Moult cycle and growth of the crab Halicarcinus planatus (Brachyura, Hymenosomatidae) in the Beagle Channel, southern tip of South America

Abstract

The crab Halicarcinus planatus is the only hymenosomatid crab that inhabits thesouthern tip of South America and is the only decapod species that reproduces twice ayear in the Beagle Channel. In this article, we study the moult cycle in the field (moultfrequency, analysis of size frequency distribution) and linked it with growth studied inthe laboratory (absolute and percent growth increment, Hiatt function). Hiatt functionswere similar for males and females. Moult frequency was seasonal: in early australspring and in austral summer. In females, the pubertal moult is the terminal moult,whereas males continue moulting after attaining the size of morphometric maturity.Moult increment was highly variable. The relationship between absolute moultincrement and crab size was described by a quadratic function. Percent growthincrement decreased with size and relationships were different for each sex: linear forfemales and quadratic for males. Seven and eight modal groups explained the sizefrequency distributions for females and males from the field, respectively, and revealedthe existence of two cohorts of recruits per year. Further modal analysis was mainlyhampered by the high variability of size increment that could make any moultingindividual fall in its own or one of two following modal groups. The antagonismbetween growth and reproduction was evident in small males. We hypothesize that theterminal pubertal moult is an advantageous feature that allows females to maximizetheir investment in reproduction after their terminal moult, which allows this species tohave two spawnings per year.