Sexual differentiation, gonad maturation, and reproduction of the Southern Ocean euphausiid Thysanoessa macrura (Sars, 1883) (Crustacea: Euphausiacea)

摘要

Thysanoessa macrura (Sars, 1883) is a small, albeit highly abundant, Southern Ocean euphausiid. It has been reported that T. macrura reproduces during late winter, with the reproductive cycle decoupled from periods of high primary productivity in the highly seasonal Southern Ocean. Despite this uncommon reproductive strategy, there has been little work to describe this cycle and quantify the reproductive potential of this species. Thysanoessa macrura collected from Antarctic waters during early September to early October 2007 were used to establish sexual developmental stages, for females and males, to provide a standard means for assessing the reproductive maturity of this species. Seven sexual developmental stages were established for females based upon the organisation of ovaries and their constituent oogonia and oocytes. Four sexual developmental stages were identified in males using secondary sexual characters. A large lipid deposit was identified in the carapace of both sexes, reducing in size with progressive gonadal development, likely fuelling egg and sperm production. Egg-batch size was determined by direct ovarian counts and scaled allometrically with total wet weight. Predicted egg-batch size ranged from 34 to 746 eggs batch(-1) for the range of individual sizes studied (30.5-165.6 mg). Using the ratios of the different oocytes remaining after the first-spawning event of females, it was predicted that up to three egg batches can be produced by T. macrura in a single reproductive season. Consequently, the fecundity of T. macrura is estimated at 60 to & 2200 eggs female(-1) year(-1). A model of T. macrura reproductive cycle confirms a winter reproductive period and further highlights the reproductive potential of this small-size Southern Ocean euphausiid, providing the first estimates of fecundity reported for this species.