Eyestalk ablation affects larval morphogenesis in the swimming crab Portunus trituberculatus during metamorphosis into megalopae

Abstract

To understand the role of the eyestalk neurosecretory system in regulation of larval morphogenesis, we performed eyestalk ablation on swimming crab Portunus trituberculatus larvae at various times during zoeal development. We measured the length of the chelae and pleopods, which become enlarged during development, and the dorsal spine and telson furcae, which are resorbed during metamorphosis in the final (fourth) stage zoeae and subsequent larval stages, including a supernumerary fifth stage zoeae and megalopae (fifth-instar larvae). The length of the chela and pleopod of fourth stage zoeae decreased when the bilateral eyestalks were ablated earlier during development. Eyestalk ablation had little effect on the zoeal dorsal spine and furcae. In fifth-instar larvae, the effects of eyestalk ablation changed radically depending on the time when the ablation was performed, and a critical period during the premoult of the third zoeal stage was identified. Ablation before this period caused retention of a large dorsal spine and furcae and resulted in moult to the supernumerary fifth zoeal stage. Ablation after this period allowed larvae to metamorphose into normal megalopae. Ablation during this period resulted in megalopae with immature morphology, whereby the larvae retained small dorsal spines and telson furcae. The results demonstrated that the eyestalk neurosecretory system most likely regulates larval morphogenesis during metamorphosis in 2 ways: the morphogenesis of body parts that are enlarged are continuously controlled throughout the zoeal stages, whereas the resorption of body parts is controlled instantaneously at a critical point during the premoult of the third zoeal stage.KEY WORDS: Larval morphogenesis · Larval metamorphosis · Morphological abnormality · Mass mortality in seed production · Moult-death syndrome OPEN PEN

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

“…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). Measures to prevent starvation of Artemia while preventing abnormal MG morphology should be developed to produce viable C1 with high survival rates.…”

“…First stage zoeae (Z1) that hatched from three females with carapace widths (CW) of 163, 174, and 190 mm were equally stocked in a cylindrical 500-L tank (total, 9800 larvae) and cultured at 25°C and 33 ppt salinity following the method described by Dan et al (2014). Then, they were given the designated treatment in cylindrical 30-L tanks the day before moulting to the third zoeal stage (Z3).…”

“…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 C1 juveniles with close-set eyes in experiments 1 and 2 were observed in some treatment groups ( Fig.…”

“…Thirty Z3 larvae were stocked in each 1-L beaker (25.2°C, 33.3 ppt) and reared according to the method of Dan et al (2014); larvae were transferred into newly prepared beakers containing new seawater and food (each treated Artemia) every morning. Artemia were hatched over 24 h at 25°C (NH Artemia group), and they were cultured for 48 h at 25°C at 200 individuals mL −1 in 10-L tanks without (starved Artemia group) or with commercially available digestible Nannochloropsis (Marine Alfa, Marinetech Co. Ltd, Aichi, Japan) at 30,000,000 cells mL −1 (dNannochloropsis Artemia group).…”

“…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).…”