Hydrodynamic facilitation of gregarious settlement of a reef-building tube worm

Diversity of cilia-based mechanosensory systems and their functions in marine animal behaviour

Sensory cells that detect mechanical forces usually have one or more specialized cilia. These mechanosensory cells underlie hearing, proprioception or gravity sensation. To date, it is unclear how cilia contribute to detecting mechanical forces and what is the relationship between mechanosensory ciliated cells in different animal groups and sensory systems. Here, we review examples of ciliated sensory cells with a focus on marine invertebrate animals. We discuss how various ciliated cells mediate mechanosensory responses during feeding, tactic responses or predator-prey interactions. We also highlight some of these systems as interesting and accessible models for future in-depth behavioural, functional and molecular studies. We envisage that embracing a broader diversity of organisms could lead to a more complete view of cilia-based mechanosensation. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Biodiversity patterns of rock encrusting fauna from the shallow sublittoral of the Admiralty Bay

The Antarctic sublittoral is one of the most demanding habitat for polar bottom-dwelling organisms, as the disturbance of this zone is highly intense. Rapid changes in the marine environment, such as increases in atmosphere and surface water temperatures, can cause dramatic changes in biodiversity, especially in glacial fjords affected by heavy melt water inputs from the retreating glaciers. In such areas, rocks are often an important support for local diversity, providing habitats for a number of encrusting organisms. Thus, understanding the patterns of diversity of shallow rock encrusting fauna and factors controlling it are particularly important. The structure and diversity patterns of rock encrusting fauna were examined from four ecologically contrasting sites in the shallow sublittoral (6-20 m) of Admiralty Bay (King George Island). The results revealed a rich and abundant encrusting community with bryozoans and polychaetes outcompeting representatives of other fauna such as foraminifera and porifera. Spatial variability in species composition, as well as biological parameters, revealed the trend of encrusting assemblages declining towards the inner fjord areas - strongly affected by high sediment input (species richness: 13.3 ± 1.2, and abundance: 68,932.99 ± 11,915.98 individuals m^-2 ± standard error). In contrast, at sites more open to the central basin, a peak of biological parameters was observed (24.8 ± 1.4 and 297,360.9 ± 30,314.72, respectively). We suggest that increased sedimentation was the major factor structuring the encrusting assemblages in Ezcurra Inlet, masking the influence of other parameters, such as food and light availability, which are important for the distribution of epifauna. Thus, if the increasing intensity of glacial processes will continue in the upcoming years, the diversity of the encrusting fauna in the shallow sublittoral could dramatically decrease.

A new flow-through bioassay for testing low-emission antifouling coatings

Current antifouling (AF) technologies are based on the continuous release of biocides into the water, and consequently discharge into the environment. Major efforts to develop more environmentally friendly coatings require efficient testing in laboratory assays, followed by field studies. Barnacles are important fouling organisms worldwide, increasing hydrodynamic drag on ships and damaging coatings on underwater surfaces, and thus are extensively used as models in AF research, mostly in static, laboratory-based systems. Reliable flow-through test assays for the screening of biocide-containing AF paints, however, are rare. Herein, a flow-through bioassay was developed to screen for diverse low-release biocide paints, and to evaluate their effects on pre- and post-settlement traits in barnacles. The assay distinguishes between the effects from direct surface contact and bulk-water effects, which are crucial when developing low-emission AF coatings. This flow-through bioassay adds a new tool for rapid laboratory-based first-stage screening of candidate compounds and novel AF formulations.

Trophic Ecology of Benthic Marine Invertebrates with Bi-Phasic Life Cycles: What Are We Still Missing?

The study of trophic ecology of benthic marine invertebrates with bi-phasic life cycles is critical to understand the mechanisms shaping population dynamics. Moreover, global climate change is impacting the marine environment at an unprecedented level, which promotes trophic mismatches that affect the phenology of these species and, ultimately, act as drivers of ecological and evolutionary change. Assessing the trophic ecology of marine invertebrates is critical to understanding maternal investment, larval survival to metamorphosis, post-metamorphic performance, resource partitioning and trophic cascades. Tools already available to assess the trophic ecology of marine invertebrates, including visual observation, gut content analysis, food concentration, trophic markers, stable isotopes and molecular genetics, are reviewed and their main advantages and disadvantages for qualitative and quantitative approaches are discussed. The challenges to perform the partitioning of ingestion, digestion and assimilation are discussed together with different approaches to address each of these processes for short- and long-term fingerprinting. Future directions for research on the trophic ecology of benthic marine invertebrates with bi-phasic life cycles are discussed with emphasis on five guidelines that will allow for systematic study and comparative meta-analysis to address important unresolved questions.

Swimming in an Unsteady World

When animals swim in aquatic habitats, the water through which they move is usually flowing. Therefore, an important part of understanding the physics of how animals swim in nature is determining how they interact with the fluctuating turbulent water currents in their environment. We addressed this issue using microscopic larvae of invertebrates in "fouling communities" growing on docks and ships to ask how swimming affects the transport of larvae between moving water and surfaces from which they disperse and onto which they recruit. Field measurements of the motion of water over fouling communities were used to design realistic turbulent wavy flow in a laboratory wave-flume over early-stage fouling communities. Fine-scale measurements of rapidly-varying water-velocity fields were made using particle-image velocimetry, and of dye-concentration fields (analog for chemical cues from the substratum) were made using planar laser-induced fluorescence. We used individual-based models of larvae that were swimming, passively sinking, passively rising, or were passive and neutrally buoyant to determine how their trajectories were affected by their motion through the water, rotation by local shear, and transport by ambient flow. Swimmers moved up and down in the turbulent flow more than did neutrally buoyant larvae. Although more of the passive sinkers landed on substrata below them, and more passive risers on surfaces above, swimming was the best strategy for landing on surfaces if their location was not predictable (as is true for fouling communities). When larvae moved within 5 mm of surfaces below them, passive sinkers and neutrally-buoyant larvae landed on the substratum, whereas many of the swimmers were carried away, suggesting that settling larvae should stop swimming as they near a surface. Swimming and passively-rising larvae were best at escaping from a surface below them, as precompetent larvae must do to disperse away. Velocities, vorticities, and odor-concentrations encountered by larvae fluctuated rapidly, with peaks much higher than mean values. Encounters with concentrations of odor or with vorticities above threshold increased as larvae neared the substratum. Although microscopic organisms swim slowly, their locomotory behavior can affect where they are transported by the movement of ambient water as well as the signals they encounter when they move within a few centimeters of surfaces.

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.

Paternity and gregariousness in the sex-changing sessile marine gastropod Crepidula convexa: comparison with other protandrous Crepidula species

In sex-changing animals with internal fertilization, gregarious behavior may increase mating opportunities and the frequency of multiple paternity, thus increasing maternal reproductive success. Crepidula convexa is a direct-developing protandrous gastropod characterized by only modest gregarious behavior compared with previously studied members of the genus: females are frequently found isolated. Using 6 microsatellite markers, we analyzed paternity profiles in 10 broods (25 embryos per mother). The number of assigned fathers varied among families from 1 to 4 fathers per brood. Interestingly, polyandry was not detected in solitary females but only in females grouped with conspecific individuals. Overall, we found an average of 1.8 fathers per brood, but this increased to 2.6 fathers per brood when considering only the nonisolated females. Among 18 unambiguously identified fathers, only 5 were collected in our samples, suggesting substantial male mobility. Comparison with previous paternity analyses in Crepidula fornicata and Crepidula coquimbensis revealed that polyandry appears as a common trait of these sex-changing gastropods despite their different grouping behaviors and life histories. As expected, the level of polyandry was nevertheless lower in the modestly gregarious C. convexa.

Effects of delayed metamorphosis on larval survival, metamorphosis, and juvenile performance of four closely related species of tropical sea urchins (genus Echinometra)

We report here, the effects of extended competency on larval survival, metamorphosis, and postlarval juvenile growth of four closely related species of tropical sea urchins, Echinometra sp. A (Ea), E. mathaei (Em), Echinometra sp. C (Ec), and E. oblonga (Eo). Planktotrophic larvae of all four species fed on cultured phytoplankton (Chaetoceros gracilis) attained metamorphic competence within 22-24 days after fertilization. Competent larvae were forced to delay metamorphosis for up to 5 months by preventing them from settling in culture bottles with continuous stirring on a set of 10 rpm rotating rollers and larval survival per monthly intervals was recorded. Larval survival was highest at 24 days, when competence was attained (0 delayed period), and there were no significant differences among the four species. Larvae that had experienced a prolonged delay had reduced survival rate, metamorphosis success, and juvenile survival, but among older larvae, Em had the highest success followed by Ea, Eo, and Ec. Juveniles from larvae of all four species that metamorphosed soon after becoming competent tended to have higher growth rates (test diameter and length of spines) than juveniles from larvae that metamorphosed after a prolonged period of competence with progressively slower growth the longer the prolonged period. Despite the adverse effects of delaying metamorphosis on growth parameters, competent larvae of all four species were able to survive up to 5 months and after metamorphosis grew into 1-month-old juveniles in lab condition. Overall, delayed larvae of Em showed significantly higher larval survival, metamorphosis, and juvenile survival than Ea and Eo, while Ec showed the lowest values in these performances. Em has the most widespread distribution of these species ranging from Africa to Hawaii, while Ec probably has the most restricted distribution. Consequently, differences in distribution may be related to differences in the ability to delay metamorphosis.

Oyster larvae settle in response to habitat-associated underwater sounds

Following a planktonic dispersal period of days to months, the larvae of benthic marine organisms must locate suitable seafloor habitat in which to settle and metamorphose. For animals that are sessile or sedentary as adults, settlement onto substrates that are adequate for survival and reproduction is particularly critical, yet represents a challenge since patchily distributed settlement sites may be difficult to find along a coast or within an estuary. Recent studies have demonstrated that the underwater soundscape, the distinct sounds that emanate from habitats and contain information about their biological and physical characteristics, may serve as broad-scale environmental cue for marine larvae to find satisfactory settlement sites. Here, we contrast the acoustic characteristics of oyster reef and off-reef soft bottoms, and investigate the effect of habitat-associated estuarine sound on the settlement patterns of an economically and ecologically important reef-building bivalve, the Eastern oyster (Crassostrea virginica). Subtidal oyster reefs in coastal North Carolina, USA show distinct acoustic signatures compared to adjacent off-reef soft bottom habitats, characterized by consistently higher levels of sound in the 1.5-20 kHz range. Manipulative laboratory playback experiments found increased settlement in larval oyster cultures exposed to oyster reef sound compared to unstructured soft bottom sound or no sound treatments. In field experiments, ambient reef sound produced higher levels of oyster settlement in larval cultures than did off-reef sound treatments. The results suggest that oyster larvae have the ability to respond to sounds indicative of optimal settlement sites, and this is the first evidence that habitat-related differences in estuarine sounds influence the settlement of a mollusk. Habitat-specific sound characteristics may represent an important settlement and habitat selection cue for estuarine invertebrates and could play a role in driving settlement and recruitment patterns in marine communities.

Mechanisms of temporary adhesion in benthic animals

Adhesive systems are ubiquitous in benthic animals and play a key role in diverse functions such as locomotion, food capture, mating, burrow building, and defence. For benthic animals that release adhesives, surface and material properties and external morphology have received little attention compared to the biochemical content of the adhesives. We address temporary adhesion of benthic animals from the following three structural levels: (a) the biochemical content of the adhesive secretions, (b) the micro- and mesoscopic surface geometry and material properties of the adhesive organs, and (c) the macroscopic external morphology of the adhesive organs. We show that temporary adhesion of benthic animals is affected by three structural levels: the adhesive secretions provide binding to the substratum at a molecular scale, whereas surface geometry and external morphology increase the contact area with the irregular and unpredictable profile of the substratum from micro- to macroscales. The biochemical content of the adhesive secretions differs between abiotic and biotic substrata. The biochemistry of the adhesives suitable for biotic substrata differentiates further according to whether adhesion must be activated quickly (e.g. as a defensive mechanism) or more slowly (e.g. during adhesion of parasites). De-adhesion is controlled by additional secretions, enzymes, or mechanically. Due to deformability, the adhesive organs achieve intimate contact by adapting their surface profile to the roughness of the substratum. Surface projections, namely cilia, cuticular villi, papillae, and papulae increase the contact area or penetrate through the secreted adhesive to provide direct contact with the substratum. We expect that the same three structural levels investigated here will also affect the performance of artificial adhesive systems.

Induction of larval metamorphosis of the coral Acropora millepora by tetrabromopyrrole isolated from a Pseudoalteromonas bacterium

The induction of larval attachment and metamorphosis of benthic marine invertebrates is widely considered to rely on habitat specific cues. While microbial biofilms on marine hard substrates have received considerable attention as specific signals for a wide and phylogenetically diverse array of marine invertebrates, the presumed chemical settlement signals produced by the bacteria have to date not been characterized. Here we isolated and fully characterized the first chemical signal from bacteria that induced larval metamorphosis of acroporid coral larvae (Acropora millepora). The metamorphic cue was identified as tetrabromopyrrole (TBP) in four bacterial Pseudoalteromonas strains among a culture library of 225 isolates obtained from the crustose coralline algae Neogoniolithon fosliei and Hydrolithon onkodes. Coral planulae transformed into fully developed polyps within 6 h, but only a small proportion of these polyps attached to the substratum. The biofilm cell density of the four bacterial strains had no influence on the ratio of attached vs. non-attached polyps. Larval bioassays with ethanolic extracts of the bacterial isolates, as well as synthetic TBP resulted in consistent responses of coral planulae to various doses of TBP. The lowest bacterial density of one of the Pseudoalteromonas strains which induced metamorphosis was 7,000 cells mm⁻² in laboratory assays, which is on the order of 0.1 –1% of the total numbers of bacteria typically found on such surfaces. These results, in which an actual cue from bacteria has been characterized for the first time, contribute significantly towards understanding the complex process of acroporid coral larval settlement mediated through epibiotic microbial biofilms on crustose coralline algae.

Hydrodynamics of larval settlement from a larva's point of view

Many benthic marine invertebrate animals release larvae that are dispersed by ocean currents. These larvae swim and can respond to environmental factors such as chemical cues. However, larvae are so small (generally 0.01-1 mm) that they are often assumed to be passive particles whose trajectories are determined by the motion of the water in which they are riding. Therefore, marine larvae are useful model organisms to study the more general question of how the locomotion of very small animals in complex, variable natural habitats is affected by the motion of the fluid (water or air) around them. Studying larval locomotion under conditions of water flow encountered in nature is challenging because measuring the behavior of an individual microscopic organism requires high magnification imaging that is difficult to do in the field. The purpose of this article is to synthesize in one place the various approaches that we have been using to address the technical challenges of studying the locomotion of microscopic larvae in realistic ambient flow. The steps in our process include: (1) measuring water flow in the field; (2) mimicking realistic water movement in laboratory flumes to measure larval scale fluctuations in velocity of flow and concentration of chemical cues; (3) mimicking fine scale temporal patterns of larval encounters with a dissolved chemical cue to record larval responses; (4) using individual-based models to put larvae back into the larger scale environmental flow to determine trajectories; and (5) mimicking fine scale spatial and temporal patterns of larval encounters with water velocities and shear to determine the instantaneous forces on larvae. We illustrate these techniques using examples from our ongoing research on the settlement of larvae onto fouling communities and from our published work on settlement of larvae onto coral reefs. These examples show that water velocities and concentrations of chemical cues encountered by microscopic organisms can fluctuate in fractions of a second and vary over scales of less than a millimeter.

Effects of semidiurnal tidal circulation on the distribution of holo- and meroplankton in a subtropical estuary

We examined how tidal changes and which physical factors affected holo- and meroplankton assemblages in a subtropical estuary in Taiwan in February 1999. A factor analysis showed that during tidal flooding, the water mass properties changed from low salinity (5-16) and high particulate organic carbon (POC, 2.6-4.5 mg L(-1)) content to increasing salinity and high total suspended matter content (29.0-104.5 mg L(-1)). With a receding tide, the water became more saline again, and its velocity increased (from non-detectable to 0.67 m s(-1)). One-way ANOVA showed that the distributions of four dominant taxa were affected by the ebb tide and exhibited two distinct groups. The first group consisted of non-motile invertebrate eggs and weakly swimming polychaete sabellid embryos and larvae (at densities of 1.25-1.40 ind. L(-1)), while the second consisted of better-swimming copepods and polychaete spionid larvae (at densities of 0.70-1.65 ind. L(-1)). A canonical correlation analysis demonstrated that the former group occurred at sites with greater freshwater input, higher POC content and greater depth, whereas the latter group was significantly associated with sites subject to seawater and faster flows. We propose that a two-layered circulation process and tidally induced oscillations in water movements might account for the distributional differences between these two groups.

FACILITATION DRIVES LOCAL ABUNDANCE AND REGIONAL DISTRIBUTION OF A RARE PLANT IN A HARSH ENVIRONMENT

The importance of facilitation to local community dynamics is becoming increasingly recognized. However, the predictability of positive interactions in stressful environments, the balance of competition and facilitation along environmental gradients, and the scaling of local positive interactions to regional distributions are aspects of facilitation that remain unresolved. I explored these questions in a habitat specialist, Delphinium uliginosum, and a moss, Didymodon tophaceus, both found in small serpentine wetlands. I tested three hypotheses: (1) moss facilitates germination, growth, and/or fecundity of D. uliginosum; (2) facilitation is stronger at the harsher ends of gradients in soil moisture, toxicity, and/or biomass; and (3) facilitation is reflected in positive associations at the levels of local abundance and regional occurrence. Although considerable competitive interactions occurred in later life stages, moss strongly facilitated D. uliginosum seedling emergence. There was no evidence that this facilitative effect weakened, or switched to competition, in benign environments. D. uliginosum was more locally abundant and more frequently present, across a large portion of its range, with than without moss, indicating a net facilitative effect in the face of competitive influences. Facilitated recruitment, possibly by seed retention, was found to be an important control on abundance and distribution in this rare species.

Effects of herbivorous snails and macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.)

This study investigated the effects of grazing by the herbivorous snail Littorina littorea (L.) and the presence of a macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.) at four sites of various distances from the mouth of Toothacher Cove on Swans Island in the Gulf of Maine in 1998. Recruitment plates were attached to the substrate at each site in all possible treatment combinations, including either presence of absence of macroalgal canopy, presence or absence of the herbivore, and presence or absence of a cage. Significant differences in recruitment were observed among sites in April and May, but not in June. The average number of recruits also varied significantly among herbivore treatments in May and June, and in the May macroalgaexherbivore interaction. Survivorship did not differ among sites, but did vary significantly among herbivore treatments. These differences were due almost entirely to consistently low recruitment and survival in the uncaged or open controls. Comparisons of the caged and open treatments explained between 88 and 99% of the variation in the respective main effects tests. The presence or absence of L. littorea or of macroalgal canopy had little effect on S. balanoides recruitment or survivorship. Results support previous conclusions that recruitment is greater on more exposed shores and suggest that exclusion cages may protect barnacle recruits from factors such as desiccation and the whiplash effect of macroalgal fronds.

Calcium control of metamorphosis in polychaete larvae

The importance of Ca2+ in the control of metamorphosis of a marine invertebrate larva was investigated. An excess of [Ca2+] in the external medium induced metamorphosis of Phragmatopoma californica (polychaete) larvae in a concentration-dependent manner. This effect is specific for calcium, and not simply the result of osmotic changes, as an excess of Mg2+ did not induce metamorphosis. Consistent with this finding, the calcium ionophore, A23187, also induced metamorphosis in a concentration-dependent manner. Paradoxically, however, the aromatic compounds diltiazem, verapamil, D600, and nifedipine, known to block Ca2+ channels in other systems, also induced metamorphosis. When exposed to diltiazem for only 20 h and subsequently washed free of this compound, 95% of the larvae metamorphosed and developed normally. Previous studies have demonstrated that the induction of metamorphosis in Phragmatopoma californica is controlled by chemosensory recognition of an exogenous morphogen and mediated by an excitatory pathway that involves adenyl cyclase and cyclic AMP. Because cellular excitation and cyclic AMP-dependent signal transduction generally involve the participation of calcium ion, the most parsimonious explanation for the results reported here include (1) direct control of the morphogenetic pathway by calcium ion, and (2) complexities of the calcium regulation of this process, or a functional similarity between the structurally related aromatic effectors tested and the natural inducer of metamorphosis.