Louis A. Gosselin

Characterizing temperate rocky shores from the perspective of an early juvenile snail: The main threats to survival of newly hatched Nucella emarginata

Mortality factors most likely to constitute substantial selective pressures for early juvenile gastropods on temperate rocky shores were identified by examining the vulnerability of hatchlings of an intertidal snail, Nucella emarginata, to heat stress, desiccation, and predation in 1992 and 1993. The highest temperature of substrata measured at tidal heights colonized by N. emarginata in Barkley Sound, British Columbia, Canada, was 28.5°C. This temperature was not lethal to hatchlings in laboratory tests. In laboratory and field desiccation experiments, all hatchlings died within 6 h of emersion. Early juveniles could not survive direct exposure to even moderate drying conditions for the duration of a low tide. Hence, intertidal microhabitats which dry up even for short periods during low tides would prove lethal. Of 45 intertidal animal species to which hatchlings were exposed in the laboratory, small decapod crustaceans were the only organisms to cause substantial hatchling mortality. Of these, Pagurus hirsutiusculus and Hemigrapsus nudus were by far the most abundant in the field, and are probably the only important predators of early juvenile N. emarginata at most sites. Total predator densities in the field were as high as 438 individuals m−2, suggesting that predation pressure may be intense. Desiccation and predation by decapod crustanceans appear to be the most significant threats to early juvenile N. emarginata. These factors commonly occur on most temperate rocky shores and undoubtedly constitute major selective agent influencing population parameters and shaping life-history strategies and early juvenile traits of intertidal invertebrates.

Feeding habits of newly hatched juveniles of an intertidal predatory gastropod, Nucella emarginata (Deshayes)

Abstract The feeding habits of juvenile Nucella emarginata ( = Thais emarginata), a marine prosobranch gastropod with direct development, were examined to determine when predatory feeding begins, and if ontogenetic shifts in mode of feeding occur. Most hatchlings did not attack live prey during the first 3 days after emerging from their egg capsule. By day 10, however, over 80% of the hatchlings had attacked prey. The late onset of predatory feeding appears to be ontogenetic, rather than a consequence of handling or experimental conditions. Hatchlings did not use alternate food types (mussel feces, algae, biofilm, or barnacle moults), as determined indirectly by growth, survival and organic content measurements of hatchlings. Once N. emarginata started attacking prey, six species of invertebrates which co-occur with the hatchlings in the field were consumed: three bivalves (Lasaea spp., Musculus taylori and Mytilus spp.) and three barnacles (Balanus glandula, Chthamalus dalli and Pollicipes polymerus). Hatchlings did not attack limpets (Lottia pelta and Lottia digitalis), a littorine (Littorina scutulata), the bivalve Hiatella arctica, or other hatchlings. In addition, hatchlings were able to survive for up to 120 days without food. Although no ontogenetic shift in mode of feeding was observed in young N. emarginata, at least two prey species consumed by hatchlings cease to be used during ontogeny. Possible reasons for the late onset of feeding are discussed.

Analysing energy content: a new micro-assay and an assessment of the applicability of acid dichromate assays

This study describes a micro-assay, based on acid dichromate oxidation, with a resolution of at least 0.5 μg organic carbon and an upper limit of ≤20 μg C. We also document several important properties of acid dichromate assays and establish their effectiveness for quantifying organic carbon and energy content of marine and freshwater organisms. Both the micro-assay and the previously described standard assay are highly sensitive to chloride: absorbance readings were significantly depressed by the presence of only 0.5–1.0 μl of seawater, and the effect of seawater was shown to be due to its chloride content. The amount of chloride contained within the bodies of very small marine organisms may therefore be sufficient to interfere with the assay. Contrary to previous claims, we found that incubating samples with phosphoric acid did not prevent chloride from interfering with the assays. The micro- and standard assays were not sensitive to inorganic carbon and were therefore specific to organic carbon. The assays were effective in estimating total energy content of carbohydrate and lipid material, but underestimated the energy content of protein material by 47–69%. This limitation can be overcome by using a protein micro-assay to correct for underestimation by the acid dichromate assays. Based on our findings, the reliability of acid dichromate oxidation assays for analysing samples of marine organisms is questionable. The assays are effective, however, for analysing chloride-free tissues or extracts. In addition, the assays have considerable potential for determining energy content of small freshwater organisms. In particular, the micro-assay is at least an order of magnitude more sensitive than the standard assay, and constitutes a relatively simple way of measuring energy content of very small samples, such as individual embryos or early juveniles of aquatic animals and plants.

A method for marking small juvenile gastropods

Methods used to identify individual organisms consistently over time have been invaluable tools in ecological studies, enabling reliable assessments of time-dependent parameters such as growth and mortality, and an accurate determination of their variance. These methods have proved to be particularly amenable to gastropods owing to the presence of an external shell on which marks or tags can be applied with little or no adverse effects on the animal. Marking and tagging techniques have enabled the study of several ecological parameters in adult marine gastropods, including growth (Frank, 1965; Hughes, 1972; Palmer, 1983; Gosselin & Bourget, 1989), mortality (Frank, 1965; Hughes, 1972), movements (Frank, 1965; Chapman, 1986), and foraging behaviour (Menge, 1974; Hughes et al. , 1992). Small organisms, however, can pose considerable problems for individual marking (Southwood, 1978). As a result, marking and tagging methods have seldom been applied to newly hatched or recently settled juvenile marine gastropods. Several methods have been developed for simultaneously labelling large numbers of invertebrate larvae (Levin, 1990), and some of these methods may be applicable to juvenile gastropods. The usefulness of these methods, however, is limited because all animals receive the same label and, consequently, individual animals cannot be recognized. To my knowledge, no method of individually marking very small juvenile marine gastropods has been documented. In fact, it is sometimes perceived that small juveniles cannot be individually marked due to their small size and sensitivity (Frank, 1965; Palmer, 1990). The object of this paper is to present a simple method of marking early juvenile gastropods, which consists of applying colour codes to the shells of individuals as small as 0·9 mm in length.

Role of Maternal Provisioning in Controlling Interpopulation Variation in Hatching Size in the Marine Snail Nucella ostrina

This study examined the role of maternal provisioning in controlling interpopulation variation in hatching size in nine isolated populations of the intertidal gastropod Nucella ostrina, in which development to the early juvenile stage takes place within an egg capsule. Variation among populations was almost entirely due to the ratio of nurse eggs to embryo, which explained 65% of the variation in hatching size. Egg size was not a significant predictor of hatching size. Differences among seven of these populations in the nurse egg/embryo ratio were entirely due to the number of nurse eggs allocated per capsule; these populations allocated different numbers of nurse eggs per capsule but allocated the same number of embryos. Intriguingly, the two most wave-sheltered populations allocated significantly more nurse eggs and more embryos to each capsule than did the seven other populations, but they maintained nurse egg/embryo ratios consistent with patterns observed in the other populations. Inter- and intrapopulation variation in hatching size appears to be controlled largely by different mechanisms: within-population variation being controlled mainly by differences in allocation of embryos per capsule, whereas most among-population variation being due to differences in allocation of nurse eggs per capsule.

Prey selection by inexperienced predators: do early juvenile snails maximize net energy gains on their first attack?

Abstract Through encounters with prey, a predator may learn to become more selective and thus forage more efficiently. This is the case for several muricid gastropods. When searching for their first prey, however, early juveniles have no such foraging experience. Prey species and prey size preferences of inexperienced Nucella emarginata (Deshayes) hatchlings from Barkley Sound, British Columbia, Canada, were therefore examined to determine if selective feeding occurs in the absence of foraging experience and, if so, whether the preferences are consistent with the energy maximization hypothesis of optimal foraging theory. When given a choice between 5 prey species (small Mytilus spp., Balanus glandula (Darwin), Chthamalus dalli (Pillsbury), Pollicipes polymerus (Sowerby) and Lasaea spp.), 80% of the hatchlings attacked Mytilus spp. on their first attack. When offered 5 size classes (1, 2, 3, 4 and 5 mm shell length) of Mytilus spp., ≈ 73% of the hatchlings attacked 1-mm or 2-mm size classes. Hatchlings were, in fact, more selective than late juveniles and adults. Hence, strong prey species and size preferences can exist without prior foraging experience even in species which use such experience later in life to make foraging decisions. But hatchlings feeding on Mytilus spp. for 25 days did not grow faster than hatchlings feeding on Balanus glandula or Chthamalus dalli , and there was no difference in energy content between Mytilus spp. and Chthamalus dalli . Thus, the strong preference for small Mytilus spp. can not be explained by considerations of energy gain. In the field, small Mytilus spp. are mostly located in microhabitats which provide hatchlings with substantial protection from mortality factors. The preference for small Mytilus spp. should keep the young snails within protective microhabitats once these sites have been reached, and might be an adaptation to help the hatchlings locate these havens from a distance, in an otherwise highly dangerous environment.

Reproduction, larval development and settlement of the intertidal serpulid polychaete Spirobranchus cariniferus

Serpulid polychaetes are often important components of marine communities, yet current knowledge of serpulid reproduction, development and settlement is largely based on the study of only a few species. This study examined whether spawning and sexual development of the mid-intertidal serpulid Spirobranchus cariniferus are typical of patterns observed in other serpulids, and also examined the effect of diet on larval development and identified cues that induce metamorphosis. Populations of S. cariniferus on the east and west coasts of New Zealands North Island had an extended spawning season with a high proportion (75–100%) of individuals carrying ripe gametes at any given time from the late spring to early autumn, consistent with other serpulid species. However, contrary to proposed patterns of sexual development in serpulids, sex-ratios and average male and female body masses suggest S. cariniferus are not protandric hermaphrodites. A diet experiment revealed that larval growth was fastest when provided with a mixed diet of three algal species. Solutions of 3-isobutyl-1-methylxanthine at a concentration of 10 −4 M were potent inducers of metamorphosis in S. cariniferus larvae, revealing that competence is reached by 13–15 days after fertilization. When exposed to surfaces coated with conspecific tube or tissue homogenates, 6% of competent larvae settled and metamorphosed on surfaces coated with tube homogenate whereas none settled on tissue homogenate or in the controls, suggesting specific cues are required for settlement and the likely presence of a settlement cue in the tubes of adult worms, consistent with the highly gregarious distribution of the species.