Comparative studies and evolution of muscles in chaetognaths

Abstract

The chaetognath locomotory muscles exhibit astonishing variations. The secondary muscle, which represents less than 1% of the body wall, has two forms. One is unique in the animal kingdom (alternation of two sarcomere types: “classical cross-striated” s1 and a unique type, s2) and has already been described; it characterizes most of the genera. The other, found only in more or less benthic species, functions by supercontraction. In the primitive genus Archeterokrohnia, it often exhibits “s1/s2 hybrid” sarcomeres, appearing as s2 sarcomeres partly containing s1 ones; moreover, the alternation of s1 and s2 sarcomeres is not regular. New sarcomeres are formed by the splitting of the Z-discs between two consecutive s1 sarcomeres. This is another unique feature of the chaetognath secondary muscle which facilitates understanding the transition from the first to the second form: suppression of s2 sarcomeres and constitution of a supercontraction capability in s1 sarcomeres. The transverse muscle, present in the less evolved genera living near or on the bottom, also has two forms: either classical cross-striated or, in more or less benthic species, supercontracting. Supercontraction would be a derived character. The peculiarities of the primary, secondary and transverse muscles of the truly benthic chaetognaths are different from those of all the planktonic or benthoplanktonic chaetognaths. The variations in the histological and cytological characteristics of the muscles agree with the phylogenetic trends previously proposed for the phylum.