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.     

How does metabolic rate scale with egg size? An experimental test with sea urchin embryos

The consequences of changes in egg size for the development of marine invertebrates have been the subject of much theoretical and experimental work. Models that explore larval developmental modes in the context of maternal investment per offspring are often couched in an energetic framework, but the relationships between egg size and the energetics of larval development are poorly understood. We used blastomere separations to examine how experimental reductions in egg size affected (1) larval metabolic rate and (2) larval resistance to starvation. We found that separating blastomeres at the 2- and 4-cell stage resulted in average reductions of 50% and 75%, respectively, in larval metabolic rate. This suggests that, in an experimental context, mass-specific metabolic rate does not change with egg size. We also found that a 50% reduction in egg volume did not reduce the resistance of larvae to starvation when particulate food was withheld. This suggests that the material supplied to larvae in the egg is used primarily for construction of the larval body, rather than as a buffer against starvation or as a means of reducing reliance on exogenous fuel to sustain maintenance metabolism.     

High variability in egg size and energetic content among intertidal mussels

Maternal investment is a fundamentally important parameter in life-history theory and models, yet the scales at which it varies (among individuals vs. among populations) is rarely reported. In this study, variability in attributes of eggs and early larvae of Mytilus californianus was examined from four sites spanning Point Conception, California, in June and September 2001. The effects of female, site, and month were examined for the following variables: egg volume (microl), egg energy content (microg carbon per egg), and initial larval size (microm). The only significant effect on both egg traits was that of female. Females differed by up to 57% in mean egg volume and 116% in mean egg energetic content. Although there were significant effects of rearing environment, female, site, and month on initial larval size, variability in larval length was small compared to the egg traits. Mean larval length was maximally 11% different among females. Neither female body weight nor length was correlated to mean offspring traits, and there were also no significant relationships between egg traits and initial larval size. The primary source of variation in maternal investment in this system appears to be among individual females rather than over space or time.     

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.     

Differential effects of offspring and maternal inbreeding on egg laying and offspring performance in the burying beetle Nicrophorus vespilloides

We investigate the effect of offspring and maternal inbreeding on maternal and offspring traits associated with early offspring fitness in the burying beetle Nicrophorus vespilloides. We conducted two experiments. In the first experiment, we manipulated maternal inbreeding only (keeping offspring outbred) by generating mothers that were outbred, moderately inbred or highly inbred. Meanwhile, in the second experiment, we manipulated offspring inbreeding only (keeping females outbred) by generating offspring that were outbred, moderately inbred or highly inbred. In both experiments, we monitored subsequent effects on breeding success (number of larvae), maternal traits (clutch size, delay until laying, laying skew, laying spread and egg size) and offspring traits (hatching success, larval survival, duration of larval development and average larval mass). Maternal inbreeding reduced breeding success, and this effect was mediated through lower hatching success and greater larval mortality. Furthermore, inbred mothers produced clutches where egg laying was less skewed towards the early part of laying than outbred females. This reduction in the skew in egg laying is beneficial for larval survival, suggesting that inbred females adjusted their laying patterns facultatively, thereby partially compensating for the detrimental effects of maternal inbreeding on offspring. Finally, we found evidence of a nonlinear effect of offspring inbreeding coefficient on number of larvae dispersing. Offspring inbreeding affected larval survival and larval development time but also unexpectedly affected maternal traits (clutch size and delay until laying), suggesting that females adjust clutch size and the delay until laying in response to being related to their mate.     

Position of egg within a clutch is linked to size at hatching in a demersal tropical fish

Size variation among propagules is ubiquitous and small initial differences in size can be critical to survival, particularly in taxa where initial survival is variable and strongly size-dependent. Despite this, the sources of size variation among fish at hatching are rarely investigated. This study examined spatial position within egg clutches as a source of size variation at hatching of the benthic spawning fish Amphiprion melanopus. We quantified within-clutch size variation at hatching and found that newly hatched larvae from the periphery (5 mm from edge) of 2-dimensional clutches were smaller in standard length, cranial depth, eye diameter and body area (7%, 8%, 4% and 11%, respectively) than larvae from the interior positions within clutches. To investigate the source of this variation, sizes of embryos at different locations with clutches were measured within 2 h of fertilisation (8 d before hatching). Newly laid embryos from the clutch periphery were smaller in length and volume than embryos from the clutch interior (> 2% and 4-6%, respectively). These eggs from the periphery also had a 33% lower rate of oxygen consumption than did embryos from the clutch interior, throughout development. The relationships between position within a clutch and egg size, oxygen consumption and larval size imply that size variation in larval fish at hatching is partly generated during early embryogenesis, either from maternal endowment or maternal nest design, and was amplified throughout development.     

Maternal effects on phenotypic plasticity in larvae of the salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis>

Maternal effects are widespread and influence a variety of traits, for example, life history strategies, mate choice, and capacity to avoid predation. Therefore, maternal effects may also influence phenotypic plasticity of offspring, but few studies have addressed the relationship between maternal effects and phenotypic plasticity of offspring. We examined the relationship between a maternally influenced trait (egg size) and the phenotypic plasticity of the induction rate of the broad-headed morph in the salamander Hynobius retardatus. The relationship between egg size and the induction of the broad-headed morph was tested across experimental crowding conditions (densities of low conspecifics, high conspecifics, and high heterospecific anuran), using eggs and larvae from eight natural populations with different larval densities of conspecifics and heterospecifics. The broad-headed morph has a large mouth that enables it to consume either conspecifics or heterospecifics, and this ability gives survival advantages over the normal morph. We have determined that there is phenotypic plasticity in development, as shown by an increase in the frequency of broad-headed morph in response to an increase in the density of conspecifics and heterospecifics. This reaction norm differed between populations. We also determined that the frequency of the broad-headed morph is affected by egg size in which larger egg size resulted in expression of the broad-headed morph. Furthermore, we determined that selection acting on the propensity to develop the broad-headed morph has produced a change in egg size. Lastly, we found that an increase in egg size alters the reaction norm to favor development of the broad-headed morph. For example, an equal change in experimental density produces a greater change in the frequency of the broad-headed morph in larvae developing from large eggs than it does in larvae developing from small eggs. Population differences in plasticity might be the results of differences in egg size between populations, which is caused by the adaptive integration of the plasticity and egg size. Phenotypic plasticity can not evolve independently of maternal effects.     

Evolutionary and experimental change in egg volume, heterochrony of larval body and juvenile rudiment, and evolutionary reversibility in pluteus form

SUMMARY Heterochronic developmental plasticity of the juvenile rudiment and larval body of sea urchin larvae occurs in response to supply of food. Evolutionary increase in egg size can also be associated with earlier development of the juvenile rudiment. We examined effects of egg volume of feeding larvae on this heterochrony and other changes in larval form. (1) Evolutionary and experimental enlargements of egg volume did not accelerate formation of the rudiment relative to the larval body. Development of the larval body and juvenile rudiment was compared for the echinoids Strongylocentrotus purpuratus (with an egg of 78–82 μm) and Strongylocentrotus droebachiensis (with an egg of 150–160 μm diameter). Development of both larval body and rudiment were accelerated in S. droebachiensis relative to S. purpuratus but with greater acceleration of the larval body, so that the rudiment of S. droebachiensi s was initiated at a later larval stage even though at an earlier age. Also, experimentally doubling the egg volume of S. purpuratus did not accelerate development of the juvenile rudiment relative to the larval body. (2) Both species exhibited similar plasticity in timing of rudiment development in response to food supplies. (3) Doubling egg volume of S. purpuratus produced a larval form more similar to that of S. droebachiensis . This result mirrors previous experiments in which larvae from half embryos of S. droebachiensis were more similar to larvae of S. purpuratus . Many of the effects of egg volume on larval form are similar against either species' genetic background and are thus evolutionarily reversible effects on larval form.     

LARVAL ECOLOGY OF MARINE BENTHIC INVERTEBRATES: PALEOBIOLOGICAL IMPLICATIONS

1. Modes of larval development play important roles in the ecology, biogeography, and evolution of marine benthic organisms. Studies of the larval ecology of fossil organisms can contribute greatly to our understanding of such roles by allowing us to race effects on evolutionary time scales. 2. Modes of development can be inferred for well preserved molluscan fossils because the size of the initial larval shell (Protoconch I in gastropods, Prodissoconch I in bivalves) reflects egg size. Other morphological criteria are also available, and a comparative approach based on related taxa with known development may be the most reliable method. By combining larval and adult traits, it is possible to recognize modes of larval development in at least some fossil bryozoans, brachiopods, and echinoderms as well. (a) Planktotrophic larvae arise from small eggs, are released in enormous numbers with little parental investment per offspring, and suffer tremendous mortality during and shortly after a planktic existence. These larvae feed on the plankton during development, and are commonly capable of a prolonged free-swimming existence, and thus wide dispersal. (b) Nonplanktotrophic larvae (which include both planktic lecithotrophic forms and ‘direct developers’) generally arise from large eggs, with relatively few young produced per parent. Relative to planktotrophic larvae, nonplanktotrophic larvae generally receive greater parental investment per larva, and larval mortality is generally lower. These larvae rely on yolk for nutrition during development, and planktic durations are generally much briefer than for species with planktotrophic larvae, so that dispersal capability is considerably less. Energetic investment per egg is generally higher than in planktotrophs, but as there are lower fecundities as well it is difficult to generalize about the total energetic cost of one mode of reproduction against the other. 3. Owing to the high dispersal capability of planktotrophic larvae, it has been suggested that species with such larvae will be geographically widespread, geologically long-ranging, and exhibit low speciation and extinction rates. Species with nonplanktotrophic larvae will tend to be geographically more restricted, geologically short-ranging, and exhibit high speciation and extinction rates (again, as a consequence of their characteristically low larval dispersal capabilities). 4. Recognition of differential dispersal capabilities can play a role in paleobiogeo-graphic analyses. Concurrent study of the distribution of groups with contrasting modes of development will permit testing of hypotheses concerning timing, magnitudes and frequencies of migration and vicariance events. 5. Larval types are not randomly distributed in the oceans, but relationships with other aspects of the organisms' biology and habitats are very complex. Mode of development varies with: (a) Ecology. A simple r–––K model of adaptive strategies is clearly insufficient to explain the observed relationships: while many ‘equilibrium’ species have nonplanktotrophic larvae, and organisms living in less prdictable environments often have planktotrophic larvae, some of the most opportunistic marine species have nonplanktotrophic larvae. Nonetheless, planktotrophic development seems most suited for exploitation of patchy but widespread habitats. (b) Latitude. At shelf depths, planktotrophy is predominant in the tropics, and decreases sharply at high latitudes. (c) Depth. Incidence of planktotrophy decreases with depth across the continental shelf, at least in some taxa. Beyond the shelf, many deep-sea organisms are nonplanktotrophic (e.g. most bivalves, peracarid crustaceans), but planktotrophic development appears to be present in other groups (prosobranch gastropods, ophiuroids, and bivalves inhabiting transient habitats such as sunken wood and hydrothermal vents). These trends in developmental types will be accompanied by trends in evolutionary rates and patterns as outlined above. The study of larval ecology by paleobiologists will yield insights into the processes that gave rise to ancient evolutionary and biogeographic patterns, and will permit the development and testing of hypotheses on the origins of the patterns observed in modern seas.     

Maternal effects on offspring growth and development depend on environmental quality in the frogBombina orientalis

Summary Differences in maternal investment and initial offspring size can have important consequences for offspring growth and development. To examine the effects of initial size variability in the frogBombina orientalis, we reared larvae (N=360) in one of two treatments representing different levels of environmental quality. We used snout-vent length at the feeding stage (stage 25, Gosner 1960) as a measure of maternal investment. In a “low quality” treatment, larvae were reared with two conspecific tadpoles and food was limited, whereas in a “high quality” treatment, larvae were reared individually and were fed ad libitum. Among tadpoles reared in the low quality treatment, individuals that were initially small had smaller body sizes through metamorphosis and longer larval periods than individuals that were initially large. Among tadpoles reared in the high quality treatment, initial size had only a weak influence on later larval size, and did not significantly affect metamorphic size of the duration of the larval period. This interaction between maternal investment and rearing conditions suggests that production of initially small offspring could be advantageous if these offspring develop in relatively benign environments, but disadvantageous if environments are more severe. These findings are discussed in light of previous studies that have demonstrated such interactions in organisms with complex life cycles.     

Optimal Egg Size in Marine Invertebrates: Theory and Phylogenetic Analysis of the Critical Relationship between Egg Size and Development Time in Echinoids

Life-history models for marine invertebrate larvae generally predict a dichotomy in egg size in different species: eggs should be either minimal in size or large enough to support development fully without larval feeding. This prediction is contradicted, however, by the empirical observation of wide, continuous variation in egg size between these extremes. The prediction of dichotomy rests on the assumption of a negative linear relationship between egg size and development time. Here, I present a simple model in which development time is inversely proportional to egg size. Incorporating this relationship into an optimality model produces predictions of intermediate rather than extreme egg size. Modeled variations in mortality, food availability, fertilization rates, and temperature all produce continuous shifts in the value of the intermediate optimal size, in direct contrast to those produced by previous models, which predict shifts between two extreme optima. Empirical data on echinoid egg size and development time strongly support the model's assumption of an inverse proportional relationship between egg size and development time. A composite phylogeny is constructed of the 37 species for which egg size, development time, water temperature, and phylogenetic relatedness are known. Independent contrasts are made of the evolutionary changes in egg size and development time. This analysis indicates that evolutionary shifts in development time are correlated with the inversely proportional shifts in egg size assumed in the model. The assumption of a negative linear relationship used in previous models is rejected. This model provides a potential explanation for intraspecific variation in egg size along environmental gradients, sympatric differences in egg size among species, and biogeographic trends in egg size and development mode across taxa.     

Differentiated egg size of the cannibalistic salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis>

Larvae of the salamander, Hynobius retardatus, are carnivorous, and even though there are two morphs, a typical morph and a broad-headed or “cannibal” morph, both are cannibalistic. They also sometimes eat other large prey, for example larvae of the frog, Rana pirica. In natural habitats, use of both conspecific and R. pirica larvae as food may contribute more strongly to high survival and substantially to fitness when larval densities are higher, because early-stage H. retardatus larvae sometimes experience scarcity of their typical prey. In cannibalistic oviparous amphibians, larger individuals that developed from larger eggs can more efficiently catch and consume larger prey and thus their survival may be better than that of smaller individuals developed from smaller eggs. Populations might therefore diverge in respect of egg size in response to variation in the density of conspecific and R. pirica larvae in natural ponds, with eggs being larger when larval density is higher. I examined how variance in hatchling size correlated with the incidence of cannibalism, and whether increasing larval density in natural ponds correlated with increasing egg size. Variance in initial larval body size facilitated cannibalism, and egg size increased as larval density in the ponds increased. In ponds with high larval density, where cannibalism and large prey consumption is a critical factor in offspring fitness, the production of fewer clutches with larger eggs, and thus of fewer and larger offspring, results in greater maternal fitness. Variation among the mean egg size in populations is likely to represent a shift in optimum egg size across larval density gradients.     

Reproductive strategies of marine benthic invertebrates revisited: facultative feeding by planktotrophic larvae

Fecundity-time models of reproductive strategies in marine invertebrates all predict that reproductive success is maximized only at the extreme levels of investment. Selection should drive egg sizes toward small eggs and planktotrophy or large eggs and lecithotrophy. The existence of two distinct larval types, feeding and nonfeeding, has been taken as confirmation of this prediction and has established the current paradigm for larval ecology. However, comparative and experimental evidence does not support the prediction that egg size is minimized in species with planktotrophic larvae. Recent discoveries have documented the existence of planktotrophs that have intermediate egg sizes, differing degrees of dependence on exogenous food, and differing capacities for facultative feeding. A fecundity-time model is presented that includes facultative larval feeding by dissociating the onset of feeding capability from the need for exogenous food. The facultative feeding model shows that reproductive success can be maximized at intermediate levels of investment per offspring between the minimum for development and the threshold for lecithotrophy, depending on the amount of food available to larvae and the intensity of planktonic mortality. A continuum of larval strategies is predicted.     

Variable shrimp in variable environments: reproductive investment within Palaemon varians

The maternal environment may influence the quantity and quality of resources invested in offspring (per-offspring provisioning) and this trait, in turn, affects larval fitness and may carry-over into early juvenile life. Here, per-offspring investment was measured across three consecutive breeding seasons for the caridean shrimp, Palaemon varians. Egg and hatchling larval dry weight as well as hatchling larval biochemical composition were measured. Results indicate that egg volume is positively correlated with dry weight, but egg volume as a proxy for dry weight should be used with caution. Correlations were identified between per-offspring investment and average breeding season temperature, but the period over which temperature was averaged was important to whether these correlations were positive or negative, highlighting the complexity of identifying environmental influence on phenotypic traits. The most important factor determining offspring size and per-offspring investment was maternal size, which may be influenced by the environment as carapace length varied significantly between breeding seasons. These data demonstrate variation in egg and larval composition, related to maternal size, which will affect early life traits and survivorship for P. varians.

     

Size-specific predation on marine invertebrate larvae

Predation on planktonic larval stages is frequently a major source of mortality for the offspring of benthic marine invertebrates. Mortality rate likely varies with larval size and developmental stage, but few experiments have measured how these factors affect predation rates. I used experimental reductions in egg size to test how variation in larval size affects the likelihood of predation during planktonic development. Blastomeres of the sand dollar Dendraster excentricus were separated at the two-cell stage to produce half-sized zygotes. Larvae resulting from this manipulation were tested for their susceptibility to predation relative to whole-sized siblings at four ages. Individuals from each size class were simultaneously presented as prey items to five predators (crab zoeae, crab megalopae, chaetognaths, solitary tunicates, and postlarval fish) in the laboratory. Four predators consumed significantly more half-sized larvae than whole-sized larvae, but one predator type (postlarval fish) consumed more whole-sized larvae. Predators that consumed more half-sized larvae also preferentially consumed younger larvae. In contrast, postlarval fish showed no significant prey preference based on larval age. These results suggest that assumptions of constant mortality rates during development should be modified to account for the effects of larval size and age.     

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.     

The effect of larval competition on development time and adult size in the seaweed fly,Coelopa frigida

Summary The effect of larval competition on adult size and egg to adult development time has been investigated in laboratory populations of the seaweed fly, Coelopa frigida. Increased larval density results in longer development times and smaller adults, suggesting a strong interference element to the competition. This may be due to a limiting rate of food supply rather than interactions between larvae. The relationship between development time and size suggests that both these characters are involved in control of the onset of pupation. C. frigida is polymorphic for an inversion on chromosome I. Differences in relative viability between the karyotypes are not related to the development time or size differences.     

Latitudinal patterns of egg size and maternal investment trade-offs in reef corals

Aim

The aims were to test the role of temperature in latitudinal patterns of egg size and investigate maternal investment trade-offs among coral taxa.

Location

Global, from 34° S to 34° N.

Time period

1981–2020.

Major taxa studied

Reef coral species from the order Scleractinia.

Methods

We compiled a comprehensive geo-referenced global dataset of egg sizes (diameter or volume) and fecundity (number of eggs per area) for colonial corals (Scleractinia; 123 species, 5359 observations and 39 localities), substantially enhanced by new field collections (>88% of observations). We used Bayesian phylogenetic multilevel models to test for Rass' rule (a hypothesized negative relationship between egg size and temperature); we also included other environmental variables and life history traits. We also tested whether a trade-off exists between egg size and fecundity in broadcast spawning hermaphroditic corals with horizontal symbiont transmission (HHT).

Results

We found a significant relationship between coral egg size and symbiont transmission. Eggs from coral species with vertical symbiont transmission were c. 18.8% smaller than those from species with horizontal symbiont transmission. We also found non-significant relationships between egg size and sea surface temperature (SST) for broadcast spawning corals and between egg size and fecundity specifically for HHT species.

Main conclusions

Contrary to recognized latitudinal patterns of egg size across taxa, our study does not provide support for Rass' rule in corals. Additionally, our findings do not support a maternal investment trade-off between egg size and fecundity for HHT species. Our study used a phylogenetic framework that should be a standard practice when studying interspecific variation, including investigation of maternal investment and identification of the influence of multiple predictors on larval fitness (egg size), in addition to trade-offs affecting propagule influx (fecundity). Both these functional traits are vital and have direct consequences for population maintenance and connectivity in sessile organisms, such as corals.     

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.     

The effects of nurse eggs and sibling interactions on the larval development of the poecilogonous annelid Boccardia proboscidea (Spionidae)

In poecilogony, different types of larvae are produced within the same species. Previous studies have suggested maternal control of the production of larval types; however, it is not clear which factors or mechanisms generate contrasting developmental patterns among siblings. The spionid polychaete Boccardia proboscidea produces within the same capsule adelphophagic larvae that eat nurse eggs and siblings and complete all or most of their development inside the capsule (Type A larvae), and larvae with little growth until they hatch as planktotrophic larvae (Type B larvae). In this study, we manipulated capsule content to explore the factors determining larval type in B. proboscidea and the role of extra‐embryonic maternal nutrition and sib–sib interaction in the developmental fate of offspring. When early larval stages were grown individually in vitro, with nurse eggs as the only food source, some of them remained small, while others continue developing into larger pre‐competent larvae by feeding on nurse eggs. This suggests that larval types in B. proboscidea are determined very early in development and are not solely the product of sib–sib interaction inside the capsule. However, our data also suggest that hatching size variability within larval types of a clutch depends on nurse egg availability. Type B larvae grew normally to metamorphosis when phytoplankton was available, but suffered high rates of cannibalism by Type A larvae. These results are consistent with the hypothesis that individual larval fates are determined very early in development and that once their fate is determined, hatching size and intracapsular survival are affected by maternal food provisioning and sibling interaction.