Multi-locus DNA metabarcoding of zooplankton communities and scat reveal trophic interactions of a generalist predator.

To understand the ecosystem dynamics that underpin the year-round presence of a large generalist consumer, the Bryde's whale (Balaenoptera edeni brydei), we use a DNA metabarcoding approach and systematic zooplankton surveys to investigate seasonal and regional changes in zooplankton communities and if whale diet reflects such changes. Twenty-four zooplankton community samples were collected from three regions throughout the Hauraki Gulf, New Zealand, over two temperature regimes (warm and cool seasons), as well as 20 samples of opportunistically collected Bryde's whale scat. Multi-locus DNA barcode libraries were constructed from 18S and COI gene fragments, representing a trade-off between identification and resolution of metazoan taxa. Zooplankton community OTU occurrence and relative read abundance showed regional and seasonal differences based on permutational analyses of variance in both DNA barcodes, with significant changes in biodiversity indices linked to season in COI only. In contrast, we did not find evidence that Bryde's whale diet shows seasonal or regional trends, but instead indicated clear prey preferences for krill-like crustaceans, copepods, salps and ray-finned fishes independent of prey availability. The year-round presence of Bryde's whales in the Hauraki Gulf is likely associated with the patterns of distribution and abundance of these key prey items.

Effects of temperature on fecundity in vitro, egg hatching and reproductive development of Benedenia seriolae and Zeuxapta seriolae (Monogenea) parasitic on yellowtail kingfish Seriola lalandi.

Globally, aquaculture industries involved with commercial culture of kingfish (Seriola spp.) experience outbreaks of monogenean parasites, which can cause heavy stock losses. In Australia and New Zealand, aquaculturists of kingfish Seriola lalandi incur financial losses caused by two monogenean species: Benedenia seriolae and Zeuxapta seriolae which parasitise the skin and gills, respectively. This study provides information on some basic temperature-dependent life-cycle parameters of these problematic monogeneans on S. lalandi. Hatching times and age at maturity were inversely related to water temperature within the range experienced by wild kingfish in New Zealand (13-21 degrees C). Mature B. seriolae in vitro laid on average 37 eggs/day that hatched over approximately 4 days; peak hatching occurred 9, 11 and 22 days post-deposition at temperatures of 21, 17.5 and 13+/-1.0 degrees C, respectively. Z. seriolae in vitro laid on average 246 eggs/day that hatched over 2 days; peak hatching occurred 7, 9 and 15 days post-deposition at these respective temperatures. B. seriolae matured within 20, 25 and 48 days p.i. at 21, 18 and 13 degrees C. Z. seriolae matured within 25, 37 and >52 days p.i. at the same temperatures. This research describes stages in the reproductive development of B. seriolae and Z. seriolae and discusses the inclusion of basic parasitic life-cycle parameters into management strategies designed to maximise treatment efficacy and limit monogenean epizootics in sea-cage kingfish culture.

Mechanical Resistance to Shear Stress: The Role of Echinoderm Egg Extracellular Layers.

Extracellular layers (jelly coats) on echinoderm eggs are composed of a fibrous network imbedded in a gelatinous material. This type of fibrous network has the potential to protect eggs from mechanical stress. To determine the effects of shear stress and the role of jelly coats in protecting eggs from these stresses, eggs of the sea urchin Lytechinus variegatus, both with and without intact jelly coats, were exposed to shear stresses ranging from 0.3 to 2 Pa in a cone and plate viscometer. The percentage of eggs remaining intact after exposure to the shear stress was assessed. The results indicate that shear stress can damage eggs and that jelly coats may play a role in decreasing the effects of these stresses. Eggs with jelly coats remained intact and fertilizable at greater shear stresses than those with the coats removed. This is the first evidence that extracellular layers on invertebrate eggs can provide protection from mechanical forces.

Are Echinoderm Egg Size Distributions Bimodal?

Marine invertebrates can be categorized into species that reproduce by producing either large numbers of small, energetically inexpensive eggs that become planktotrophic larvae, or fewer, larger eggs with more yolk and lecithotrophic development (1). The selective advantages of these alternative strategies were considered in a series of simple mathematical models by Vance (2, 3). These models predicted that intermediate egg sizes should have lower reproductive efficiency, and that only extreme egg sizes should be evolutionarily stable (2, 3). Specifytally, Vance's models (2, 3) and later modifications (4-7), predict that eggs of marine invertebrates should have bimodal size distributions, rejecting the contrast between small egg/feeding and large egg/nonfeeding modes of development and the selection against intermediate egg sizes. Evidence for bimodality in egg size distributions is, however, equivocal, with unimodal distributions seen in the majority of comparative studies that are appropriate tests of the hypothesis (8-13). Bimodal distributions have been described only in a few groups of molluscs (4) and asteroid and echinoid echinoderms (14). Here we test the prediction of bimodality in the holothuroid and ophiuroid echinoderms and show that although the natural log-transformed egg size distributions are visually unimodal, the holothurian egg size distribution is statistically composed of two discrete modes. Moreover, reexamination of the asteroid and echinoid egg size distributions (14) with the addition of data from more recent literature confirms that there are two statistical modes in the egg size distributions of these classes. Thus, in the phylum Echinodermata, there is a bimodal egg size distribution three of the four classes in which this prediction can tested.

Ovarian Development in the Class Holothuroidea: a Reassessment of the "Tubule Recruitment Model".

The "tubule recruitment model" for the development of the holothurian gonad was proposed (a) to connect the stages of oogenesis with ovarian morphology in holothurians throughout the reproductive season and (b) to emphasize the potential for the holothurian ovary as a model system for cytological and biochemical study of echinoderm oogenesis. To reassess the evidence for this model, we have examined published accounts and unpublished observations on gonad development in holothurians from both temperate and tropical habitats, in shallow water and in the deep sea. A very limited number of species were found to conform to the predictions of the tubule recruitment model. The patterns of gonad development vary substantially in holothurians, even at the individual level, and with taxonomic position, geographical location, and habitat. The tubule recruitment model can be applied to only a small subset of holothurians, specifically those in the families Stichopodidae and Holothuriidae that have gonad morphology similar to that of Parastichopus californicus. However, the tubule recruitment model is invalid for many other aspidochirotes, and does not have wider applicability within the class Holothuroidea.

Mortality of Pentactulae During Intraovarian Brooding in the Apodid Sea Cucumber Leptosynapta clarki.

Estimates of mortality during development in brooders have been restricted by the lack of a reliable method for determining the number of embryos at the start of brooding. The calcareous ring of the pentactulae of Leptosynapta clarki can be used to determine initial embryo number because the ring is present for about 93% of the brooding period, is retained within the ovary, and is not resorbed after embryo death. Pentactulae mortality was found to be variable in female Leptosynapta clarki, with up to 100% mortality (= total brood loss) in some females. Estimates of instantaneous mortality in L. clarki ranged from 0 to -0.0114 d-1 and were of similar magnitude to those for other brooding marine invertebrates. The resorption of dead pentactulae in the ovary may provide additional material to the ovarian fluid for the nutrition of living pentactulae. The retention of hard embryonic structures (e.g., calcareous parts, shells) in other brooding marine invertebrates may allow further estimates of mortality during brooding, and provide an insight into the importance of embryo mortality in the evolution of marine invertebrate life histories.

Small Size, Brooding, and Protandry in the Apodid Sea Cucumber Leptosynapta clarki.

The apodid sea cucumber Leptosynapta clarki exhibits the three commonly associated traits of small adult size (max. length 113 mm), brooding (intraovarian = viviparity), and hermaphroditism (protandric). Juvenile L. clarki are released from the ovary at a length of 1-2 mm in the early spring (April-May) and are reproductively active as males in the reproductive season (November) following their birth. In their second year, some individuals continue to reproduce as males, but others undergo protandric sex change to reproduce as females. Analysis of the relationship between size and sex revealed a "critical" size for sex change at a weight of 200-400 mg with a 1:1 sex ratio above 500 mg total weight. Transitional gonads with previtellogenic oocytes and mature spermatozoa were observed, suggesting that sex change is initiated prior to reproducing in the current reproductive season. A test of the allometric hypothesis on the association between small size and brooding found no evidence for scaling constraints on brood size in L. clarki. These allometric constraints may be avoided because of potentially low fertilization success and brooding within a distensible structure. The sequential hermaphroditism in L. clarki may additionally be a method to reduce inbreeding in a species with limited dispersal.

Kinetics of Fertilization in the Sea Urchin Strongylocentrotus franciscanus: Interaction of Gamete Dilution, Age, and Contact Time.

Determining fertilization success of free spawning organisms in the field requires knowledge of how eggs and sperm interact under varying encounter frequencies and durations. In the laboratory, we investigated the relative influence of sperm concentration, egg concentration, sperm-egg contact time, and sperm age on fertilization in the sea urchin Strongylocentrotus franciscanus. Our results indicated that sperm concentration, sperm-egg contact time, sperm age, and individual variability were sequentially the most important factors influencing fertilization success. Egg concentration was not significant over the range tested. A theoretical model of fertilization (Vogel-Czihak-Chang-Wolf model) was used to estimate the two rate constants of fertilization kinetics: the rate constant of sperm-egg encounter and rate constant of fertilization. This model explained 91% of the variation in fertilization success, provided estimates of the rate constants involved in fertilization, and indicated the proportion (3%) of sperm-egg contacts that result in fertilization. Estimates of sperm swimming velocity and egg diameter were used to independently calculate the rate of sperm-egg encounter and confirm the predictions of the model. This model also predicts the non-significant effect of egg concentration on fertilization success found empirically.