Female gonad morphology of small copepods Oithona similis and Microsetella norvegica

Abstract

Small marine copepods, such as Oithona similis and Microsetella norvegica, form the core of polar planktonic communities. However, little is known about the gonad morphology of these species and their reproductive strategies are still debated. We studied the female genital system and reproductive traits of these common copepods collected from the White Sea during their main reproductive period (June 2012 and May 2014) and in a pre-winter season (November 2013). The applicability of the borax carmine staining method for the gonad investigation of these small species was successfully tested for the first time. We proposed the classification of O. similis gonad developmental stages that could be used to estimate the reproductive status of the population. The gonad maturation types of O. similis (Cyclopoida) and M. norvegica (Harpacticoida) are in general similar to calanoid copepods, with Pseudocalanus-type and Aetidius-type gonads, respectively [classified by Niehoff (Prog Oceanogr 74:1–47, 2007)]. Both the studied species form large oocytes at advanced maturing stage, before the hatching of the eggs carried in the sac, thereby shortening the time needed for the development of the new clutch. The percentage of fertilised O. similis females in natural population was also assessed. Many of the O. similis females were found to have spermatophores (39 % in May, 68 % in June), and almost half of them carried two or even three pairs of spermatophores. The simultaneous presence of unmated and multiple-mated adult females showed high mating opportunities and selectivity of O. similis males in some of the field populations obtained from the White Sea. Our findings do not support the “fertilisation limitation hypothesis” proposed for Oithona (Kiorboe in Limnol Oceanogr 52:1511–1522, 2007).