Abstract
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.
Highlights
Egg size has long played an important role in the conceptual framework built around the evolution of life histories of marine organisms [1,2,3,4,5,6,7,8]
We addressed the following question: are there predictable relationships between egg size, egg composition, and egg energy in planktotrophic species? We investigated this question in the context of geminate sea urchins, which are sister taxa separated by the Central American Isthmus (CAI) that rose approximately 2–4 million years ago [34] and divided a once continuous marine environment into the tropical western Atlantic (WA) and eastern Pacific (EP) oceans [35]
Ec. lucunter (WA) eggs were significantly less carbohydrate-rich than Ec. vanbrunti (EP); there was no significant difference in the proportions of carbohydrate in eggs of Ec. viridis (WA) and Ec. vanbrunti (EP) (Tables 1,2)
Summary
Egg size has long played an important role in the conceptual framework built around the evolution of life histories of marine organisms [1,2,3,4,5,6,7,8]. Though much attention has focused on the relationship between egg size and key larval life history characters like those listed above, we understand considerably less about the associations among egg size, egg composition, and egg energy. The three primary biochemical constituents of eggs of marine invertebrates are protein, lipid, and carbohydrate [10,21,22]; these provide the major energetic and structural elements for larval morphogenesis and development and can be measured in a variety of ways (see [23] for review). Little is known about the extent to which egg size and egg composition are functionally or evolutionarily linked
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