Eggs as Energy: Revisiting the Scaling of Egg Size and Energetic Content Among Echinoderms

Abstract

Marine organisms exhibit substantial life-history diversity, of which egg size is one fundamental parameter. The size of an egg is generally assumed to reflect the amount of energy it contains and the amount of per-offspring maternal investment. Egg size and energy are thought to scale isometrically. We investigated this relationship by updating published datasets for echinoderms, increasing the number of species over those in previous studies by 62%. When we plotted egg energy versus egg size in the updated dataset we found that planktotrophs have a scaling factor significantly lower than 1, demonstrating an overall trend toward lower energy density in larger planktotrophic eggs. By looking within three genera, Echinometra, Strongylocentrotus, and Arbacia, we also found that the scaling exponent differed among taxa, and that in Echinometra, energy density was significantly lower in species with larger eggs. Theoretical models generally assume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life-history evolution. These data suggest that the evolution of egg size and egg energy content can be decoupled, possibly facilitating response to selective factors such as sperm limitation which could act on volume alone.

Evolutionary Transitions in Mode of Development

“…In this case, however, it would be mode in what is generally interpreted as an adaptive way. As one broad example, the eggs of lecithotrophic species generally contain proportionally more lipid, which is thought to provide energy for the metamorph as well as the nonfeeding larva (Emlet and Hoegh-Guldberg, 1997;Mora n and Manahan, 2003;Prowse et al, 2008;Moran et al, 2013;Falkner et al, 2015) and has also been implicated in buoyancy (Arai et al, 1993). Feeding larvae, in contrast, contain proportionally more protein and build lipid reserves through feeding (Jaeckle, 1995;Moran and Manahan, 2004;Sewell, 2005;Moran et al, 2013).…”

“…As one broad example, the eggs of lecithotrophic species generally contain proportionally more lipid, which is thought to provide energy for the metamorph as well as the nonfeeding larva (Emlet and Hoegh-Guldberg, 1997;Mora n and Manahan, 2003;Prowse et al, 2008;Moran et al, 2013;Falkner et al, 2015) and has also been implicated in buoyancy (Arai et al, 1993). Feeding larvae, in contrast, contain proportionally more protein and build lipid reserves through feeding (Jaeckle, 1995;Moran and Manahan, 2004;Sewell, 2005;Moran et al, 2013). Lipid profiles also differ by taxon and mode of development; for example, in asterinid sea stars, triglycerides are relatively more abundant in eggs of most lecithotrophic species than in planktotrophic species (Prowse et al, 2008), while some lecithotrophic ophiuroids supply eggs with wax esters, a lipid class not found in planktotrophs (Falkner et al, 2015).…”

Phenotypic Plasticity of Feeding Structures in Marine Invertebrate Larvae

“…Alternatively, comparative studies among multiple species of ophiuroids (Podolsky and McAlister, 2005) and echinoids (Reitze l and Heyland, 2007) have supported the alternate hypothesis. Egg size is likely a coarse measure of egg energetic content, however, it provides no information about egg biochemical composition (Moran and McAlister, 2009;Moran et al, 2011). Increases in egg size can be obtained through increased maternal provisioning or through simple hydration (Podolsky and Strathman n, 1996;McAlister and Moran, 2012).…”

“…Indeed, egg size varies greatly (>80-fold difference in volume as calculated from values below) among species of echinoids with planktotrophic larvae. In some species mothers provision their offspring with a small portion of the energy needed to complete larval development and metamorphose into a juvenile (e.g., Arbacia stellata, egg volume 0.13 nl, egg energy 1.0 mJ; Moran et al, 2011); whereas in other species, mothers provide all of the energy needed to complete larval development and metamorphosis (e.g., the facultative demonstrations and documentation of the expression of feeding-structure plasticity. These studies have included assessments of the presence, magnitude, variability, and timing of the response within specific taxa (Table 8.1).…”

Complex offspring size effects: variations across life stages and between species

“…Both assumptions have been explored from numerous angles, in countless taxa and environments, with variable outcomes. In aquatic systems, some studies have determined that offspring (egg) size reflects parental investment (Quattro and Weeks 1991;Jaeckle 1995) and the amount of reserves available for early growth, while others have shown that energetic content and egg size are not always directly related (Moran et al 2013). Contrary to terrestrial models such as insects and birds (Fox and Czesak 2000;Krist 2011), support for the assumption that larger offspring perform better has been inconsistent in aquatic models (Sogard 1997;Moran 1999;Dziminski and Roberts 2006;Dibattista et al 2007).…”