Egg-trading in simultaneous hermaphrodites
Studying seabasses (Serranidae) may reduce measurements that
help determine whether Axelrod's payoff matrix is satisfied. In these shallow
water fishes, simultaneous hermaphroditism is a common feature (Fischer
1988, Conner 1992). They are known to be obligate outbreeders with external
fertilisation and planktonic eggs. Such a system almost certainly prevents
kin selection (Fischer 1988). During mating, each individual divides its
clutch of eggs into parcels and, subsequent to courtship displays, alternates
with its mate in offering parcels of eggs for fertilisation.
Owing to anisogamy, producing eggs is probably more costly than producing
sperm. Thus the 'temptation to defect' i.e. fertilising eggs without giving
any to be fertilised, is presumably high. Why do serranids still cooperate?
Fischer (1988) suggested the fishes were using TFT and the system would
apply to an IPD. However, there are several contradictions to this hypothesis:
Fischer himself already pointed out (1988) that by dividing the clutches
into a large number of parcels, the benefit b (i.e. T) of fertilising a
parcel of eggs is small. He also remarked that defection consists of the
absence of an act. i.e. failure to offer eggs, rather than a specific behaviour.
In his experiments (Fischer 1988) he referred to retaliation as waiting
"significantly longer to release a batch of eggs than [...] if the
partner did reciprocate". This is an obvious violation of the TFT
rule. He concluded that the fish were playing a nicer version of TFT e.g.
TF2T or GTFT. Yet, considering the evolution of the mating system, Connor
(1992) proposed that an alternative explanation for cooperation in seabasses
is more likely. It was suggested that simultaneous hermaphroditism evolved
originally at a low density of conspecifics where this reproductive system
has an advantage (Fischer 1988, Conner 1992). At that time the clutches
were not parcelled. Changing ecological conditions increased the abundance
of mates and parcelling appeared initially because it extended male mating
success (Fischer 1988): a parcelling individual in a population of non-parcellers
would therefore obtain more eggs from additional matings. Once parcelling
became common, the seabasses found themselves in an IPD (Fischer 1988),
possibly susceptible for a cheating strategy such as FR or Rover. Further
decreasing each investment may ultimately have led to R>T (Connor 1992)
and thus the system escaped the IPD. This still remains to be tested.
If Conner's model applies, both individuals will cooperate until the last
move, until there is only one parcel left. Not. offering this parcel will
favour the withholding of the parcel on the penultimate move and so on.
In Serranid seabasses, short spawning periods prevent cheating on the last
move, as eggs must be spawned on the same day they are produced or they
will become inviable. Since the chance of finding a new mate late in the
spawning period is low, it is better to get its eggs fertilised than not
to mate at all (Fischer 1988, Conner 1992). Moreover, Lima (1989) reported
that cooperation may be stable in a finite IPD whose end point is known
to all players.