Role of the embryo in crab terrestrialisation: an ontogenetic approach

Abstract

Strategies permitting amphibious brachyurans to successfully occupy land environments have long been studied, with a focus on both the adult-terrestrial and larval-water-dependent stages. However, the ontogenetic approach to terrestrial adaptations in crabs has not considered the strategies of embryos, even though natural selection should act on all stages of development. We review the state of the field of reproductive adaptations of terrestrial crabs through both an extensive meta-analysis, aiming at testing the current hypotheses suggesting evolutionary trends towards an increase in egg size and decrease in total egg clutch during the conquest of land, and the presentation of novel data on bimodal respiration of crab embryos. Published studies on the morphological characteristics of eggs and on the reproductive traits of 121 marine, freshwater and terrestrial species of brachyurans could not confirm the currently hypothesized trends. Our meta-analysis confirms that the conquest of land by brachyurans implies strong selective pressures at all developmental levels, leading towards fundamental evolutionary steps, such as the air respiration of embryos. Our novel data on aquatic and aerial respiration in the embryos of an intertidal and a shallow subtidal species confirm recent data showing that early life stages of crabs are able to breathe in air, although they are apparently water dependent. An ontogenetic approach is needed to formulate new hypotheses regarding trends in terrestrial adaptations at all brachyuran life stages and to highlight other embryonic processes related to terrestrialisation, such as excretion, which is of the utmost importance in terrestrial adult crabs.