Colour polymorphism in the sea snake Emydocephalus annulatus

Evolutionary theory suggests that polymorphic traits can be maintained
within a single population only under specific conditions, such as
negative frequency-dependent selection or heterozygote advantage.
Non-venomous turtle-headed sea snakes (Emydocephalus annulatus) living in
shallow bays near Noumea in New Caledonia exhibit three colour morphs:
black, black-and-white banded, and an intermediate (grey-banded) morph
that darkens with age. We recorded morph frequencies during 18 consecutive
years of surveys, and found that the numbers of recruits (neonates plus
immigrants) belonging to each morph increased in years when that morph was
unusually rare in the population, and decreased when that morph was
unusually common. Thus, morph frequencies are maintained by negative
frequency-dependent selection. We interpret the situation as Batesian
mimicry of highly venomous sea snakes (Aipysurus, Hydrophis, Laticauda)
that occur in the same bays, and range in colour from black-and-white
banded to grey-banded. Consistent with the idea that mimicry may protect
snakes from attack by large fish and sea eagles, behavioural studies have
shown that smaller fish species in these bays flee from banded snakes but
attack black individuals. As predicted by theory, mimetic (banded) morphs
are less common than the cryptically-coloured melanic morph.