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.MethodsThree adjacent populaions were surveyed annually, in January, using mark-recapture methods. The colour phase of every individual was scored into one of three categories. The analyis looks at how the relative numbers of snakes of eachcolour morph changed through time in each population.进化理论表明，多态性状仅能在特定条件下维持于单一种群之中，例如负频率依赖选择（negative frequency-dependent selection）或杂合子优势（heterozygote advantage）。栖息于新喀里多尼亚努美阿附近浅湾的无毒龟头海蛇（Emydocephalus annulatus）具有三种体色型：黑色型、黑白相间带纹型，以及一种随年龄增长而体色加深的中间型（灰纹型）。我们在连续18年的野外调查中记录了各体色型的频率，结果发现：当某一色型在种群中异常罕见时，该色型的新生个体（幼蛇+迁入个体）数量会随之增加；而当该色型异常常见时，其数量则会下降。由此可见，该种群的体色型频率由负频率依赖选择维持。 我们将该现象解释为对同海湾内分布的剧毒海蛇（涵盖Aipysurus、Hydrophis、Laticauda三属）的贝茨拟态（Batesian mimicry），这些剧毒海蛇的体色范围覆盖黑白带纹至灰纹类型。已有行为研究证实，拟态可使海蛇免遭大型鱼类与海雕的攻击：该区域的小型鱼类会主动躲避带纹型海蛇，却会捕食黑色型个体。正如理论预测的那样，拟态型（带纹型）的种群丰度低于具有隐蔽体色的黑化型个体。 方法 我们于每年1月对三个相邻种群开展年度调查，采用标记-重捕（mark-recapture）法。每一个体的体色阶段被划分为前述三类之一。本分析旨在探究各体色型的相对数量在每个种群中随时间的变化规律。