Effects of maternal investment on larvae and juveniles of the annelid Capitella teleta determined by experimental reduction of embryo energy content

Experimental manipulations of the energy content of marine invertebrate embryos have been useful in testing key assumptions of life history theory, especially those concerning relationships between egg size, length of the planktonic period, and juvenile size and quality. However, methods for such “allometric engineering” experiments have been available for only a limited set of taxa (those with regulative early development, e.g., cnidarians and echinoderms). Here, we describe a method for the reduction of embryo energy content in the spirally cleaving embryos of a marine annelid, Capitella teleta, by targeted deletion of endodermal precursor cells. Embryos of C. teleta in which up to three cells (the macromeres 3A, 3B, and 3C) were deleted formed morphologically normal lecithotrophic larvae that were much smaller than larvae developing from control embryos. Experimental larvae metamorphosed at high rates, forming juveniles that were smaller than control juveniles. Juveniles derived from treated embryos had functional midguts, ingested and digested food, and grew into sexually mature adults. These results are consistent with those from previous allometric engineering studies of echinoid echinoderms, which suggest that in facultatively planktotrophic or lecithotrophic species, little maternally derived energy is used for construction of the larval body; instead, the majority is allocated to the formation of a large, high‐quality juvenile. Cleavage programs are highly conserved among divergent spiralian taxa (e.g., molluscs, nemerteans, and platyhelminths), so this method will likely be applicable to a diverse set of embryos. Similar experiments carried out in these diverse taxa will be extremely useful for evaluating inferences on relationships between egg size, length of the planktonic period, and juvenile size and quality previously based only on experiments on echinoid echinoderms.     

Relationships among Egg Size, Composition, and Energy: A Comparative Study of Geminate Sea Urchins

Egg size is one of the fundamental parameters in the life histories of marine organisms. However, few studies have examined the relationships among egg size, composition, and energetic content in a phylogenetically controlled context. We investigated the associations among egg size, composition, and energy using a comparative system, geminate species formed by the closure of the Central American Seaway. We examined western Atlantic (WA) and eastern Pacific (EP) species in three echinoid genera, Echinometra, Eucidaris, and Diadema. In the genus with the largest difference in egg size between geminates (Echinometra), the eggs of WA species were larger, lipid rich and protein poor compared to the smaller eggs of their EP geminate. In addition, the larger WA eggs had significantly greater total egg energy and summed biochemical constituents yet significantly lower egg energy density (energy-per-unit-volume). However, the genera with smaller (Eucidaris) or no (Diadema) differences in egg size were not significantly different in summed biochemical constituents, total egg energy, or energy density. Theoretical models generally assume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life history evolution. We show that even among closely-related taxa, large eggs cannot be assumed to be scaled-up small eggs either in terms of energy or composition. Although our data comes exclusively from echinoid echinoderms, this pattern may be generalizable to other marine invertebrate taxa. Because egg composition and egg size do not necessarily evolve in lockstep, selective factors such as sperm limitation could act on egg volume without necessarily affecting maternal or larval energetics.     

Effects of egg size reduction and larval feeding on juvenile quality for a species with facultative-feeding development

In free-spawning marine invertebrates, larval development typically proceeds by one of two modes: planktotrophy (obligate larval feeding) from small eggs or lecithotrophy (obligate non-feeding) from relatively large eggs. In a rare third developmental mode, facultative planktotrophy, larvae can feed, but do not require particulate food to complete metamorphosis. Facultative planktotrophy is thought to be an intermediate condition that results from an evolutionary increase in energy content in the small eggs of a planktotrophic ancestor. We tested whether an experimental reduction in egg size is sufficient to restore obligate planktotrophy from facultative planktotrophy and whether the two sources of larval nutrition (feeding and energy in the egg) differentially influence larval survival and juvenile quality. We predicted, based on its large egg size, that a reduction in egg size in the echinoid echinoderm Clypeaster rosaceus would affect juvenile size but not time to metamorphosis. We reduced the effective size of whole (W) zygotes by separating blastomeres at the two- or four-cell stages to create half- (H) or quarter-size (Q) “zygotes” and reared larvae to metamorphosis, both with and without particulate food. Larvae metamorphosed at approximately the same time regardless of food or egg size treatment. In contrast, juveniles that developed from W zygotes were significantly larger, had higher organic content and had longer and more numerous spines than juveniles from H or Q zygotes. Larvae from W, H and Q zygotes were able to reach metamorphosis without feeding, suggesting that the evolution of facultative planktotrophy in C. rosaceus was accompanied by more than a simple increase in egg size. In addition, our results suggest that resources lost by halving egg size have a larger effect on larval survival and juvenile quality than those lost by withholding particulate food.     

How do changes in parental investment influence development in echinoid echinoderms?

SUMMARY Understanding the relationship between egg size, development time, and juvenile size is critical to explaining patterns of life-history evolution in marine invertebrates. Currently there is conflicting information about the effects of changes in egg size on the life histories of echinoid echinoderms. We sought to resolve this conflict by manipulating egg size and food level during the development of two planktotrophic echinoid echinoderms: the green sea urchin, Strongylocentrotus droebachiensis and the sand dollar, Echinarachnius parma . Based on comparative datasets, we predicted that decreasing food availability and egg size would increase development time and reduce juvenile size. To test our prediction, blastomere separations were performed in both species at the two-cell stage to reduce egg volume by 50%, producing whole- and half-size larvae that were reared to metamorphosis under high or low food levels. Upon settlement, age at metamorphosis, juvenile size, spine number, and spine length were measured. As predicted, reducing egg size and food availability significantly increased age at metamorphosis and reduced juvenile quality. Along with previous egg size manipulations in other echinoids, this study suggests that the relationship between egg size, development time, and juvenile size is strongly dependent upon the initial size of the egg.     

EVOLUTIONARY LOSS OF LARVAL FEEDING: DEVELOPMENT,FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA,BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the intermediate form (a facultatively feeding larva) is rare. Knowledge of facultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary loss of larval feeding are as follows: (1) large eggs evolve before modifications in larval development, and (2) the intermediate form (facultatively feeding larva) is evolutionarily short-lived. I show that larvae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae appears to have little effect on larval growth and development. The development, form, and suspension feeding mechanism of these larvae are similar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feeding echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, independence from larval food, and relatively low feeding rate have evolved before the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests that hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection for large eggs, rather than on the evolution of heterochronies and developmental novelties in particular clades. I also discuss alternative hypotheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.     

DEVELOPMENTAL CONSEQUENCES OF AN EVOLUTIONARY CHANGE IN EGG SIZE: AN EXPERIMENTAL TEST

Larvae of two species of sea urchins (Strongylocentrotus droebachiensis and S. purpuratus) differ in initial form and in the rate of development. To determine whether these differences are attributable to the large interspecific difference in egg size, we experimentally reduced egg size by isolating blastomeres from embryos. The rate of development of feeding larvae derived from isolated blastomeres was quantified using a novel morphometric method. If the differences early in the life histories of these two species are due strictly to differences in egg size, then experimental reduction of the size of S. droebachiensis eggs should yield an initial larval form and rate of development similar to that of S. purpuratus. Our experimental manipulations of egg size produced three clear results: 1) smaller eggs yielded larvae that were smaller and had simpler body forms, 2) smaller eggs resulted in slower development through the early feeding larval stages, and 3) effects of egg size were restricted to early larval stages. Larvae from experimentally reduced eggs of the larger species had rates of development similar to those of the smaller species. Thus, cytoplasmic volumes of the eggs, not genetic differences expressed during development, account for differences in larval form and the rate of form change. This is the first definitive demonstration of the causal relationship between egg size (parental investment per offspring) and life-history characteristics in marine benthic invertebrates. Because larval form influences feeding capability, the epigenetic effects of egg size on larval form are likely to have important functional consequences. Adaptive evolution of egg size may be constrained by the developmental relationships between egg size and larval form: evolutionary changes in egg size alone can result in concerted changes in larval form and function; likewise evolutionary changes in larval form and function can be achieved through changes in egg size. These findings may have broader implications for other taxa in which larval morphology and, consequently, performance may be influenced by changes in egg size.     

Life history trade-offs and response to selection on egg size in the polychaete worm Hydroides elegans

In order to examine the genetic relationships among life-history traits in a hermaphroditic species we used artificial selection for increased egg size and measured correlated responses across the life cycle of the serpulid polychaete Hydroides elegans, a protandrous sequential hermaphrodite. We recorded sex ratios across generations, and measured egg size, egg energy, larval volume at two time points, juvenile tube length, adult dry weight and fecundity after selection. Selection for larger eggs produced positive correlated responses in egg energy, fecundity and larval size at competence. Selection for increased egg size was also manifested by earlier sex change and this resulted in selected individuals spending less time as males relative to controls. We propose that egg size is negatively correlated with duration of andromorphy, that is, that female fitness trades off with male fitness.     

Egg size and the evolution of phenotypic plasticity in larvae of the echinoid genus Strongylocentrotus

Planktotrophic larvae grow by utilizing energy obtained from food gathered in the plankton. Morphological plasticity of feeding structures has been demonstrated in multiple phyla, in which food-limited larvae increase feeding structure size to increase feeding rates. However, before larvae can feed exogenously they depend largely on material contained within the egg to build larval structures and to fuel larval metabolism. Thus, the capacity for plasticity of feeding structures early in development may depend on egg size. Using the congeneric sea urchins Strongylocentrotus franciscanus and S. purpuratus, which differ in egg volume by 5-fold, I tested whether the degree of expression of feeding structure (larval arm length) plasticity is correlated with differences in the size of the egg. I experimentally manipulated egg size of S. franciscanus (the larger-egged species) by separating blastomeres at the 2-cell stage to produce half-sized larvae. I reared half-size and normal-size larvae under high and low food treatments for 20 days. I measured arm and body lengths at multiple ages during development and calculated the degree of plasticity expressed by larvae from all treatments. Control and unmanipulated S. franciscanus larvae (from ∼ 1.0 nl eggs) had significantly longer arms relative to body size and a significantly greater degree of plasticity than half-sized S. franciscanus larvae (from < 0.18 nl eggs), which in turn expressed a significantly greater degree of plasticity than S. purpuratus larvae (from ∼ 0.3 nl eggs). These results indicate that egg size affects larval arm length plasticity in the genus Strongylocentrotus; larger eggs produce more-plastic larvae both in an experimental and a comparative context. However, changes in egg size alone are not sufficient to account for evolved differences in the pattern of plasticity expressed by each species over time and may not be sufficient for the evolutionary transition from feeding to non-feeding.     

Morphological and ultrastructural characterization of sea urchin immune cells

The free circulating coelomocytes in the coelomic cavity of echinoderms are considered to be immune effectors by phagocytosis, encapsulation, cytotoxicity, and by the production of antimicrobial agents. Although echinoderms (especially sea urchin embryo) have been used as a model organisms in biology, no uniform criteria exist for classification of coelomocytes in echinoderms, and few studies have reported about the biological functions of their coelomocytes. Hence, we study the coelomocytes in the echinoid sea urchin, Paracentrotus lividus, and describe their morphological and ultrastructural features using light and transmission electron microscopes. We classify the coelomocytes of P. lividus into red spherule and colorless spherule cells, small cells, vibratile cells, and phagocytic cells; petaloid and filopodial cells. To our knowledge, this is the first report describing ultrastructural details of the coelomocytes of P. lividus. J. Morphol. 276:583–588, 2015. © 2015 Wiley Periodicals, Inc.     

Genome‐wide microarray screening for Bombyx mori genes related to transmitting the determination outcome of whether to produce diapause or nondiapause eggs

The bivoltine silkworm Bombyx mori (Lepidoptera: Bombycidae) exhibits a maternally controlled embryonic diapause. Maternal silkworms decide whether to lay diapause or nondiapause eggs depending on environmental factors such as the temperature and photoperiod during the egg and larval stages, and then induce diapause eggs during the pupal stage. However, little is known about the molecular mechanism that conveys the outcome of whether to produce diapause or nondiapause eggs from the egg or larval stages to the pupal stage. This study used microarray analysis to investigate differentially expressed genes in the larval brains of diapause‐ and nondiapause‐egg producers, to which bivoltine silkworms were destined by thermal or photic stimulation during the egg stage. The cytochrome P450 18a1 and Krüppel homolog 1 genes were upregulated in producers of diapause eggs compared with those of nondiapause eggs under both experimental conditions. Cytochrome P450 18a1 encodes a key enzyme for steroid hormone inactivation and Krüppel homolog 1 is an early juvenile hormone‐inducible gene that mediates the repression of metamorphosis. The upregulation of these genes during the larval stage might be involved in the signaling pathway that transmits information about the diapause program from the egg stage to the pupal stage in the silkworm.     

An initial comparative genomic autopsy of wasting disease in sea stars

Beginning in 2013, sea stars throughout the Eastern North Pacific were decimated by wasting disease, also known as “asteroid idiopathic wasting syndrome” (AIWS) due to its elusive aetiology. The geographic extent and taxonomic scale of AIWS meant events leading up to the outbreak were heterogeneous, multifaceted, and oftentimes unobserved; progression from morbidity to death was rapid, leaving few tell‐tale symptoms. Here, we take a forensic genomic approach to discover candidate genes that may help explain sea star wasting syndrome. We report the first genome and annotation for Pisaster ochraceus, along with differential gene expression (DGE) analyses in four size classes, three tissue types, and in symptomatic and asymptomatic individuals. We integrate nucleotide polymorphisms associated with survivors of the wasting disease outbreak, DGE associated with temperature treatments in P. ochraceus, and DGE associated with wasting in another asteroid Pycnopodia helianthoides. In P. ochraceus, we found DGE across all tissues, among size classes, and between asymptomatic and symptomatic individuals; the strongest wasting‐associated DGE signal was in pyloric caecum. We also found previously identified outlier loci co‐occur with differentially expressed genes. In cross‐species comparisons of symptomatic and asymptomatic individuals, consistent responses distinguish genes associated with invertebrate innate immunity and chemical defence, consistent with context‐dependent stress responses, defensive apoptosis, and tissue degradation. Our analyses thus highlight genomic constituents that may link suspected environmental drivers (elevated temperature) with intrinsic differences among individuals (age/size, alleles associated with susceptibility) that elicit organismal responses (e.g., coelomocyte proliferation) and manifest as sea star wasting mass mortality.     

A limit on the extent to which increased egg size can compensate for a poor postnatal environment revealed experimentally in the burying beetle,Nicrophorus vespilloides

It is often assumed that there is a positive relationship between egg size and offspring fitness. However, recent studies have suggested that egg size has a greater effect on offspring fitness in low‐quality environments than in high‐quality environments. Such observations suggest that mothers may compensate for poor posthatching environments by increasing egg size. In this paper we test whether there is a limit on the extent to which increased egg size can compensate for the removal of posthatching parental care in the burying beetle, Nicrophorus vespilloides. Previous experiments with N. vespilloides suggest that an increased egg size can compensate for a relatively poor environment after hatching. Here, we phenotypically engineered female N. vespilloides to produce large or small eggs by varying the amount of time they were allowed to feed on the carcass as larvae. We then tested whether differences between these groups in egg size translated into differences in larval performance in a harsh postnatal environment that excluded parental care. We found that females engineered to produce large eggs did not have higher breeding success, and nor did they produce larger larvae than females engineered to produce small eggs. These results suggest that there is a limit on the extent to which increased maternal investment in egg size can compensate for a poor posthatching environment. We discuss the implication of our results for a recent study showing that experimental N. vespilloides populations can adapt rapidly to the absence of posthatching parental care.     

Identification and characterization of PlAlix,the Alix homologue from the Mediterranean sea urchin Paracentrotus lividus

The sea urchin provides a relatively simple and tractable system for analyzing the early stages of embryo development. Here, we use the sea urchin species, Paracentrotus lividus, to investigate the role of Alix in key stages of embryogenesis, namely the egg fertilization and the first cleavage division. Alix is a multifunctional protein involved in different cellular processes including endocytic membrane trafficking, filamentous (F)‐actin remodeling, and cytokinesis. Alix homologues have been identified in different metazoans; in these organisms, Alix is involved in oogenesis and in determination/differentiation events during embryo development. Herein, we describe the identification of the sea urchin homologue of Alix, PlAlix. The deduced amino acid sequence shows that Alix is highly conserved in sea urchins. Accordingly, we detect the PlAlix protein cross‐reacting with monoclonal Alix antibodies in extracts from P. lividus, at different developmental stages. Focusing on the role of PlAlix during early embryogenesis we found that PlAlix is a maternal protein that is expressed at increasingly higher levels from fertilization to the 2‐cell stage embryo. In sea urchin eggs, PlAlix localizes throughout the cytoplasm with a punctuated pattern and, soon after fertilization, accumulates in larger puncta in the cytosol, and in microvilli‐like protrusions. Together our data show that PlAlix is structurally conserved from sea urchin to mammals and may open new lines of inquiry into the role of Alix during the early stages of embryo development.     

Comparative larval energetics of an ophiuroid and an echinoid echinoderm

The morphologically convergent larvae of the echinoderm classes Ophiuroidea and Echinoidea have been suggested to be functionally dissimilar when it comes to their capacities to feed, but little is known about whether these larvae are similar in terms of energetics. Here, we compare the energetics of early development of a tropical ophiuroid, Ophiocoma alexandri, and a temperate to tropical echinoid, Arbacia punctulata, two species with similarly sized eggs. Measurements of respiration and constituent analyses were performed on eggs and unfed larvae of both species. Members of both species showed an increase in oxygen consumption during morphogenesis followed by a lower, static rate once morphogenesis was complete (3 d for O. alexandri and 1.3 d for A. punctulata). Compared to the echinoid larvae, the ophiuroid larvae developed more slowly and had peak respiration rates that were 3.1× lower. Eggs of O. alexandri contained significantly more protein and significantly less triacylglycerol than eggs of A. punctulata. Energy utilization, as calculated via respiration measurements, closely matched decreases in energy content from the eggs to larvae as measured with biochemical constituent assays. Larvae of A. punctulata used 1.4× more energy to reach the pluteus stage than larvae of O. alexandri, and used 4× more energy during the first 9 d of larval life. These data suggest that echinoid larvae require more energy to develop to the feeding stage than ophiuroid larvae, and likewise have higher requirements for maintenance metabolism. Ophiuroid larvae may be more tolerant of low food levels due to their very low metabolic rates, but this advantage may be offset by their slower rate of development.     

Evolutionary ecology of pipefish brooding structures: embryo survival and growth do not improve with a pouch

For animals that reproduce in water, many adaptations in life‐history traits such as egg size, parental care, and behaviors that relate to embryo oxygenation are still poorly understood. In pipefishes, seahorses and seadragons, males care for the embryos either in some sort of brood pouch, or attached ventrally to the skin on their belly or tail. Typically, egg size is larger in the brood pouch group and it has been suggested that oxygen supplied via the pouch buffers the developing embryos against hypoxia and as such is an adaptation that has facilitated the evolution of larger eggs. Here, using four pipefish species, we tested whether the presence or absence of brood pouch relates to how male behavior, embryo size, and survival are affected by hypoxia, with normoxia as control. Two of our studied species Entelurus aequoreus and Nerophis ophidion (both having small eggs) have simple ventral attachment of eggs onto the male trunk, and the other two, Syngnathus typhle (large eggs) and S. rostellatus (small eggs), have fully enclosed brood pouches on the tail. Under hypoxia, all species showed lower embryo survival, while species with brood pouches suffered greater embryo mortality compared to pouchless species, irrespective of oxygen treatment. Behaviorally, species without pouches spent more time closer to the surface, possibly to improve oxygenation. Overall, we found no significant benefits of brood pouches in terms of embryo survival and size under hypoxia. Instead, our results suggest negative effects of large egg size, despite the protection of brood pouches.     

Comparative size,biomass, elemental composition (C,N, H), and energy concentration of caridean shrimp eggs

Summary

In the evolution of decapod crustaceans, interspecific variation in egg size is considered as an important life-history trait that is linked with the duration of embryonic and larval development, the number and type of larval stages, and with juvenile size. Aiming to provide a quantitative characterization of reproductive traits in related decapod taxa differing in lifestyle (freshwater, estuarine, marine) and geographic-climatic distribution (tropical-temperate), we compared size, biomass, and elemental composition of eggs of caridean shrimps from three families: seven species of Palaemonidae (three congeners of Macrobrachium: M. olfersii, M. carcinus, M. acanthurus; four species of Palaemon: P. northropi, P. pandaliformis, P. elegans, P. adspersus), two Atyidae (Potimirim potimirim, Atya scabra), and one Pandalid (Pandalus montagui). Egg size was measured as larger and smaller diameter (D₁ D₂), volume was calculated from D₁ and D₂, and biomass was measured as dry mass (W), carbon (C), nitrogen (N), hydrogen (H), and energy (E, estimated from C) contents. The smallest size and lowest biomass were found in the eggs of two freshwater atyids (both originating from Brazil); the largest size occurred in a marine species, P. montagui (from the North Sea); and intermediate values in freshwater, estuarine, and marine palaemonid species (from Brazil and the Baltic Sea, respectively). Among the Palaemon species, the most limnic (P. pandaliformis) showed a significantly larger egg size and volume (P<0.001) than the estuarine and marine congeners, P. elegans, P. adspersus and P. northropi. This suggests that the generally postulated relationship between egg size and lifestyle (freshwater vs. estuarine or marine) may appear at a generic but not at the family level. On the other hand, individual biomass (in μg or Joules per egg) of early eggs was significantly higher in P. elegans and P. adspersus, indicating interspecific variability in biomass and energy concentration (in μg or Joules per unit volume, mm³). Generally lower biomass concentrations in early eggs of freshwater shrimps may be caused by a higher average water content. Eggs in late embryonic stages were generally larger than earlier eggs of the same species, reflecting an increase in the water content, while an increasing D₁:D₂ quotient indicated an increasingly elongated egg shape. The biomass per egg decreased during embryonic development due to metabolic degradation of organic reserves. As a consequence of inverse ontogenetic changes in size and organic biomass of developing eggs, the mass-specific biomass values (C, N, H in percent of W; E in Joules per mg W) and volume-specific concentrations (μg or Joules per unit volume) decreased. This change was consistently stronger in the C, H, and E contents than in other measures of biomass (W, N). In consequence, the C:N mass ratio also decreased, suggesting that lipid degradation rather than protein utilization was the principal fuel for embryonic development. Our results indicate high intra- (mainly developmental) and interspecific variation in reproductive traits of closely related species. While volume-specific biomass and energy concentrations of early eggs appear to be associated with variation in habitat salinity (freshwater, brackish, marine), individual egg size and biomass may be related more with the climatic-geographic distribution (temperate, tropical) of different taxa.     

Size‐dependent reproductive success of wild zebrafish Danio rerio in the laboratory

Size‐dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size‐selective harvesting of the F₁‐offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small‐sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L_S)‐at‐hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area‐at‐hatch and greater yolk‐sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L_S between days 60 and 90 post‐fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large‐ and small‐sized spawners by disentangling size‐dependent maternal and paternal effects on reproductive variables in D. rerio.     

Eel protection measures within the Manapouri hydro-electric power scheme,South Island,New Zealand

Variation in developmental rate from fertilization to swim-up, and body size at swim-up, may affect the growth and survival of young-of-the-year fish. Fish egg size (diameter) is often positively correlated to adult female size, but whether increased egg size equates to higher egg nutrient content and subsequently improved embryo/larval performance, remains unclear. Artificially fertilized northern pike eggs from individual females (total length 400–800 mm) were cultured under temperature controlled laboratory conditions to test the hypothesis that female body size positively influences egg size and the amount of specific nutrients (carbon, nitrogen, phosphorus, and fatty acids) allocated to eggs. We further hypothesized that greater egg nutrient content would positively influence egg survival, developmental rate, and the size of swim-up fry. These hypotheses were investigated in the context of two different northern pike spawning strategies (early season tributary vs. late season deep shoal spawning). Nutrients were allocated conservatively in northern pike eggs, showing very little variation in nutrient concentrations, but the total mass of all egg nutrients increased linearly with egg dry mass. Neither egg dry mass nor nutrient content (concentration or mass), were related to either egg diameter or female body size. The mass of individual egg nutrients was, however, strongly correlated with egg dry mass (r ² range 0.62 to 0.99 for individual nutrients) and positively related to the total length of swim-up fry (r ² = 0.58). The eggs of late spawning pike had significantly greater dry mass (average = 3.02 mg/egg) and developed more rapidly to swim-up (average = 17.89 days) than did those of early spawners (average = 2.28 mg/egg, 19.05 days). Our results indicate that egg quality may be best assessed by egg dry mass, which was correlated with greater nutrient mass and increased swim-up fry body length, as opposed to egg diameter or female body size which showed no correlation to egg nutrient composition and egg/larval performance. Guest editors: J. M. Farrell, C. Skov, M. Mingelbier, T. Margenau & J. E. Cooper International Pike Symposium: Merging Knowledge of Ecology, Biology, and Management for a Circumpolar Species