Ovarian development in Argentinean shortfin squid Illex argentinus: group-synchrony for corroboration of intermittent spawning strategy

Modern living cephalopod species have evolved a wide phenotypic diversity and flexibility of reproductive strategies, which is closely linked to the pattern of oogenesis and oocytes ovulation of species. Although it has been suggested that Argentinean shortfin squid Illex argentinus lay eggs intermittently, there is still little evidence for the mode of oocyte production and development. In this study, the ovarian development of I. argentinus was investigated by using histological analysis of ovaries, and six distinct histological stages of ovarian development were found among the ovaries. For each histological stage of the ovary, the frequency distribution of both number and occupied areas by each oogenesis stage was unimodal, and that gradually progressed along with ovarian development. The oocyte size distribution in ovaries before reaching vitellogenic stage was unimodal, bimodal in vitellogenic ovaries, and polymodal in ripe and partially spent ovaries. This evidence indicates that I. argentinus undergoes group-synchronous ovarian development, with a single dominant oocyte stock being produced to develop in several batches for a multiple-batch group-synchronous ovulation and confirms the intermittent spawning strategy of this species.