Abnormal morphology in megalopae of the swimming crab,Portunus trituberculatusduring seed production: causes and prevention

Abstract

The occurrence of morphologically immature megalopae, which retain zoeal features such as dorsal spines and furcae of telson, is closely correlated with larval mass mortality during seed production of the swimming crab Portunus trituberculatus in Japanese hatcheries. To determine the cause of immature megalopal morphology, zoeae were reared with various supplementary schedules and density of diets (rotifer, Artemia and phytoplanktons including Chlorella vulgaris and Nannochloropsis oculata). In addition, to assess the relationship between immature morphology and endocrine control, the effect of causative dietary factor was compared with the effect of eyestalk ablation at various timing during zoeal development. Megalopal immature morphology was more distinct in Chlorella‐supplemented groups than in Nannochloropsis‐supplemented groups. High density Chlorella supplementation was associated with the highest incidence of immaturity and resulted in larval mass mortality. The premoult of the third zoeal stage was identified as a critical period at which Chlorella supplementation led to the highest incidence of immaturity. This critical period coincided with the critical period at which larval metamorphosis was regulated by the eyestalk neurosecretory system. Our results suggested that the occurrence of immature megalopal morphology under culture conditions is most likely caused by phytoplankton (especially, Chlorella) supplementation, which disrupts the endocrine regulation. On the basis of our results, we successfully prevented the occurrence of immature megalopal morphology in 500 L tanks by excluding the influence of phytoplankton before the critical period (i.e. discontinuing phytoplankton supplementation and supplying rotifer cultured with non‐phytoplankton materials).

Starvation of Artemia in larval rearing water affects post-larval survival and morphology of the swimming crab, Portunus trituberculatus (Brachyura, Portunidae)

“…Supplementing larval rearing water with digestible Nannochloropsis is suggested to be effective based on our results. In contrast, supplementing larval rearing water with phytoplankton, such as Nannochloropsis and Chlorella, may cause morphologically abnormal MG resulting in mass mortality via assimilation of these phytoplankton by rotifers (Dan et al, 2013(Dan et al, , 2014(Dan et al, , 2015. The critical period at which phytoplankton induce an abnormal MG is estimated to be the Z3 premoult (Dan et al, 2014(Dan et al, , 2015, and this period coincides with the Artemia feeding period during seed production (Dan et al, 2013).…”

“…In addition to the absolute length of the chela, the ratio of the chela length (relative chela length, CHLr) to the CL was calculated (as percentages) as an index of abnormal morphology in Z4 (Arai et al, 2004(Arai et al, , 2007Hamasaki et al, 2011). CL and length of the dorsal spine (DSL) retained on the MG carapace were measured, and the frequency of megalopae retaining the dorsal spine (DSf) and furcae on their telsons (TELf) were calculated as indices of abnormal immature morphology in MG (Dan et al, 2013(Dan et al, , 2015 in experiments 1 and 2. These measurement methods are described in Dan et al (2013Dan et al ( , 2014.…”

“…The occurrence of morphologically immature megalopae with zoeal features, such as a small dorsal spine and furcae of telson, has been associated with decreased survival during the last zoeal to the megalopal stages; these morphologically immature megalopae die within a few days after metamorphosis (Dan et al, 2013). The immature morphology of the megalopae is induced by phytoplankton, such as Chlorella vulgaris and Nannochloropsis oculata, supplemented in larval rearing water as food for rotifers (Dan et al, 2015). The critical period at which the phytoplankton induce immature megalopae was identified as premoult of the third zoeal stage (Dan et al, 2014(Dan et al, , 2015.…”

“…The immature morphology of the megalopae is induced by phytoplankton, such as Chlorella vulgaris and Nannochloropsis oculata, supplemented in larval rearing water as food for rotifers (Dan et al, 2015). The critical period at which the phytoplankton induce immature megalopae was identified as premoult of the third zoeal stage (Dan et al, 2014(Dan et al, , 2015. Based on these results, the occurrence of immature megalopal morphology could be prevented by excluding phytoplankton before a critical period; i.e., exchanging the majority of larval culture water before the critical period, discontinuing phytoplankton supplementation, and alternative feeding with rotifers enriched with a non-phytoplankton source after water exchange (Dan et al, 2015).…”

“…However, the cause of low survival from the megalopal stage to the juvenile crab stage remains uncertain. Although we used these measures and succeeded in preventing abnormal morphology in megalopae, post-larval survival to reach the first crab stage remains low (Dan et al, 2015), suggesting that other underlying factors are retarding larval survival. We found that larval survival after the megalopal stage was reduced in almost all cases when larvae were reared without phytoplankton supplementation after the third zoeal stage to prevent the immature megalopal morphology.…”