Late Juvenile Stages Research Articles

Whole-body replacement of larval myofibers generates permanent adult myofibers in zebrafish

Drastic increases in myofiber number and size are essential to support vertebrate post-embryonic growth. However, the collective cellular behaviors that enable these increases have remained elusive. Here, we created the palmuscle myofiber tagging and tracking system for in toto monitoring of the growth and fates of ~5000 fast myofibers in developing zebrafish larvae. Through live tracking of individual myofibers within the same individuals over extended periods, we found that many larval myofibers readily dissolved during development, enabling the on-site addition of new and more myofibers. Remarkably, whole-body surveillance of multicolor-barcoded myofibers further unveiled a gradual yet extensive elimination of larval myofiber populations, resulting in near-total replacement by late juvenile stages. The subsequently emerging adult myofibers are not only long-lasting, but also morphologically and functionally distinct from the larval populations. Furthermore, we determined that the elimination-replacement process is dependent on and driven by the autophagy pathway. Altogether, we propose that the whole-body replacement of larval myofibers is an inherent yet previously unnoticed process driving organismic muscle growth during vertebrate post-embryonic development.

Rearing performance of juvenile yellowtail snapper, Ocyurus chrysurus, in a sea water recirculation system at two different stocking densities

AbstractEvaluation of viability of new marine species in aquaculture systems is important to reduce negative social and environmental externalities, offering sustainable options for supply to fish markets. The objective was to describe the performance indicators of late juvenile yellowtail snapper (243.2 ± 57.1 mean weight of total fish) in an experimental recirculating aquaculture system (RAS) system supplied by seawater to determine its pre‐feasibility. Two densities were stocked, D1: 10 fish/tank (2.6 kg/m3, mean weight 258.3 ± 57 g) and D2: 20 fish/tank (4.7 kg/m3, mean weight 235.6 ± 55.9 g). Fish were fed to satiety with an extruded diet containing 46% crude protein and 12% crude lipid. After 238 days, survival rate was 93% for both densities, and mean weight and biomass were 534.5 ± 19.4 g and 5.7 kg/m3 for D1, and 606.9 ± 34.5 g and 10 kg/m3 for D2. Final feed conversion rate (3.13 for D1 and 3.03 for D2) did not differ significantly between densities. The exponents (b) of the length–weight relationship calculated for D1 (3.15) versus D2 (3.10) and condition factor (2.12 ± 0.07 for D1, and 2.03 ± 0.04 for D2) demonstrated a higher condition factor than those of their wild counterparts. This study is one of the few reports on this species’ performance in late juvenile stage in RAS.

Investigating the Applicability of Ichthyoplanktonic Indices in Better Understanding the Dynamics of the Northern Stock of the Population of Atlantic Hake Merluccius merluccius (L.)

Abundance indices are essential data for the application of stock assessment models to obtain fish abundance estimates. Abundance indices have usually been derived from fishery-dependent data, yet the increase in fisheries-independent surveys is now offering new opportunities for these calculations. In this study, we explored the usefulness of ichthyoplankton indices derived from scientific surveys in estimating spawning biomass. In addition, we also investigated whether the strength of the year–class of the commercial cohort of Atlantic hake, as a determinant, could be defined at an early life stage. We used samples collected during the triennial mackerel and horse mackerel egg surveys (MEGS), which cover the hake spawning area in the Bay of Biscay. The biomass indices were determined as the abundance of eggs in the early development stage (stage 1) when transformed into egg production (EP) from 1995 to 2019 in the months of March and April—which is considered a period of high spawning activity for hake in this area. Additionally, we built a metric for larval abundance and converted larval length into age. This was in addition to constructing a pre-recruit year-class index (YCI) while using the EVHOE bottom trawl abundance database for hake for the period of 1997 to 2016. The results of regression analysis of egg production and spawning stock biomass indicate that both parameters are significantly correlated (r = 0.76). By connecting the abundance of eggs and larvae in the adjoining stages, we are able to identify two periods of high mortality associated with the transition from “yolk-sac-first” to “feeding larvae” and “late larvae-YCI10”, but we were unable to discover when the strength of the recruitment year–class is determined. As such, it appears that for the northern stock of hake, recruitment is established in the late juvenile stages.

Exogenous myo-inositol increases salt tolerance and accelerates CAM induction in the early juvenile stage of the facultative halophyte Mesembryanthemum crystallinum but not in the late juvenile stage.

Mesembryanthemum crystallinum L. (ice plant) develops salt tolerance during the transition from the juvenile to the adult stage through progressive morphological, physiological, biochemical, and molecular changes. Myo-inositol is the precursor for the synthesis of compatible solute D-pinitol and promotes Na+ transport in ice plants. We previously showed that supplying myo-inositol to 9-day-old seedlings alleviates salt damage by coordinating the expression of genes involved in inositol synthesis and transport, affecting osmotic adjustment and the Na/K balance. In this study, we examined the effects of myo-inositol on physiological parameters and inositol-related gene expression in early- and late-stage juvenile plants. The addition of myo-inositol to salt-treated, hydroponically grown late juvenile plants had no significant effects on growth or photosynthesis. In contrast, supplying exogenous myo-inositol to salt-treated early juvenile plants increased leaf biomass, relative water content, and chlorophyll content and improved PSII activity and CO2 assimilation. The treatment combining high salt and myo-inositol synergistically induced the expression of myo-inositol phosphate synthase (INPS), myo-inositol O-methyltransferase (IMT), and inositol transporters (INTs), which modulated root-to-shoot Na/K ratio and increased leaf D-pinitol content. The results indicate that sufficient myo-inositol is a prerequisite for high salt tolerance in ice plant.

Knockout of microphthalmia-associated transcription factor (mitf) confers a red and yellow tilapia with few pigmented melanophores

As the most important genetic factor for melanophore differentiation and melanogenesis, mitf has been studied for over 30 years. The phenotypes of mitf mutants have also received continuous attention for nearly a century. Due to the third round of genome duplication, teleosts have two mitf genes. Mutation analysis demonstrates that mitfa plays an important role in zebrafish pigmentation. However, there have been no functional studies on body color regulation by mitfb or studies of mitfa;mitfb double mutants. In the present study, we mutated both mitf genes in tilapia using CRISPR/Cas9. Disruption of mitfa resulted in light yellow body color with weak gray vertical bars due to significantly reduced numbers of melanophores and increased sizes of xanthophores, while disruption of mitfb led to slight hypo-pigmentation. Double mutation of mitfa and mitfb resulted in dramatic hypo-pigmentation in trunk and fins due to loss of additional melanophores and increase in the number of iridophores compared to the single mutants, but had no influence on retinal pigment epithelium (RPE) pigmentation. The mitfa−/−;mitfb−/− mutants were yellow with black spots at early juvenile stage (60 dpf), yellow-reddish at late juvenile stage (120 dpf) and red and yellow (including iris) at adult stage (180 dpf), due to increased erythrophores and enlarged xanthophores. Our results demonstrated that both mitf genes are important for body color formation in tilapia, but mitfa plays a more important role than mitfb. Additionally, by comparing the phenotypes of the mitfa−/−;mitfb−/−, pmela−/−;pmelb−/− and hps4−/− mutants, we found that disruption of genes with different functions in different aspects of melanogenesis could lead to different body colors in tilapia. Mitf is the most important gene for melanophore differentiation and is probably also critical for differentiation of erythrophores, xanthophores and iridophores. To our knowledge, this is the first report showing that both mitf genes are involved in body color formation by loss of function study, and the first report showing that erythrophore numbers and xanthophore sizes were affected by mitf. Our research not only serves as a model for studying mitf function in tilapia and the closely related cichlids, but also provides new strategies for breeding red and yellow tilapia for aquaculture.

Impact of squid predation on juvenile fish survival

Predation is a major source of mortality during the early life stages of marine fishes; however, few studies have demonstrated its impact—especially that of squid predation—on survival processes. Here, we examined the feeding habits and predation impacts of swordtip squid on a major prey fish, juveniles of jack mackerel, in the East China Sea. Otoliths of the juveniles extracted from the squid stomach were used to reconstruct the age–length relationship and the growth trajectory of the consumed juveniles, and they were compared to those of juveniles collected with a net using a newly developed statistical framework. The juveniles consumed by squid had significantly shorter body lengths and smaller body sizes during the late larval and early juvenile stages than the netted juveniles, suggesting that smaller juveniles with slower growth rates have a higher probability to be selected. The body mass ratio of the predator squid to prey juveniles (predator–prey mass ratio, PPMR) ranged from 7 to 700, which was remarkably lower than the PPMR reported in various marine ecosystems based on analyses of fishes. Our findings demonstrate that squid predation can significantly impact the early life survival of fish and the trophodynamics in marine ecosystems.

Tank-Based Nursery Production of the Sea Cucumber Holothuria scabra in Various Seaweed Feed Regimens

This study explored the feasibility of Laurencia papillosa, Sargassum spp, and Gracilaria bailinae crude extracts as feeds for the juvenile sea cucumber Holothuria scabra in a tank-based system. Post-metamorphic juveniles (~1 mm) were harvested from larval tanks and were reared until the late juvenile stage in indoor tank systems at 300 ind./tank stocking density for 45 days. Growth and survival rates of H. scabra juveniles were monitored to assess the performance of each seaweed treatment. L. papillosa consistently yielded the best growth (SGR: 7.63-11.32%.d-1) and highest survival rates (54.6–56.8%) of H. scabra juveniles. The performance of L. papillosa did not differ from that of Sargassum spp., while G. bailinae yielded poor growth (SGR of 5.01–9.38%.d-1) and low survivorship of juveniles (8.77–19.77%). Growth and survival rates were similar between fresh and dried seaweeds (p>0.05), suggesting dried seaweeds' applicability as feeds for juvenile H. scabra. Moreover, increased feed rations resulted in better growth of juveniles, but survival rates among feed rations (4.5, 6.0, 7.5, and 9.0 L.d-1) did not differ significantly (p>0.05). The monthly mean water temperature ranged from 26.6°C to 28.8°C, salinity of 33.2–35.1 ppt, dissolved oxygen of 6.1–8.3 mg/L, and pH of 8.1–8.3. High yields of sea cucumber juveniles were observed during the dry season, while low survival rates (<20%) were observed during wet months. The success of this study could set forth the application of tank-based nursery systems for H. scabra, especially during the season of high rainfall and extreme weather conditions that heavily affects cage rearing in the open-sea setting.

A new wing skeleton of the Jehol tapejarid Sinopterus and its implications for ontogeny and paleoecology of the Tapejaridae

The tapejarid pterosaurs flourished in the Jehol Biota with an abundance of immature individuals and a rarity of individuals at skeletal maturity. Most of these individuals plot well on an ontogenetic series based on the proportions of limb elements, but this has lacked histological evidence until now. Here, a new wing skeleton of Sinopterus was thin-sectioned to provide the first histological data about the ontogeny of the Jehol tapejarids. Histologically, the new specimen is an immature individual at a late juvenile stage prior to sexual maturity. It is grouped with medium-sized and medium-crested individuals, which are distinct from the small-sized and crestless individuals as well as the rare large-sized and large-crested individuals at skeletal maturity, supporting the presence of the premaxillary crest as an ontogenetic feature in the Jehol tapejarids. Furthermore, this histology indicates that the largest skeletally immature individuals might have reached the sexual maturity. Enigmatically, there is a size gap between sexual and skeletal maturity, which is at about 79% of the large size, implying a ontogenetic strategy comparable with Pteranodon and possibly with the Brazilian tapejarid Caiuajara. This size gap is consistent with lack of the larger sexually mature individuals in the Jehol Biota, which is hypothesized to be a migratory habitat for the Jehol tapejarids.

Effects of Diet on the Gut Microbial Communities of Nile Tilapia (Oreochromis niloticus) Across Their Different Life Stages

Nile tilapia (Oreochromis niloticus) is one of the most important commercial freshwater fish in China, and dietary transition occurs in their different life stages. The gut microbiota is important to host health. The relationships among the diet, gut microbiota, and development of tilapia are not well known. In the present study, we attempted to understand how diet is associated with microbiota community dynamics during the development of tilapia. The first experiment was performed under standard laboratory feeding operation to determine the effect of diet transition on intestinal microbiota. In the second experiment, tilapia were fed with Artemia or plant-based dry (PBD) food from the fish started feeding to their late juvenile stage (90 days post-fertilization). The results in the first experiment showed that feeding habit transition in juvenile fish had a low effect on the microbiota of the tilapia intestine. In the second experiment, plant-based food negatively affected the survival rate and intestinal development of tilapia. The phylum Planctomycetes was dominant in juvenile fish fed PBD food. The phylum Fusobacteria was dominant in the juvenile fish fed Artemia. At the genus level, Gemmobacter, Pirellula, and Planctomyces, belonging to the phylum Planctomycetes, were significantly abundant in the guts of fish fed the PBD food diet. Cetobacterium of the phylum Fusobacteria was dominant in juvenile fish fed Artemia. Thus, we can conclude that diet types have a great effect on the microbiota of tilapia intestine in their early life stages. The intestinal microflora of tilapia was established in juvenile tilapia, approximately 2 months after hatching. Our results provide useful information for the experimental design of studies on the microbial community of the tilapia gut. We suggest that modulation of gut microbiota of tilapia could be performed in their early life.

Osteohistology of Greererpeton provides insight into the life history of an early Carboniferous tetrapod.

The vertebrate transition to land is one of the most consequential, yet poorly understood periods in tetrapod evolution. Despite the importance of the water-land transition in establishing modern ecosystems, we still know very little about the life histories of the earliest tetrapods. Bone histology provides an exceptional opportunity to study the biology of early tetrapods and has the potential to reveal new insights into their life histories. Here, we examine the femoral bone histology from an ontogenetic series of Greererpeton, an early tetrapod from the Middle-Late Mississippian (early Carboniferous) of North America. Thin-sections and micro-CT data show a moderately paced rate of bone deposition with significant cortical thickening through development. An interruption to regular bone deposition, as indicated by a zone of avascular tissue and growth marks, is notable at the same late juvenile stage of development throughout our sample. This suggests that an inherent aspect to the life history of juvenile Greererpeton resulted in a temporary reduction in bone deposition. We review several possible life history correlates for this bony signature including metamorphosis, an extended juvenile phase, environmental stress, and movement (migration/dispersal) between habitats. We argue that given the anatomy of Greererpeton, it is unlikely that events related to polymorphism (metamorphosis, extended juvenile phase) can explain the bony signature observed in our sample. Furthermore, the ubiquity of this signal in our sample indicates a taxon-level rather than a population-level trait, which is expected for an environmental stress. We conclude that movement via dispersal represents a likely correlate, as such events are a common life history strategy of aquatically bound vertebrates.

Use of soil pools to prolong the breeding period and culture of the clam, Mactra quadrangularis (Reeve, 1854)

In this study, the development of gonads of the clam, Mactra quadrangularis, in soil pools was evaluated using histological methods, and the growth and survival of larvae and juveniles were investigated. The results showed that compared to wild samples from the marine region, the breeding period of M. quadrangularis was extended from 15–30 days to more than 60 days, which greatly improved the controllability of artificial breeding of the clam. The growth rate of individuals in the larval and metamorphosis stages was 7.83 ± 1.33 µm, and the survival rate was relatively high (80.24% ± 2.36%). The population metamorphosis duration was 5–6 days, the metamorphosis rate of larvae was 73.33% ± 1.67%, and their size was 188.33 µm ± 2.92 µm. The survival rate of both early and late juvenile stages was > 90%, and the growth rate (127.67 ± 37.50 μm ~ 268.21 ± 55.33 μm) was also faster than that of the larval and metamorphosis stages. The 90-day-old juveniles reached a size of 16.17 ± 0.43 mm. These results show that culture in the soil pool effectively prolonged the breeding season of M. quadrangularis, which provides a theoretical foundation for improving artificial breeding of M. quadrangularis.

Non-thalamic origin of zebrafish sensory nuclei implies convergent evolution of visual pathways in amniotes and teleosts.

Ascending visual projections similar to the mammalian thalamocortical pathway are found in a wide range of vertebrate species, but their homology is debated. To get better insights into their evolutionary origin, we examined the developmental origin of a thalamic-like sensory structure of teleosts, the preglomerular complex (PG), focusing on the visual projection neurons. Similarly to the tectofugal thalamic nuclei in amniotes, the lateral nucleus of PG receives tectal information and projects to the pallium. However, our cell lineage study in zebrafish reveals that the majority of PG cells are derived from the midbrain, unlike the amniote thalamus. We also demonstrate that the PG projection neurons develop gradually until late juvenile stages. Our data suggest that teleost PG, as a whole, is not homologous to the amniote thalamus. Thus, the thalamocortical-like projections evolved from a non-forebrain cell population, which indicates a surprising degree of variation in the vertebrate sensory systems.

Bone histology of the non-iguanodontian ornithopod Jeholosaurus shangyuanensis and its implications for dinosaur skeletochronology and development

ABSTRACT Bone histology has provided valuable information on the life history of dinosaurs, and the presence of growth lines provides useful information for age estimation, growth variation, and the reconstruction of paleobehavior. Here, we present new data recovered from five individuals of the non-iguanodontian ornithopod dinosaur Jeholosaurus shangyuanensis from the Early Cretaceous Jehol Biota. These specimens, ranging in body length from 16 to 62 cm, represent early juvenile, late juvenile, and subadult ontogenetic stages. The bones of Jeholosaurus mainly consist of fibrolamellar tissue, which is similar to that of other non-iguanodontian ornithopods; however, parallel-fibered bone and lamellar bone tissues were also deposited in early juvenile through subadult individuals, suggesting relatively slow growth rates. Parallel-fibered bone is only regionally present in the juvenile but is well developed throughout the outermost cortex of the subadult. Skeletochronology indicates that these specimens range in age from one to five years old. Analyzing bone tissue distribution and lines of arrested growth (LAGs) in these specimens, we estimate that Jeholosaurus reached sexual maturity at two to four years old. The largest individual (IVPP V15939) displays an apparently higher growth rate during the first two years, which is abruptly reduced in the following years, suggesting a distinct growth pattern that may be related to sexual dimorphism or variable environmental conditions. Finally, the largest specimen displays parallel-fibered bone tissue but lacks an external fundamental system (EFS) near the periphery, suggesting that it was still growing but was approaching somatic maturity at death.

Changes in bioactive compounds in spiny coriander leaves in response to inflorescence pruning at different growth stages

The objective of the study was to evaluate the effect of inflorescence pruning on spiny coriander and determine the levels of antioxidants at different growth stages. Inflorescence pruning increased leaf biomass by 188%, although total biomass did not vary among treatments. Pruning altered the biomass partition, induced the earmarking of a higher percentage of photosynthates for the production of new leaves, and increased the leaf contents of reducing sugars, sucrose and soluble proteins. The contents of bioactive compounds and antioxidant activity were not affected by inflorescence pruning. Regarding the growth stages, the polyphenol and flavonoid contents were high during the juvenile stage, and highest levels of carotenoids, chlorophyll a and chlorophyll b were produced at the beginning of plant maturation. From the full maturity stage onwards, peroxidase (POD) and polyphenoloxidase (PPO) were highest, coinciding with a decrease in bioactive compounds. Considering the production of bioactive compounds and antioxidant, POD and PPO activities, the ideal harvesting time would be in the late juvenile and early maturation stages of spiny coriander, between 90 and 120 days after germination. Moreover, inflorescence pruning is recommendable for spiny coriander cultivation, for increasing the leaf biomass without affecting the leaf levels of bioactive compounds and antioxidant activity. Our results highlight the relevance of the growth stages for the production of bioactive compounds of this crop, contributing to improve the cultivation practices, with higher yields and an optimized use of the therapeutic properties of the leaves.

Viability and fertility of hybrid New Zealand tree wētā Hemideina spp. (Orthoptera: Anostostomatidae)

Natural hybridization between species provides an opportunity to study the mechanisms that maintain independent lineages and may help us understand the process of speciation. The New Zealand tree wētā speciesHemideinathoracicaproduces F1hybrids where it lives in sympatry with two closely related species:HemideinacrassidensandHemideinatrewicki. This study looked at the viability and fertility of F1hybrid wētā betweenH.thoracicaandH.crassidensthat were collected from the wild and kept in captivity. The hybrids appeared to have normal viability from the late juvenile stage, with all male wētā maturing at a late instar. Male F1hybrids displayed normal mating behavior and one male produced offspring in captivity. In contrast to Haldane’s rule, female F1hybrids appeared to be infertile; they refused to mate and did not produce eggs. No evidence ofWolbachiainfection was identified in any of the three North IslandHemideinaspecies.

Limited flexibility in female Pacific field cricket (Teleogryllus oceanicus) exploratory behaviors in response to perceived social environment

AbstractHow populations adapt, or not, to rapid evolution of sexual signals has important implications for population viability, but is difficult to assess due to the paucity of examples of sexual signals evolving in real time. In Hawaiian populations of the Pacific field cricket (Teleogryllus oceanicus), selection from a deadly parasitoid fly has driven the rapid loss of a male acoustic signal, calling song, that females use to locate and evaluate potential mates. In this newly quiet environment where many males are obligately silent, how do phonotactic females find mates? Previous work has shown that the acoustic rearing environment (presence or absence of male calling song) during late juvenile stages and early adulthood exposes adaptive flexibility in locomotor behaviors of males, as well as mating behaviors in both sexes that helps facilitate the spread of silent (flatwing) males. Here, we tested whether females also show acoustically induced plasticity in walking behaviors using laboratory‐reared populations of T. oceanicus from Kauai (HI; >90% flatwings), Oahu (HI; ~50% flatwings), and Mangaia (Cook Islands; no flatwings or parasitoid fly). Though we predicted that females reared without song exposure would increase walking behaviors to facilitate mate localization when song is rare, we discovered that, unlike males, female T. oceanicus showed relatively little plasticity in exploratory behaviors in response to an acoustic rearing environment. Across all three populations, exposure to male calling song during development did not affect latency to begin walking, distance walked, or general activity of female crickets. However, females reared in the absence of song walked slower and showed a marginally non‐significant tendency to walk for longer durations of time in a novel environment than those reared in the presence of song. Overall, plasticity in female walking behaviors appears unlikely to have facilitated sexual signal loss in this species.

Long-term PIT tag retention rates in Atlantic salmon (Salmo salar)

BackgroundPassive integrated transponder (PIT) tags are widely used in studies of various aspects of fish survival, movement and behaviour. Quality of such studies depends on the fish retaining the tags over the study period and that the tagging procedure or the tag does not influence behaviour or survival. Here we document PIT tag retention rates in hatchery-reared Atlantic salmon over a 533-day period from the late juvenile freshwater stage (pre-smolts) to young adults after 1 year in seawater. Fish were marked with 12-mm PIT tags, injected into their body cavity, as age 1+ pre-smolt and scanned for presence of PIT tags five times during the study.ResultsTag retention for the entire period was 91% and varied between periods (96.09–99.89%). For individual time steps, daily retention rate was lowest in the first period following tagging (days 0–49). After this period retention rate increased substantially, before again dropping close to initial levels at the two last periods (days 173–533). Length of fish was only significantly related to retention during the first period. No difference in retention rates was found between males and females. A subset of fish without detected tags was X-rayed to verify that lack of PIT detection was due to tag loss.ConclusionThe retention rates observed in this study clearly show that tag loss needs to be accounted for when analysing PIT tag data on Atlantic salmon. Further, the temporal changes in retention rate clearly challenge previous assertions that tag loss is something that primarily occurs shortly after tagging or during spawning.

Larval dietary protein complexity affects the regulation of muscle growth and the expression of DNA methyltransferases in Senegalese sole

Due to its high protein synthesis and deposition rates, skeletal muscle protein deposition is a major determinant of fish growth. Dietary protein complexity is likely to influence protein utilization and deposition in skeletal muscle, possibly affecting fish myogenesis. In this study, three microdiets were formulated with different degree of hydrolysis of dietary protein as the changing factor: one diet contained a mix of intact protein sources targeting a peptide with molecular weight >20 kDa (Intact); a second diet contained a hydrolysate with polypeptides ranging from 5 to 70 kDa (PartH); and a third diet contained a high level of a protein hydrolysate mostly composed of small peptides (<5 kDa) (HighH). A possible effect on the regulation of muscle growth in Senegalese sole larvae was evaluated through white muscle cellularity and the expression of muscle growth-related genes at 16 and 36 DAH. The PartH diet promoted white muscle growth during the metamorphosis climax (16 DAH), which was reflected on increased body weight. At 36 DAH, different diets induced different expression patterns of genes encoding for the myogenic regulatory factors, which affected muscle growth dynamics, ultimately promoting growth potential in the Intact group. A lower recruitment of small-sized fibres in the PartH and HighH groups led to reduced potential for muscle growth, which resulted on further reduced somatic growth. Accordingly, fish fed the Intact diet grew better up to a late juvenile stage (60 DAH) and were still heavier than others even after 30 days of feeding all groups on the same commercial diet, at 90 DAH. The up-regulation in the transcript levels of genes encoding for de novo DNA methyltransferases in the HighH group suggest a potential for nutritional programming in this species.

Biology and ecology of the world’s largest invertebrate, the colossal squid (Mesonychoteuthis hamiltoni): a short review

The colossal squid Mesonychoteuthis hamiltoni (Robson 1925) is the largest (heaviest) living invertebrate and although it is preyed upon by many top predators, its basic biology and ecology remain one of the ocean’s great mysteries. The present study aims to review the current biological knowledge on this squid. It is considered to be endemic in the Southern Ocean (SO) with a circumpolar distribution spreading from the Antarctic continent up to the Sub-Antarctic Front. Small juveniles (<40 mm mantle length) are mainly found from the surface to 500 m, and the late juvenile stages are assumed to undergo ontogenetic descent to depths reaching 2000 m. Thus, this giant spends most of its life in the meso- and bathypelagic realms, where it can reach a total length of 6 m. The maximum weight recorded so far was 495 kg. M. hamiltoni is presently reported from the diets of 17 different predator species, comprising penguins and other seabirds, fishes and marine mammals, and may feed on various prey types, including myctophids, Patagonian toothfish, sleeper sharks and other squid. Stable isotopic analysis places the colossal squid as one of the top predators in the SO. It is assumed that this squid is not capable of high-speed predator–prey interactions, but it is rather an ambush predator. Its eyes, the largest on the planet, seem to have evolved to detect very large predators (e.g., sperm whales) rather than to detect prey at long distances. The study of this unique invertebrate giant constitutes a valuable source of insight into the biophysical principles behind body-size evolution.

The Physiology and Ecology of Diapause in Marine Copepods.

Diapause is a type of dormancy that requires preparation, typically precedes the onset of unfavorable conditions, and necessitates a period of arrest before development can proceed. Two ecologically important groups of copepods have incorporated diapausing stages into their life histories. In freshwater, estuarine, and coastal environments, species within the Centropagoidea superfamily can produce resting eggs containing embryos that may be quiescent, diapausing, or in some intermediate state. Resting eggs sink into the sediments, remain viable over months to years, and form a reservoir from which the planktonic population is reestablished. In coastal and oceanic environments, copepods within the Calanidae and Eucalanidae families can enter diapause during late juvenile (copepodid) or adult stages. These copepods accumulate large amounts of lipids before they migrate into deep water and diapause for several months. Through respiration, diapausing copepods may sequester more carbon in the deep ocean than any other biogeochemical process, and changes in diapause phenology associated with climate change (particularly reduction in diapause duration) could have a significant impact not only on regional ecosystems, but on global climate as well.