Our methodological approach was strong enough to also
find intra-generic differences in oxygen uptake for both
Chiromantes spp. and Perisesarma spp. across stages and
in both respiratory media. A comprehensive discussion
regarding these differences is a difficult task at the present
time, since there is no detailed information on the biological,
reproductive and ecological differences between the
two species of each genus we analysed. Indeed, the East
African Chiromantes spp. and Perisesarma spp. widely
overlap, both at the micro- and macro-scale, and further
ecological studies are required to understand the true ecological
niche and habits of these species.
Another interesting trend we observed was the smaller
egg dimension of the more land-adapted species, which does
not accord with the hypothesis that more terrestrial species
should have larger eggs. The general assumption is that the
combined effects of surface tension and gravity should
generate an anoxic microenvironment, by creating a bubble
of water surrounding the whole egg mass and filling its
interstices with standing water, ultimately causing problems
of hypo- and anoxia. Therefore, in evolutionary terms, larger
eggs should be selected, as they generate enlarged interstices
and permit enhanced oxygen diffusion, reducing viscosity, in
the egg mass (Strathmann and Chaffee 1984; Strathmann and
Hess 1999; Anger 1995; Seymour 1999). However, our
results do accord with the work of Cannicci et al. (2011)
which refutes this hypothesis based on a meta-analysis performed
on 121 species comprising marine, intertidal, freshwater
and terrestrial species, finding that egg dimension
correlates with reproductive strategy and not with the level of
terrestrialization.We suggest that the higher diffusiveness of
oxygen in air than in water permits normoxic conditions in
the central part of the egg mass. Consequently, the small
volume of Chiromantes spp. eggs allows the egg mass to
remain moist for a longer period, by means of reducing the
interstices within the mass that act as capillarity tubes and
maintain standing water due to surface tension and gravity,
ultimately limiting complete desiccation and avoiding the
consequent physiological stress.
In conclusion, our comparative approach allowed us to
demonstrate an adaptive trend towards terrestrialization in
the embryos of supralittoral East African sesarmids. In fact,
the bimodal late-stage embryos of the land-adapted Chiromantes
spp. displayed an enhanced metabolism when
immersed in sea water that we interpreted as the need to
fulfil the metabolic requirements of the pelagic larvae
while embryos approach hatching. Moreover, with respect
to the less land-adapted species, Chiromantes spp. produced
smaller eggs that maintain moisture in the egg mass
for longer periods and can cope with prolonged emersion
periods, showing a morphological adaptation to terrestrial
life. However, we confirm the water dependence of the
embryos of semi-terrestrial sesarmids, providing strong
support for the hypothesis that they evolved bimodal respiration;
yet, they are lacking other physiological characteristics
typical of adults, such as terrestrial adaptations to
ammonium excretion, acid–base balance and osmoregulation.
Therefore, we can confirm that the selective pressure
for terrestrial adaptation acts at all ontogenetic levels, and
that the embryos of land-adapted species, although the end
product is a fully marine larva, are not a strong limiting
factor for semi-terrestrial females, which are not restricted
in their emersion period by the oxygen requirements of
their embryos.