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13 pages, 8 figures, 1 tables, aupplementary data https://doi.org/10.1016/j.jembe.2021.151607, The feeding behaviour of deep-sea animals is largely understudied despite being of relevance to assess changes in ecosystems functioning in the face of anthropogenic impacts and climate regime shifts. Here, time-lapse videography in dark, cold-water, flow-through laboratory settings was used to study the behaviours of a gastropod (Buccinum scalariforme) and three sea stars (Ceramaster granularis, Hippasteria phrygiana, Henricia lisa) from the bathyal Northwest Atlantic, to build knowledge on their foraging strategies (i.e. food search, assessment and handling). In all species tested, the presence/absence of a palatable food source modulated the speed and directionality of movements. Approach paths were erratic in the absence of palatable food and targeted in its presence: B. scalariforme, C. granularis and H. phrygiana, moved in straight line to the food, while H. lisa moved either in straight line or counterclockwise loop. The four species traveled at mean speeds of 2.9, 0.2, 0.7, and 0.6 cm min−1, respectively. At the finer scale, unprecedented pulses in displacement speed were detected, varying in amplitude and frequency depending on the food item. The results also support more generalist diets and broader scavenging habits in deep-sea sea gastropods and sea stars than assumed from previous studies and knowledge of shallower-water counterparts. Depth-related food limitation may drive greater flexibility in foraging strategies and acceptable food items, This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) via grants awarded to A.M., J.A. is a member of the Tecnoterra Associated Research Unit (ICM-CSIC/UPC) and supported by the Spanish Government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

When modeling life-history evolution of marine benthic invertebrates, it has been common to assume constant mortality rate of offspring during the pre-settlement phase and greater propagule vulnerability in the pelagic than in the benthic habitat. However, recent findings have challenged these assumptions, emphasizing the need for further empirical tests. Here we present the results of a multifactorial experimental investigation of predation rates on propagules of various taxa by benthic invertebrates. Planktotrophic and lecithotrophic propagules of echinoderms, cnidarians and annelids (i.e. sea cucumbers, sea stars, sea urchins, corals, sea anemones, jellyfishes, and polychaetes) were tested at various stages of development to examine the effects of ontogeny, size and behavior on predation rates by common filter feeders (mussels, tunicates) and suspension feeders (sea anemones). Overall, propagule survival was positively correlated with their size, although on closer inspection this was essentially due to the size difference between developmental modes (larger lecithotrophic propagules being less vulnerable). A slight inverse relationship between survival and age was detected in both lecithotrophic and planktotrophic propagules; however ingestion rates were not systematically higher on more advanced life stages within prey species due to predator-specific responses to ontogenetic changes. Filter feeders were generally more effective predators than suspension feeders. Tunicates expressed greater selectivity based on size and age of propagules than on their behavior, while the inverse occurred in the two other predators. A combination of factors, i.e. size, buoyancy and chemical defenses, presumably underlie overall higher survival rates in lecithotrophic than in planktotrophic propagules, supporting the hypothesis that the former but not the latter may have evolved due to increased epibenthic predation.

While epibiotic associations between macrobenthic invertebrates are common, the role they play in the feeding ecology of intervening species is often incompletely understood. The diets of epibiotic sea anemones Allantactis parasitica and their gastropod hosts were analyzed using digestive tract contents, lipid biomarkers and observations of live specimens in an attempt to detect dietary feedback from the facultative association. Comparisons were made using symbiotic individuals and asymbiotic counterparts collected at depths of 191–627 m from three neighbouring areas in the northwest Atlantic. Gastropods carrying one or two epibionts had higher stomach indices than those harbouring three epibionts or no epibiont. The diet of symbiotic gastropods was also more diversified based on stomach contents and lipid analysis. Among other things, symbiotic gastropods contained four times more lipids and a greater proportion of Σn−3 fatty acids. Gastrovascular cavity content indices of asymbiotic sea anemones were generally lower than those of symbiotic counterparts. Their cavities were more often empty, and their diet less diversified with fewer benthic items, suggesting that foraging of gastropods through the sediments makes more food available to sea anemones living as epibionts. Lipid analysis showed some disparities between symbiotic and asymbiotic sea anemones at the regional scale, including in percent phospholipids and in the proportion of certain fatty acids. Together these findings indicate that mutual protection against predators leads to prolonged and more efficient foraging for gastropods and increased time spent deployed (feeding) in food-rich areas for sea anemones, thus enabling both partners to fully exploit food resources that may be limited at bathyal depths.

The hormathiid sea anemone Allantactis parasitica was found living as an epibiont on numerous species of gastropods at depths of 725–1100 m along the continental slope of eastern Canada. The proportion of bathyal gastropods hosting 1–6 sea anemones reached 72.5% in a single trawl. Although A. parasitica was occasionally found on other substrata (i.e. empty shells, pebbles), laboratory trials confirmed that they preferably associate with living gastropods. Settlement of planula larvae occurred significantly more often on the shells of live bathyal gastropods than on all other substrata present in the tanks. Juvenile sea anemones (∼ 1 mm diameter) readily moved from the mud or other inert substrata onto shells of burrowed bathyal gastropods. Conversely, larvae, juveniles and adults of A. parasitica never associated with any shallow-water gastropods when given the opportunity. Trials exposing predatory sea stars (Leptasterias polaris) from shallow and bathyal depths to bathyal gastropods (Buccinum undatum) with epibiotic A. parasitica, and to asymbiotic bathyal and shallow-water B. undatum, revealed adaptive behaviours in both prey and predator. Shallow-water gastropods (devoid of epibionts) reacted defensively to L. polaris, whereas bathyal gastropods relied mostly on their epibionts to protect them, thus falling prey to L. polaris when the epibionts were removed. L. polaris from bathyal depths typically ignored symbiotic gastropods, but they consistently preyed on asymbiotic ones, while L. polaris from shallow areas initially attempted to prey on all gastropods, but learned to avoid those harbouring sea anemones. Furthermore, living as epibionts afforded sea anemones a means to escape one of their own predators, the sea star Crossaster papposus. The mutualistic relationship between hormathiid sea anemones and bathyal gastropods from the NW Atlantic may have evolved in response to predation pressure.

This study investigated the daily activities of juvenile sea cucumbers Holothuria scabra Jaeger. All individuals exhibited daily burrowing and feeding rhythms in response to environmental factors. The cycle of the smallest juvenile, >10–40 mm, was linked to light; they began to burrow around sunrise and emerged close to sunset. Their burrowing activity was inhibited by continuous darkness. Juveniles >40–140 mm responded to changes in temperature; they burrowed earlier around 03:30 h, seeking shelter in the sediment as temperature declined, and emerged around mid-day. The maintenance of a constant warm temperature prevented them from burrowing. For all juveniles, time spent on the surface corresponded with feeding and periods of locomotion; while burrowed they remained stationary, did not feed and had a low intestinal transit. Organic matter content in the intestine was also found to vary with the daily cycle. Decreases in salinity from 35 to 30, 25 and 20‰ induced the burrowing of all juveniles within minutes, but they began to re-emerge after a few hours. Acclimation occurred most rapidly at salinity 30‰ and was slowest at salinity 20‰. Conversely, nearly 40% of the juveniles were unable to cope with a decrease to salinity 15‰. Juveniles of all sizes demonstrated a strong selectivity for sediment characteristics. In two substrate selection experiments, their preference for sand with a grain size around 0.4 mm and for organically rich material was firmly expressed within an hour. The general behavior of juvenile H. scabra reflects their ability to inhabit shallow sandy areas with high terrigenous inputs and variable environmental conditions.

Settlement and post-settlement processes of the sea cucumber Holothuria scabra Jaeger were studied in the laboratory. Independent and paired choice experiments revealed that several substrates could induce metamorphosis into pentactulae, but that specific substrates favoured settlement. Leaves of seagrass Thalassia hemprichii, with or without their natural bio-film, yielded the highest settlement rates (4.8-10.5%). T. hemprichii was preferred as a settlement substrate over sand, crushed coral, several other plant species and artificial seagrass leaves with or without a bio-film. Only settlement on the seagrass, Enhalus acoroides, was similar to that recorded for T. hemprichii. In the absence of a substrate, the larvae delayed settlement for nearly 96 h and survival was less than 0.5%. Sand and crushed coral, either alone or together, induced settlement from1.5% of the available larvae. The pentactulae found on sand, coral and in bare containers were 10-35% smaller than those on T. hemprichii leaves. Soluble extracts from T. hemprichii and E. acoroides successfully induced metamorphosis and settlement on clean plastic surfaces. Newly settled juveniles remained on the seagrass leaves for 4-5 weeks before migrating to sand at around 6 mm in length. Prior to this, the juveniles spent 4-5 days moving on and off the leaves. Once on the sand, the juveniles became deposit-feeders, but did not show the typical burrowing behaviour of older specimens until they reached around 11 mm in length. The larvae of H. scabra appear to actively select seagrass leaves, possibly through chemical detection. We hypothesise that larvae settling on seagrass have an increased chance of growth and survival because they are provided with a suitable substrate on which to grow, and a bridge to sand substrates as they become deposit-feeders.