Successful Larval Development Research Articles

Assessing concrete nest boxes for cavity-nesting bees

AbstractArtificial nest boxes for solitary bees and other cavity-nesting Hymenoptera are increasingly used for a variety of purposes, including ecological research, crop pollination support and public outreach. Their attractivity and colonization success by cavity-nesting solitary bees depend on their design and placement, including hole dimensions, orientation and the neighboring habitats and available resources. While most bee nest boxes are made of wooden materials, we assessed here the suitability of perennial, concrete nest boxes for cavity-nesting bees. We carried out a three-year nesting survey of 52 custom-made nest boxes located in 11 different sites throughout France and totaling 2912 available holes of 6, 8, 10 or 12 mm in diameter. Concrete nest boxes successfully attracted reproductive females of solitary bee species and supported successful larval development until the emergence of new individuals. Preferred cavities were the smallest ones (6-8 mm), located at the lowest tested positions above ground (31-47 cm) and oriented southward. Local bee populations established in nest boxes steadily increased throughout the three successive seasons in nearly all study sites. The cavity-nesting bee communities were mostly composed of rather common and generalist species, but also comprised a foraging specialist. Additionally, two cleptoparasitic bee species were detected. All species belonged to the Megachilidae. We further discuss the effects of neighboring urban and natural habitats as potential source or sink of nesting bees, as well as opportunities of concrete nest boxes as tools for urban agriculture and more generally for the new biomimetic urban designs to restore local ecosystem services in cities.

Dragonflies from hot springs in Russia with a country-level checklist of species known to occur in geothermal environments

Geothermal springs are known to harbor unusual assemblages of dragonflies and damselflies worldwide. A review of original records and the body of available literature revealed that 27 Odonata species were recorded from hot springs in Russia so far and that the successful larval development in geothermal environments was discovered for 17 species. Among them, four species exclusively inhabit hot springs, i.e. Mnais costalis, Anotogaster sieboldii, Orthetrum melania (Kunashir Island), and O. albistylum (Eastern Siberia). In Russia, these southern species are unable to develop beyond geothermally heated water bodies due to cold climate, and they exist as local geothermal populations there. Here, we report on several novel records of Odonata species from geothermal springs in eastern Russia. Four species were recorded on the Kunashir Island (Kurile Archipelago): Mnais costalis (adults), Anotogaster sieboldii (adults and larvae), Orthetrum melania (adults), and Sympetrum pedemontanum elatum (adults and larvae). Two species were found in the Kamchatka Peninsula, i.e. Libellula quadrimaculata (freshly emerged imago and exuvia) and Aeshna juncea (larvae). To explain the origin of isolated geothermal populations of Odonata, three alternative hypotheses can be proposed as follows: (1) pre-glacial relicts; (2) mid-Holocene relicts (since the Holocene Climate Optimum); and (3) recent (late Holocene) populations founded by long-distance dispersal events. These scenarios are yet to be tested by means of a molecular approach.

Exploring larval phenology as predictor for range expansion in an invasive species

Predicting range expansion of invasive species is one of the key challenges in ecology. We modelled the phenological window for successful larval release and development (WLR) in order to predict poleward expansion of the invasive crab Hemigrapsus sanguineus along the Atlantic coast of North America and north Europe. WLR quantifies the number of opportunities (in days) when larval release leads to a successful completion of the larval phase; WLR depends on the effects of temperature on the duration of larval development and survival. Successful larval development is a necessary requirement for the establishment of self‐persistent local populations. WLR was computed from a mechanistic model, based on in situ temperature time series and a laboratory–calibrated curve predicting duration of larval development from temperature. As a validation step, we checked that model predictions of the time of larval settlement matched observations from the field for our local population (Helgoland, North Sea). We then applied our model to the North American shores because larvae from our European population showed, in the laboratory, similar responses to temperature to those of a North American population. WLR correctly predicted the northern distribution limit in North American shores, where the poleward expansion of H. sanguineus appear to have stalled (as of 2015). For north Europe, where H. sanguineus is a recent invader, WLR predicted ample room for poleward expansion towards NE England and S Norway. We also explored the importance of year‐to‐year variation in temperature for WLR and potential expansion: variations in WLR highlighted the role of heat waves as likely promoters of recruitment subsidising sink populations located at the distribution limits. Overall, phenological windows may be used as a part of a warning system enabling more targeted programs for monitoring.

Symbiosis disruption in the olive fruit fly, Bactrocera oleae (Rossi), as a potential tool for sustainable control.

The olive fruit fly Bactrocera oleae (Rossi) (OLF) is a major agricultural pest, whose control primarily relies on the use of chemical insecticides. Therefore, development of sustainable control strategies is highly desirable. The primary endosymbiotic bacterium of OLF, 'Candidatus Erwinia dacicola', is essential for successful larval development in unripe olive fruits. Therefore, targeting this endosymbiont with antimicrobial compounds may result in OLF fitness reduction and may exert control on natural populations of OLF. Here, we evaluate the impact of compounds with antimicrobial activity on the OLF endosymbiont. Copper oxychloride (CO) and the fungal metabolite viridiol (Vi), produced by Trichoderma spp., were used. Laboratory bioassays were carried out to assess the effect of oral administration of these compounds on OLF fitness and molecular analyses (quantitative polymerase chain reaction) were conducted to measure the load of OLF-associated microorganisms in treated flies. CO and Vi were both able to disrupt the symbiotic association between OLF and its symbiotic bacteria, determining a significant reduction in the endosymbiont and gut microbiota load as well as a decrease in OLF fitness. CO had a direct negative effect on OLF adults. Conversely, exposure to Vi significantly undermined larval development of the treated female's progeny but did not show any toxicity in OLF adults. These results provide new insights into the symbiotic control of OLF and pave the way for the development of more sustainable strategies of pest control based on the use of natural compounds with antimicrobial activity. © 2020 Society of Chemical Industry.

Complexity within an oil palm monoculture: The effects of habitat variability and rainfall on adult dragonfly (Odonata) communities

AbstractRecent expansion of oil palm agriculture has resulted in loss of forest habitat and forest‐dependent species. However, large numbers of species—particularly insects—can persist within plantations. This study focuses on Odonata (dragonflies and damselflies): a charismatic indicator taxon and a potentially valuable pest control agent. We surveyed adult Odonata populations biannually over three years within an industrial oil palm plantation in Sumatra, Indonesia. We assessed the effects of rainfall (including an El Niño Southern Oscillation‐associated drought), the role of roadside ditches, and the importance of understory vegetation on Odonata populations. To assess the impacts of vegetation, we took advantage of a long‐term vegetation management experiment that is part of the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Programme. We found 41 Odonata species, and communities varied between plantation core and roadside edge microhabitats, and between seasons. Abundance was significantly related to rainfall levels four months before surveys, probably indicating the importance of high water levels in roadside ditches for successful larval development. We found no significant effect of the BEFTA understory vegetation treatments on Odonata abundance, and only limited effects on community composition, suggesting that local understory vegetation structure plays a relatively unimportant role in determining communities. Our findings highlight that there are large numbers of Odonata species present within oil palm plantations and suggest that their abundance could potentially be increased by maintaining or establishing waterbodies. As Odonata are predators, this could bring pest control benefits, in addition to enhancing biodiversity within intensive agricultural landscapes.Abstract in Indonesian is available with online material.

Seasonal windows of opportunity in milkweed-monarch interactions.

Many organisms experience seasonal windows of opportunity for growth and reproduction. These windows represent intervals in time when organisms experience improved prospects for advancing their life history objectives, constrained by the combined effects of seasonally variable biotic and abiotic conditions acting independently or in combination. Although seasonal windows of opportunity are likely to be widespread in nature, relatively few studies have conducted the repeated observations necessary to identify them or suggest the factors that structure them in time. Here, we present the results of three experimental studies conducted at different field sites in three different years in which we manipulated the phenology of monarch caterpillars (Danaus plexippus) throughout the growing season. The primary aims of these experiments were (1) to identify seasonal windows of opportunity for successful larval development on milkweed (Asclepias spp.), and (2) to suggest which factors are most likely to constrain these windows of opportunity in time. We found strong seasonal windows of opportunity in the developmental success of monarchs, with distinct periods of higher developmental prospects during each study year. We evaluated the role of seasonal variation in abiotic thermal stress, host plant density, host plant defensive traits, and natural enemy risk as potential factors that may limit seasonal windows of opportunity. By comparing the seasonal patterns of larval success and potential explanatory factors across all 3 yr, we find patterns that are consistent with seasonally variable abiotic conditions, host plant availability, host plant traits, and natural enemy risk factors. These results suggest the potential for seasonal variation in the factors that limit monarch larval development and population growth. More generally, this study also highlights the value of temporally explicit experimental studies that can identify and examine seasonal patterns in species interactions.

The effects of in-vitro pH decrease on the gametogenesis of the red tree coral, Primnoa pacifica.

Primnoa pacifica is the most ecologically important coral species in the North Pacific Ocean and provides important habitat for commercially important fish and invertebrates. Ocean acidification (OA) is more rapidly increasing in high-latitude seas because anthropogenic CO2 uptake is greater in these regions. This is due to the solubility of CO2 in cold water and the reduced buffering capacity and low alkalinity of colder waters. Primnoa pacifica colonies were cultured for six to nine months in either pH 7.55 (predicted Year 2100 pH levels) or pH 7.75 (Control). Oocyte development and fecundity in females, and spermatocyst stages in males were measured to assess the effects of pH on gametogenesis. Oocyte diameters were 13.6% smaller and fecundities were 30.9% lower in the Year 2100 samples. A higher proportion of vitellogenic oocytes (65%) were also reabsorbed (oosorption) in the Year 2100 treatment. Lower pH appeared to advance the process of spermatogenesis with a higher percentage of later stage sperm compared to Control. There was a laboratory effect observed in all measurement types, however this only significantly affected the analyses of spermatogenesis. Based on the negative effect of acidification on oogenesis and increased rate of oosorption, successful spawning could be unlikely in an acidified ocean. If female gametes were spawned, they are likely to be insufficiently equipped to develop normally, based on the decreased overall size and therefore subsequent limited amount of lipids necessary for successful larval development.

Seasonality overrides differences in butterfly species composition between natural and anthropogenic forest habitats

AbstractAnthropogenic disturbances that alter habitat structures can lead to strong changes in species composition. The importance of certain habitat features for structuring species composition, however, may be determined by the availability of resources that can seasonally fluctuate. The Kenyan Arabuko Sokoke coastal forest consists of three pristine forest types and provides various anthropogenic forest‐like habitats, such as plantations and transition zones between forest and agricultural land. In parallel, this region experiences pronounced climatic seasonality which causes differences in resource availability. To test for potential differences in butterfly communities between natural and anthropogenic forest habitats, and whether they vary seasonally, we assessed butterfly species along transects in all five habitat types during the rainy and the consecutive dry season. We compiled ecological characteristics for each transect and for each butterfly species. We found highest species richness and abundances in the three natural forest habitats during the rainy season. Our data show distinct species clusters for each habitat type during the rainy season. In contrast, butterfly species composition does not form distinct community groups during the dry season. During this time period, highest species richness and abundances are found in plantations. These temporal differences in species richness, abundance and species composition between the two seasons are assumed to be driven by limited nectar availability inside of the forest during the dry season. This situation might lead to increased mobility and the invasion of anthropogenic habitats, where nectar sources are still available during the dry season. Thus, plantations might represent a surrogate habitat for forest butterflies during their imaginal stage, but do not represent a suitable habitat providing all resources needed for successful larval development.

Spatial variation in pollinator gall failure within figs of the gynodioecious Ficus hirta

Figs, the inflorescences of Ficus species (Moraceae), contain numerous uni-ovulate flowers. Male trees of gynodioecious Ficus have figs that support development of pollinator fig wasp offspring (Agaonidae) and rarely produce seeds. Pollinator larvae develop inside galled ovules that expand rapidly after eggs are laid to fill the available space. Galls that fail to support successful larval development can be abundant and failures may influence oviposition behavior and modify realized offspring sex ratios. We examined pollinator reproductive success in figs of the Asian Ficus hirta where we had allowed entry by either one or two foundresses and prevented attack by parasitoids. At the developmental stage when adult offspring were about to emerge from their galls, we recorded where in the figs their galls were located, the distributions of sons and daughters in the galls and whether galls that developed closest to the periphery of the figs were more likely to fail. Foundress number had an effect on gall location, but not total offspring numbers. No spatial variation in the distribution of male and female adult offspring was detected. Overall, over 25% of the galled ovaries failed to support offspring development, and failure rates were independent of foundress number. More peripheral galls were more likely to fail in figs entered by two foundresses. Gall location in gynodioecious figs is determined largely by the extent to which their basal pedicels expand after galling. Competition for nutrients between galls, with those developing shorter pedicels being at a disadvantage, may explain the results. If pedicel length is related to timing of oviposition, then pollinator eggs laid later are less likely to survive.

Photophase Duration Affects Immature Black Soldier Fly (Diptera: Stratiomyidae) Development.

This study tested the effect of photophase duration on black soldier fly, Hermetia illucens (L.; Diptera: Stratiomyidae), development. Successful larval eclosion, development time and adult emergence were measured for individuals exposed to 0 h, 8 h, and 12 h of light, at approximately 27°C and 70% relative humidity. Accumulated degree hours (ADH) were calculated to correct for differences in temperature across treatments. Larvae successfully eclosed in all treatments, with larvae in 12 h light requiring 5.77% and 4.5% fewer ADH to eclose than larvae in 0 h and 8 h, respectively. Overall, larvae in 0 h required 39.34% and 37.78% more ADH to complete their development from egg to adult than larvae in 8 h and 12 h, respectively. The effect of photophase duration on juvenile development was largest in the post-feeding stage, and smallest in the pupal stage. Specifically, post-feeding larvae in 0 h required 80.02% and 90.08% more ADH to pupate than larvae in 8 h and 12 h, respectively, but pupae in 8 h required 9.63% and 7.52% fewer ADH to eclose than pupae in 0 h and 12 h, respectively. Lastly, larval mortality was significantly higher in 0 h, with 72% survivorship, and 96% and 97% in 8 h and 12 h, respectively. However, 17.8% of mortality in the absence of light is hypothesized to be a result of predation by Arachnidae and Blattidae. These data could prove valuable for optimizing industrial processes for mass-production of this species for use as alternative protein in feed for livestock, poultry, and aquaculture.

Influence of maize kernel state and type on life history of Plodia interpunctella (Lepidoptera: Pyralidae)

The objective of this study was to investigate the influence of different mechanical states (whole, broken and ground kernels) and types (dent, semi-flint and flint) of maize kernels on life history parameters of Plodia interpunctella (Hübner), the Indian meal moth. These parameters included larval development and its dynamics, adult emergence, mean development duration (MDD) and fecundity. Since the larvae are the developmental stage that causes the most damage on maize, additional data on larval development helps identify conditions that promote development. The results of our study indicate that broken maize kernels are the most suitable for P. interpunctella development, where the most successful larval development, the highest number of emerged adults, the shortest MDD and the highest total fecundity were recorded. On the contrary, whole kernels were the least suitable and the most resistant to infestation by P. interpunctella. The type of kernel also significantly affects P. interpunctella developmental parameters. Females reared on flint kernels (the hardest kernel type tested in our study) laid the largest total number of eggs, which implies that kernel hardness, i.e. the type of kernel should be an important parameter when choosing maize hybrids for cultivation and for storage. Our study warrants further investigation of maize types and their susceptibility to P. interpunctella.

Effect of salinity on the embryonic development of Macrobrachium acanthurus (Decapoda: Palaemonidae)

The effect of salinity level on the embryonic development of Macrobrachium acanthurus was analyzed under laboratory conditions, considering characteristics of the egg (size, volume, and water content) and of the embryo (eye index). The experimental design was completely randomized, with five repetitions (ovigerous females) per treatment (0, 10, 17 and 20 ppt). During embryonic development, two eggs per female were taken daily for analyses of size, volume, water content, and eye index. Our results showed that salinity of 20 ppt leads to death and/or abortion of the embryo in all females. The size, volume and water content of eggs increased according to embryonic development, providing space in the egg for formation and organization of embryo. Salinity affected these egg characteristics, causing water loss to the hypertonic medium. Neither the duration of embryonic development nor embryo formation were affected by saltwater content. The results of the present study indicate that ovigerous females of M. acanthurus can survive in freshwater rivers as well as in low-salinity environments during incubation period and the successful larval development is not likely to rely on female migrating to estuaries. Larvae can easily be incubated in freshwater and complete development at higher salinities after hatch.

Specialization in the yucca-yucca moth obligate pollination mutualism: A role for antagonism?

Specialized brood pollination systems involve both mutualism and antagonism in the overall interaction and have led to diversification in both plants and insects. Although largely known for mutualism, the role of the antagonistic side of the interaction in these systems has been overlooked. Specialization may be driven by plant defenses to feeding by the insect larvae that consume and kill developing plant ovules. The interaction among yuccas and yucca moths is cited as a classic example of the importance of mutualism in specialization and diversification. Pollinators moths are very host specific, but whether this specificity is due to adult pollination ability or larval feeding ability is unclear. Here, I test the potential role of antagonism in driving specialization among yuccas and yucca moths. I examined the ability of the most-polyphagous yucca moth pollinator, Tegeticula yuccasella, to pollinate and develop on five Yucca species used across its range. Yucca species endemic to the Great Plains and Texas were transplanted to a common garden in Syracuse, New York and exposed to the local pollinator moth population over 3 years. Local moths visited all but one of the Yucca species, but had drastically lower rates of successful larval development on non-natal Yucca species in comparison to the local host species. Specialization in many brood pollination systems may be strongly influenced by the antagonistic rather than the mutualistic side of the overall interaction, suggesting that antagonistic coevolution is a possible source of diversification.

Importance of the natural flow regime to an amphidromous shrimp: a case study

Amphidromous shrimp inhabiting large river systems are thought to migrate downstream to hatch larvae close to the estuary or sea. Recent work on Macrobrachium spinipes in northern Australia challenged this hypothesis, with the discovery of abundant females in reproductive condition over 400km upstream. We investigated the early life history of M. spinipes by using a combination of field sampling, laboratory experiments and modelling. We determined the spatial and temporal production of larvae, larval salinity requirements, and, finally, the potential for transport of larvae to the estuary during the wet season. Larvae occurred in high abundances across a 400-km river reach. However, successful larval development was contingent on access, by drift, to saltwater within 7 days. This interval is longer than that found for the majority of amphidromous shrimp species and we propose that larvae in the study river are able to successfully moult through the early stages in freshwater. Large, wet-season discharge events were critical for transporting larvae from all sampled sites to the estuary. The present study confirmed the amphidromous life history of M. spinipes, highlighted the importance of natural flows for maintaining existing populations and provided evidence of a novel type of amphidromous life-history strategy.

Vegetation and climate determine ant-mound occupancy by a declining herbivorous insect in grasslands

There is considerable research on the effects of soil-disturbing ecosystem engineers in semi-natural Central European grasslands in terms of plant species diversity, soil properties and soil microorganisms. However, knowledge on the importance of such ecosystem engineers for animal diversity is still relatively rare. In this study we analyse the role of Lasius flavus nest mounds as habitats for larvae of the declining burnet moth species Zygaena purpuralis in calcareous grasslands. We assumed that anthills act as preferred larval habitats in dependence on climate or vegetation structure. The analysis considers a macroclimatic gradient by comparing sites with differing elevation and, hence, local climates. Additionally, we analysed the importance of anthills for Z. purpuralis along a microclimatic gradient by comparing sites with different vegetation structures. The study clearly shows that anthills are important larval habitats when general microclimatic conditions in the grasslands are unfavourable due to a cool climate at higher elevation or dense vegetation. The likelihood of detecting larvae of the burnet moth species on sites where anthills were occupied was higher on the mounds than within the surrounding vegetation. Anthills were preferred as larval habitats by Z. purpuralis as they combine two essential elements for successful larval development: (i) a favourable microclimate thanks to an open vegetation structure and (ii) a sufficient amount of food. The study underlines the importance of L. flavus as an ecosystem engineer for maintaining habitat heterogeneity and biodiversity within semi-natural grasslands.

Leaf traits influencing oviposition preference and larval performance of Cameraria ohridella on native and novel host plants

AbstractLeaf traits are known to influence the preference and performance of phytophagous insects. Traits that influence oviposition can differ from traits that favour larval development, but in native hosts the association between traits usually leads to positive preference‐performance relationships. However, when herbivores interact with novel hosts, traits that influence oviposition and successful larval development can become decoupled, leading to poor preference‐performance relationships. We investigated traits associated with preference and performance of the horse‐chestnut leaf miner, Cameraria ohridella Deschka & Dimic (Lepidoptera: Gracillariidae), on its native host, Aesculus hippocastanum L. (Sapindaceae), and on other species of Aesculus that are novel hosts for the leaf miner. Cameraria ohridella laid eggs on all species of Aesculus, but showed preferences for A. hippocastanum and the phylogenetically closely related Japanese horse‐chestnut, Aesculus turbinata Blume. Successful mine development, however, was observed only on A. hippocastanum and A. turbinata, which indicates a partial mismatch between female oviposition and larval performance. The two susceptible Aesculus species were very similar in their leaf traits, which might explain why larvae performed equally well on these species. In contrast, species resistant to C. ohridella [Aesculus chinensis Bunge, Aesculus indica (Camb) Hook, and Aesculus flava Aiton] showed considerable variation in leaf traits, which suggests that different species of Aesculus might have different mechanisms of resistance to C. ohridella, supporting the idea of different defensive syndromes in different species of plants.

Molehills as important larval habitats for the grizzled skipper, Pyrgus malvae (Lepidoptera: Hesperiidae), in calcareous grasslands

Small-scale soil disturbance, such as animal mounds created by subterraneous ecosystem engineers are important microhabitats for species of conservation concern. There are many studies on the effects of ecosystem engineers on plant species diversity and soil properties, but the influence of such organisms on other taxa, such as insects, are rarely analyzed. In this paper, we demonstrate for the first time the role of molehills produced by the European Mole (Talpa europaea) (Linnaeus, 1758) as a larval habitat for a threatened butterfly species within central European calcareous grasslands. We selected the grizzled skipper (Pyrgus malvae) (Linnaeus, 1758) as a study species as previous studies revealed that it prefers warm and open microhabitats for oviposition. Our study clearly showed that host plants occurring on or in the close vicinity of molehills were regularly occupied by immature stages of P. malvae. Occupied host plants at these disturbed sites were characterized by a more open vegetation structure with a higher proportion of bare ground or stones and a lower cover of the herb layer compared with available host plants (control samples). Among molehills those were preferred that had the highest cover of host plants, especially Agrimonia eupatoria. Molehills are important larval habitats for P. malvae in calcareous grasslands on deeper soils. At these sites succession occurs rapidly and the amount of bare ground is usually low, even when they are managed. In contrast to other known breeding sites in calcareous grasslands, molehills with a high cover of host plants provide ideal conditions for the successful larval development of P. malvae: (i) sufficient food during the larval period and (ii) warm microclimatic conditions.

Host Specificity ofHypena opulenta: A Potential Biological Control Agent ofVincetoxicumin North America

A biological control program has been initiated against the European swallow-worts Vincetoxicum nigrum (L.) Moench and V. rossicum (Kleopow) Barbar. (Family Apocynaceae) that have become invasive in North America. The leaf-feeding moth, Hypena opulenta Christoph (Lepidoptera: Erebidae), originating from eastern Europe, has been under measurement as a potential biological control agent of swallow-worts since 2006. In this study we measured the host range of H. opulenta by screening 82 potential host plant species for larval development under no-choice conditions. In addition, we also monitored female fecundity, longevity, and oviposition preference among suitable larval hosts. Successful larval development occurs only on Vincetoxicum spp. Partial larval development by one larva was observed on Boehmeria cyclindrica (L.) Sw. (Urticaceae) to the final instar, but this individual failed to pupate. Exploratory feeding occurred on Gonolobus stephanotrichus Griseb. (Apocynaceae) and Urtica dioica L. (Urticaceae), but all larvae failed to develop past the first and second instar, respectively. Additional testing with mature larvae on a subset of the plant species demonstrates that no species outside the genus Vincetoxicum are suitable for complete larval development of H. opulenta. The longevity and fecundity of females raised on each target weed are similar and gravid females do not display an oviposition preference among Vincetoxicum spp. Hypena opulenta does not present a risk to any native plant species or species of economic importance in North America. Petitions have been submitted for experimental open-field releases of H. opulenta in the United States and Canada.

Biochemical compounds’ dynamics during larval development of the carpet-shell clam Ruditapes decussatus (Linnaeus, 1758): effects of mono-specific diets and starvation

Successful larval growth and development of bivalves depend on energy derived from internal (endotrophic phase) and external (exotrophic phase) sources. The present paper studies survival, growth and biochemical changes in the early developmental stages (from egg to pediveliger) of the clam Ruditapes decussatus in order to characterize the nutritional requirements and the transition from the endotrophic to the exotrophic phase. Three different feeding regimes were applied: starvation and two mono-specific microalgal diets (Isochrysis aff galbana and Chaetoceros calcitrans). A comparison between fed and unfed larvae highlighted the importance of egg lipid reserves, especially neutral lipids, during a brief endotrophic phase of embryonic development (first 2 days after fertilization). Egg reserves, however, may energetically contribute to the maintenance of larvae beyond the embryonic development. In fed larvae, the endotrophic phase is followed by a mixotrophic phase extending to days 5–8 after fertilization and a subsequent exotrophic phase. Metamorphosis starts around day 20. The intense embryonic activities are supported by energy derived from lipids, mainly from neutral lipids, and the metamorphic activities are supported by energy derived essentially from proteins accumulated during the planktonic phase and depend on the nutritional value of diets. The diet of I. aff galbana proves to be more adequate to R. decussatus larval rearing. The results provide useful information for the successful production of R. decussatus aquaculture.

Energetic changes throughout lecithotrophic larval development in the deep-sea lithodid crab Paralomis spinosissima from the Southern Ocean

Adult populations of the lithodid crab Paralomis spinosissima thrive at the lower temperature boundary of about 1 °C for the Lithodidae, and are endemic to deep waters off South Georgia, Southern Ocean. Here, we present evidence for lecithotrophic development throughout the early ontogeny of this species over a period of 14 months. At constant 5 °C (± 0.5), development of the Zoea I and II, the megalopa, and the crab I instar lasted about 12, 24, 108, and 290 days, respectively. Successful development and moult at lower temperatures could not be observed. Changes in biomass and elemental composition ( W, C, and N) were studied throughout larval and early megalopa development. Initial biomass was about twice as high as in lecithotrophic larval developments previously reported for subantarctic lithodids, and only a minimal shift (9.2 to 8.9) in C:N mass ratio was observed from the Zoea I to the megalopa stage; at the same time an even decrease of both C and N indicated no bias toward the depletion of lipids. Very thin exuviae cast at moult support the view of energy-saving adaptations throughout larval development. The cost of higher maternal investment into single offspring and the lack of evidence for successful larval development at temperatures typical of Southern Ocean deep waters, pose questions with regard to the frequency of successful recruitment events of lithodid crabs at their lower physiological temperature tolerance. Significant intraspecific variability in larval energetic fitness at hatching between females, suggesting variability in offspring fitness, may additionally constrain on offspring survival.