examine how habitat structure affected drift and benthic resettlement of larval hydropsychid caddisflies (Ceratopsyche sparna). In the first experiment, we quantified drift distance and the number of times larvae re-entered the drift in 9.0 × 0.51-m channels with contiguous patches (ea. 2.5-m long) of biofilm-covered gravel, biofilm-covered cobbles (– Cladophora), and Cladophora-covered cobbles (+ Cladophora). In the second experiment, we tracked nocturnal movements of larvae after benthic settlement in 2.8 × 0.1-m channels, each containing one of the three habitat types. In experiment 1, drift distance was (1) greatest in gravel and lowest in cobbles + Cladophora, (2) inversely related to hydraulic roughness of habitats, (3) independent of body size, and (4) similar for live and dead larvae. Average drift distance was relatively short (<2.5 m), regardless of habitat type. Number of drift re-entries also varied among habitats, being greatest in gravel and lowest in cobbles + Cladophora. No larvae re-entered the drift after settling in Cladophora patches. Results from experiment 2 revealed that drift propensities were higher for larvae in biofilm-covered gravel and cobbles than in cobbles + Cladophora. Larvae remaining in substrate patches (i.e. not drifting) laid fewer draglines in biofilm-covered stones than in Cladophora patches. Extent of benthic movement (i.e., crawling) by non-drifting larvae did not differ significantly among habitats. However, distance moved did differ with flow direction, being 4× greater downstream than upstream. These results highlight how local substrate and hydraulic conditions interact to affect small-scale movements of caddisfly larvae.     

The influence of resident adults on larval settlement: experiments with four species of ascidians

Residents within any community can affect the larval settlement of both their own and other species. In marine sessile communities resident adults can affect larval settlement by preying on settling larvae, removing or adding space for the larvae to colonize, or stimulating or prohibiting larval settlement on available substratum nearby. To examine those processes by which residents affect settlement, we exposed experimental substrata with three densities of adults of a single species at a site in eastern Long Island Sound, USA for a 24-h period. Four species of common ascidians, Botryllus schlosseri (Pallas), Botrylloides diegensis Ritter and Forsyth, Diplosoma macdonaldi Herdman, and Molgula manhattensis (De Kay), were used in 11 separate experiments. Few individuals of any species settling attached to the surfaces of these species and this resulted in the main effect of these residents being the usurpation of space and the restricting of settlement to unoccupied areas. A model is also presented to explain the apparent aggregated settlement of several species in open areas adjacent to the resident ascidians. From this model we suggest that the aggregated settlement can result from limited larval mobility such that some larvae that contact and reject the resident species as settlement sites may subsequently contact open surfaces of the same substratum and increase settlement densities there over those observed on control substrata. Finally, settlement data for several species indicate that Molgula may influence settlement by preying on larvae.     

Natural Populations of Shipworm Larvae Are Attracted to Wood by Waterborne Chemical Cues

The life cycle of many sessile marine invertebrates includes a dispersive planktonic larval stage whose ability to find a suitable habitat in which to settle and transform into benthic adults is crucial to maximize fitness. To facilitate this process, invertebrate larvae commonly respond to habitat-related chemical cues to guide the search for an appropriate environment. Furthermore, small-scale hydrodynamic conditions affect dispersal of chemical cues, as well as swimming behavior of invertebrate larvae and encounter with potential habitats. Shipworms within the family Teredinidae are dependent on terrestrially derived wood in order to complete their life cycle, but very little is known about the cues and processes that promote settlement. We investigated the potential for remote detection of settling substrate via waterborne chemical cues in teredinid larvae through a combination of empirical field and laboratory flume experiments. Natural populations of teredinid larvae were significantly more abundant close to wooden structures enclosed in plankton net compared to empty control nets, clearly showing that shipworm larvae can sense and respond to chemical cues associated with suitable settling substrate in the field. However, the flume experiments, using ecologically relevant flow velocities, showed that the boundary layer around experimental wooden panels was thin and that the mean flow velocity exceeded larval swimming velocity approximately 5 mm (≈ 25 larval body lengths) from the panel surface. Therefore, we conclude that the scope for remote detection of waterborne cues is limited and that the likely explanation for the higher abundance of shipworm larvae associated with the wooden panels in the field is a response to a cue during or after attachment on, or very near, the substrate. Waterborne cues probably guide the larva in its decision to remain attached and settle, or to detach and continue swimming and drifting until the next encounter with a solid substrate.     

Prevalence of Nosema sp. (Microsporidia: Nosematidae) during an outbreak of the jack pine budworm in Ontario

Microsporidia are believed to play little or no role in outbreaks of the jack pine budworm, Choristoneura pinuspinus Freeman (Lepidoptrera: Tortricidae), because the short duration (2–4 years) of those outbreaks may not permit significant build-up of the pathogen. We conducted the first survey of Nosema sp. (Microsporidia: Nosematidae) over the course of a recent jack pine budworm outbreak in Ontario. Between 2004 and 2010 the outbreak defoliated a cumulative total of 1.78 million ha. Microscopic examination of ∼15,000 overwintering larvae collected over 6 years in sites with densities of 3 larvae per branch or more revealed widespread occurrence of Nosema at generally high infection intensities. The pathogen was present in 69.5% of the 518 plots that were monitored. Prevalence of infection was generally low (below 40% in 84% of plots with infected larvae) but reached high levels (80–95%) locally and increased rapidly in most infestations within 1–2 years of onset. We hypothesize that the habit of early-instar larvae to feed on developing male flowers (pollen cones) after spring emergence is critical in allowing rapid build-up of Nosema by increasing efficiency of horizontal transmission (higher density of both infected larvae and egested spores). Nosema infection may contribute to the complexity of jack pine budworm outbreak patterns by affecting egg recruitment and early-instar survival at the stand level in concert with known effects of budworm-induced reductions in pollen cone production on those processes.     

Mapping of serotonin-immunoreactive neurons in the larval nervous system of the flies Calliphora erythrocephala and Sarcophaga bullata

Summary Serotonin-immunoreactive (5-HTi) neurons were mapped in the larval central nervous system (CNS) of the dipterous flies Calliphora erythrocephala and Sarcophaga bullata. Immunocytochemistry was performed on cryostat sections, paraffin sections, and on the entire CNS (whole mounts).The CNS of larvae displays 96–98 5-HTi cell bodies. The location of the cell bodies within the segmental cerebral and ventral ganglia is consistent among individuals. The pattern of immunoreactive fibers in tracts and within neuropil regions of the CNS was resolved in detail. Some 5-HTi neurons in the CNS possess axons that run through peripheral nerves (antenno-labro-frontal nerves).The suboesophagealand thoracico-abdominal ganglia of the adult blowflies were studied for a comparison with the larval ventral ganglia. In the thoracico-abdominal ganglia of adults the same number of 5-HTi cell bodies was found as in the larvae except in the metathoracic ganglion, which in the adult contains two cell bodies less than in the larva. The immunoreactive processes within the neuropil of the adult thoracico-abdominal ganglia form more elaborate patterns than those of the larvae, but the basic organization of major fiber tracts was similar in larval and adult ganglia. Some aspects of postembryonic development are discussed in relation to the transformation of the distribution of 5-HTi neurons and their processes into the adult pattern.     

Processes affecting newly-settled juveniles and the consequences to subsequent community development

Summary

Processes affecting the growth and mortality of the juvenile benthic life-stages that immediately follow larval metamorphosis and settlement are as important as those processes controlling the supply of settling larvae or later interactions among established adults. In addition, the ecology of juveniles is of ten distinctly different from that of other life-stages, including differences in interactions with predators and competitors and responses to the physical environment. In particular, newly-settled stages of ten experience quantitatively or qualitatively different predation than older life-stages. We have documented this in a New England hard substrate community where the wrasse, Tautogolabrus adspersus, and two species of tiny gastropods, Mitrella lunata and Anachis lafresnayi, prey on newly-settled andjuvenile ascidians but not on adults. An extensive series of field experiments was conducted using artificial pilings placed subtidally. Results demonstrated that (1) the predators were extremely active and fairly specific in their prey, (2) predators could eliminate prey species regardless of settlement densities, (3) predation varied drastically with life-stage, and (4) predators control community structure and composition by altering the number of settling larvae that survived their first several weeks to become identifiable recruits. Because of differences in predator abundances the development and species dominance within the community varied drastically between sites.