Dissertation - HQ

Oriented swimming and passive advection 159
decisions more evenly in time. In addition, the recovery capacity of
older larvae would probably be higher than that of younger stages.
Therefore, swimming at maximum speed may become optimal at the
end of the larval phase. Second, the relationship defining the energetic
cost of swimming was identical for young and old larvae. The reasoning
behind this null-hypothesis is detailed on page 146. Yet, the eventuality
that swimming would be relatively more costly for young stages than
for old ones — as the authors initially suggested — has to be considered.
With a similar criterion of minimum energy expenditure, this should
result is less swimming at the beginning of larval life. Third, early larvae
may not have the sensory abilities to detect even local cues and orient in
consequence. Though there is evidence that even clownfish embryos can
detect sounds waves in the range of those produced by coral reef communities
255 , knowledge is currently lacking in this area. Nevertheless,
the two energetic constraints can be discussed. Even if early swimming
is frequently observed among optimal trajectories (Figure 6.18), it does
not mean that all larvae swim repeatedly for the first days after hatching.
On the contrary, swimming decisions along any single optimal
trajectory are quite sparse (Figure 6.17). In addition, if some larvae
swim at, or close to, their maximum U crit, many also swim at speeds
equivalent to only a fraction of it (Figure 6.18). Therefore, there is room
for more constraints on early swimming speed and endurance before
these optimal strategies become impossible energetically. Furthermore,
taxa which differ in early life history (i.e. demersal vs. pelagic eggs) are
usually distributed differently in space 130,162,168,274 (see also chapter 4).
This supports the hypothesis that events occurring early in the larval
phase have a large influence on its outcome 71 — though it might be
confounded by systematic differences during the rest of the pelagic interval:
species with demersal eggs are usually only passable swimmers
while species with pelagic eggs are, on average, very good swimmers by
the end of the larval phase 25 . Anyhow, it was already pointed out that
energy not invested in swimming is available for growth, and that larval
growth is particularly important for survival 48,50,170,237,238 . So a strategy
minimising overall energetic expenditure, such as early swimming here,
should at least be regarded as a selective advantage, if anything else.
Finally, the importance of early swimming decisions is highlighted
by the differences between the tropical and temperate larvae considered
here. The temperate species has very weak swimming abilities,
even late in larval life (U crit < 5 cm s -1 ), and, in addition, has pelagic
eggs. As a consequence, in the model, temperate larvae are initially
advected passively by currents and are not capable of coming back to
the recruitment zone if they are entrained too far away. This explains,
in part, why tropical larvae achieve self-recruitment rates of 95% and
temperate larvae only 40 to 70%, and makes early retention particularly
important for the latter. A natural development of this study would
be to investigate the case of tropical taxa with pelagic eggs, which
are passive at first, have longer pelagic lives, but are very capable
. . . but is probably an
evolutionary optimum