A Possible Mechanism for the Evolution of Aposematic Colouration in Solitary Nemerteans (Phylum Nemertea)

Abstract

A two-locus model for the evolution of warning colouration in a distasteful nemertean (phylum Nemertea) is presented. The crucial assumptions of the model are: (1) There are three possible colour forms in the population, coded by one of the two loci concerned: red, cryptic, and intermediate. The fitness of each morph depends on the water depth where it lives since the physical properties of the water will influence the absorption of red light whereby the degree of conspicuousness will change with depth. (2) A mutation arises in a population of worms living in the sublittoral, or deeper, causing the worms to seek more shallow water. (3) A change in the sublittoral environment increases the stress for those worms remaining there, while the relative fitness of mutants that seek shallow water increases. (4) Both the cryptic and red forms have higher fitness than the intermediate form in a population living in the littoral. This is based on the assumption that probability of discovery by a predator increases with increasing prey conspicuousness but is balanced by an enhanced avoidance learning ability which reduces the number of future attacks. Further, a threshold value of conspicuousness is assumed below which predator learning does not take place. Hence, the intermediate form has reduced fitness because its degree of conspicuousness is below this threshold, while still easier to detect than the cryptic forms. The model shows that warning colouration can evolve in a distasteful nemertean through individual selection, although I do not claim that it is the only, or general, way for this to happen. The population may arrive at two different genetic compositions (aposematic or cryptic), irrespective of the relative fitness of these states and only depending on the initial frequencies of the alleles at the locus coding for colouration. Although the model is explicitly designed for a hypothetical nemertean, the conclusions are applicable to any marine invertebrate living under similar conditions.