The evolutionary puzzle of egg size, oxygenation and parental care in aquatic environments.
Offspring fitness generally improves with increasing egg size. Yet, eggs of most aquatic organisms are small. A common but largely untested assumption is that larger embryos require more oxygen than they can acquire through diffusion via the egg surface, constraining egg size evolution. However, we found no detrimental effects of large egg size on embryo growth and survival under hypoxic conditions. We tested this in the broad-nosed pipefish, Syngnathus typhle, whose males provide extensive care (nourishment, osmoregulation and oxygenation) to their young in a brood pouch on their bodies. We took advantage of this species' pronounced variation in egg size, correlating positively with female size, and tested the effect of hypoxia (40% dissolved oxygen) versus fully oxygenated (100%) water on embryo size and survival of large versus small eggs after 18 days of paternal brooding. Egg size did not affect embryo survival, regardless of O2 treatment. While hypoxia affected embryo size negatively, both large and small eggs showed similar reductions in growth. Males in hypoxia ventilated more and males with large eggs swam more, but neither treatment affected their position in the water column. Overall, our results call into question the most common explanation for constrained egg size evolution in aquatic environments.
Institute of Evolutionary Biology and Environmental Studies, Animal Behaviour, University of Zurich, Winterthurerstrasse 190, Zürich 8057, Switzerland Department of Biological and Environmental Sciences, University of Gothenburg, PO Box 463, Gothenburg 40530, Sweden ines.goncalves@ieu.uzh.ch.
Ines
http://orcid.org/0000-0003-0659-9029
I
Braga Goncalves
Department of Ecology and Genetics/Animal Ecology, Uppsala University, Norbyvägen 18D, Uppsala 75236, Sweden.
Ingrid
I
Ahnesjö
Department of Biological and Environmental Sciences, University of Gothenburg, PO Box 463, Gothenburg 40530, Sweden.
Charlotta
http://orcid.org/0000-0001-8983-2900
C
Kvarnemo
D000332:Aerobiosis; D000693:Anaerobiosis; D000818:Animals; D005075:Biological Evolution; D047108:Embryonic Development; D005260:Female; D008297:Male; D010063:Ovum / Q000502:physiology*; D010100:Oxygen / Q000378:metabolism*; D010332:Paternal Behavior; D012098:Reproduction; D012119:Respiration; D023701:Smegmamorpha / Q000196:embryology / Q000254:growth & development / Q000502:physiology*
D010100:Oxygen
Syngnathidae; egg size; embryo survival; fish; hypoxia; paternal care