Mysid crustaceans as standard models for the screening and testing of endocrine-disrupting chemicals

Author Posting. © Springer, 2007. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Ecotoxicology 16 (2007): 205-219, doi:10.1007/s10646-006-0122-0.Investigative efforts into the potential endocrine-disrupting effects of chemicals have mainly concentrated on vertebrates, with significantly less attention paid to understanding potential endocrine disruption in the invertebrates. Given that invertebrates account for at least 95% of all known animal species and are critical to ecosystem structure and function, it remains essential to close this gap in knowledge and research. The lack of progress regarding endocrine disruption in invertebrates is still largely due to: (1) our ignorance of mode-of-action, physiological control, and hormone structure and function in invertebrates; (2) lack of a standardized invertebrate assay; (3) the irrelevance to most invertebrates of the proposed activity-based biological indicators for endocrine disruptor exposure (androgen, estrogen and thyroid); (4) limited field studies. Past and ongoing research efforts using the standard invertebrate toxicity test model, the mysid shrimp, have aimed at addressing some of these issues. The present review serves as an update to a previous publication on the use of mysid shrimp for the evaluation of endocrine disruptors (Verslycke et al., 2004a). It summarizes recent investigative efforts that have significantly advanced our understanding of invertebrate-specific endocrine toxicity, population modeling, field studies, and transgeneration standard test development using the mysid model.Supported by a Fellowship of the Belgian American Educational Foundation

REPRODUCTION IN NORTHERN KRILL (MEGANYCTIPHANES NORVEGICA SARS)

International audienceThis review presents the current state of knowledge with regard to the reproductive biology of Northern krill (Meganyctiphanes norvegica) Reproduction is limited to a distinct period of the year First development of the ovary occurs at the onset of the season when the stock of primary oocytes issued from the germinal zone starts to accumulate glycoproteic yolk Previtellogenesis continues through out the entire reproductive season but oosorption (the retrieval by the ovary of the yolk constituents from the growing oocytes) may occur in unfavourable conditions and represents an important metabolic process for sustaining females during such periods Oosorption also occurs at the onset of the resting season It has been established that individual females may perform several cycles of reproduction each year Each reproductive cycle spans two moult cycles one in which lipid yolk is accumulated (vitellogenesis) and another when spawning occurs The time of spawning does not coincide with the moult (ecdysis) but with the onset of moult preparation (C Do moult stages) The complete egg batch is spawned well before the moult Storage lipids are accumulated preferentially in the ovary with distinctly high levels of omega 3 polyunsaturated fatty acids in the polar lipid fraction as well as phosphatidylcholine a key component in the development of the embryo There is no difference concerning lipid storage between resting females males and Juvenile krill Beside the ovary the fat body is an important organ involved in the metabolism and storage of the glycoproteins and lipids that will be transformed into the lipoglycoproteins of the yolk platelets in the ovary M norvegica produce large egg batches with the number of mature oocytes in one batch being proportional to the size of the female with a mean number of 1000-1200 eggs per batch The number of reproductive cycles per year is a function of the trophic conditions with the first reproductive cycle being triggered by the first phytoplankton bloom Other reproductive features reflect specific adaptations of krill to a pelagic life like swarming and vertical migration behaviour M norvegica segregate at night for moulting and mating or spawning while swimming constantly during their diet vertical migration (DVM) Key questions concerning krill reproduction remain particularly in identifying the cues that switch krill in and out reproductive development or between egg building and oosorption New molecular tools are now available to tackle such question

Seasonal reproduction, multiple spawning, and fecundity in northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba

2nd International Symposium on Krill, UNIV CALIF SANTA CRUZ, SANTA CRUZ, CALIFORNIA, AUG 23-27, 1999Both northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba, release eggs seasonally in multiple spawning events. Fecundity estimates were determined based on counts of mature oocytes in the ovary of females in preserved field samples. Principal components analysis and allometry were used to analyze the covariation of egg-batch size with female characteristics. In both krill species, egg-batch size scaled isometrically with ovarian weight but showed a positive allometry with body size. Predicted egg-batch size ranged from 200 to 4000 eggs for M. norvegica of 25.5-35 mm body length and from 1500 to 6000 eggs for E. superba of 36-55 mm. Allometric relationships can be used to predict annual fecundity from size structure data of krill populations. In this study, the effect of temperature on the duration of molting and spawning cycles was used to infer a number of reproductive cycles per year. taking into account climate and seasonal variation

Life cycle strategies of Northern krill (Meganyctiphanes norvegica) for regulating growth, moult, and reproductive activity in various environments: the case of fjordic populations

Adaptive strategies of two fjord populations of Northern krill (Meganyctiphanes norvegica Sars) were studied and compared with other populations from different climatic and oceanic conditions. The Gullmarsfjord (West Sweden) and the Clyde Sea (West Scotland) resident populations followed the same basic pattern of development as the Kattegat (between Denmark and Sweden) and the Ligurian Sea (Northwest Mediterranean) populations, but the fjord krill reached a comparatively larger body size during their second year of life. The positive relationship between body size and fecundity means that fjord populations are potentially more productive than those of the open sea. High rates of moulting and spawning activity were limited to the spring and summer despite the fact that trophic conditions still seemed favourable in early autumn. We show here that, in autumn, the adult moult cycle became longer and growth stopped, large 2-year-old krill disappeared from the population and ovarian development was arrested. Resources already accumulated ill the oocytes (glycoproteic and lipid yolk) were recovered by oosorption and ovaries regressed for a winter rest. Autumn trophic conditions were still favourable in both sites, especially in terms of copepod abundance. However, the phytoplankton community changed from a dominance of diatoms in early season to dinoflagellates in late summer. We suggest that these changes triggered the autumn arrest of krill production

Ovarian development and spawning in relation to the moult cycle in Northern krill, Meganyctiphanes norvegica (Crustacea : Euphausiacea), along a climatic gradient

Adaptive processes linked to reproduction were studied comparatively for three populations of Northern krill, Meganyctiphanes norvegica (M. Sars, 1857), sampled during winter and summer cruises in the Clyde Sea (W Scotland), the Kattegat (E Denmark), and the Ligurian Sea (NW Mediterranean). The aim was to investigate the functional relationship between egg production and moulting under contrasted climatic and environmental conditions. A staging system for female sexual development established for live krill was complemented by a histological study of the ovary at various developmental steps. During the reproductive season, all adult female krill were engaged in cyclical egg production. During experiments, female krill released one batch of mature oocytes in one or two spawning events. The ovary of postspawn female krill still contained developing oocytes for another egg batch. In the non-reproductive period, all female krill had a resting ovary. Ovarian structure and pattern of egg production were identical in the three populations, but seasonal timing of egg production was different. The model proposed for the Ligurian population of the annual cycle of ovarian development can be extended to the other two populations, taking into account the seasonal characteristics of each site. Random field samples were staged simultaneously for moult cycle and for sexual development. Moult stages and the seasonal variation of the intermoult period were studied for the Kattegat population using multi-year data and compared to data obtained during summer/winter cruises in the Clyde and the Ligurian Sea. At the three sites, intermoult period was shorter and temperature-dependent during the reproductive period, concurrent with the season of greatest food availability. During most of the year and the period of sexual rest, moulting activity was reduced. The relationship between spawning and the moult cycle was studied comparatively for the three populations. Eggs were released during the premoult phase of a ``spawning moult cycle'', in one or two spawnings associated with apolysis and Moult Stage D1, respectively. Yolk accumulation for the next egg batch was completed during an alternating ``vitellogenic moult cycle''. A model for the timing of cyclical egg production in relation to moulting, as proposed for the Kattegat, can be extended to the other populations, taking into account intermoult period variation with temperature. Temperature appeared to be the principal environmental factor controlling growth (through moulting) and egg production during the reproductive season, in connection with favourable trophic conditions

Synchronization in the molting and spawning activity of northern krill (Meganyctiphanes norvegica) and its effect on recruitment

The molting, spawning, and recruitment of northern krill (Meganyctiphanes norvegica) were analyzed over an annual cycle (1999-2000) in the Clyde Sea (west coast of Scotland). Results supported the hypothesis of a functional relationship between egg production and molt development for the duration of the reproductive season (March to August), with one reproductive cycle being made up of two molt cycles. Females remained in reproductive condition throughout the reproductive season, and the timing of their spawning and molting was synchronized at the population level throughout this period. A semiempirical model predicted that the krill population produced an egg pulse every 20 to 26 d (depending on temperature), and three cohorts were evident in net samples taken later in the year. The likely date on which the first cohort was spawned was around 26 d after the main phytoplankton bloom, suggesting that the bloom triggered egg development in all adult females. Such a synchronized spawning period was observed directly in adult females 26 d after a bloom in March 2000. A total of three cohorts over the 6-month reproductive season is less than the maximum of seven that would be possible if spawning occurred at a periodicity of between 20 and 26 d, suggesting that larval recruitment was not always successful. Analysis showed that successful recruitment was only achieved when chlorophyll a levels were adequate during both the period of egg maturation in the ovary and the subsequent development of larvae, especially the furcilia stages

External parasite infestation depends on moult-frequency and age in Antarctic krill (Euphausia superba)

Levels of infestation by the suctorian Ephelota sp. were determined in Antarctic krill taken in the north-eastern region of the Scotia sea, close to the island of South Georgia. Individuals caught in summer between 2002 and 2004 were measured for body length, eye diameter, moult stage, sex, sexual maturity and number of suctorian parasites. All krill samples were found to contain at least two modal size-classes and the complete range of maturity- and moult stages. Sample analysis for the different characters identified moult stage as an important determinant of infestation by Ephelota sp., with 66% of those about to moult (pre-moult individuals) being infested compared with 0% of post-moult individuals. Multiple Correspondence Analysis also identified a strong link between age and degree of suctorian infestation with large-eyed (and hence old) males having very high numbers of suctorian bodies. Moulting is costly in terms of energy and vulnerability, but it is believed that krill moult at relatively high rates throughout their life to facilitate growth and "shrinkage". Here, we demonstrate that the control of external parasitism is also a major advantage of moulting at high frequency. The capacity of krill to maintain a high moult rate decreases with age, leading to older krill enduring higher parasitic loads