The myotome formula of the lanceletEpigonichthys lucayanum(Acrania): can variations be related to larval dispersion patterns?

Variation in the myotome formula of the acraniate Epigonichthys lucayanum (Andrews) is discussed in relation to that of its putative larval stage Amphioxides pelagicus (Günther). In certain adult populations, two main forms can be recognized: typically, one form has 58–61 myotomes, of which 38–40 are pre-atrioporal, and the other form has 65–69 myotomes, of which 43–45 are pre-atrioporal. These two adult forms are thought to be derived from different larval stages, the former from an amphioxus larva-like stage that has metamorphosed after a short planktonic phase and the latter from the amphioxides stage which has undergone a long, more widely dispersed, planktonic existence. The adaptations for a prolonged planktonic life developed in the amphioxides, especially gill-slit development, appear to be reflected to some extent in the meristic characters of the resulting adult.