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

Survival of the swimming crab, Portunus trituberculatus, during the post-larval period from the megalopal to the first crab stage was low compared with successive larval stages in seed production trials conducted in Japan. To clarify the cause for the low survival, we explored whether rotifers, Artemia, or phytoplankton have negative effects during this period. The effects of the causative factor on larval survival and morphology were verified, and measures to overcome low survival were investigated. We conducted three experiments. In experiment 1, larvae were reared with different types of food (rotifers or newly hatched Artemia) with or without Nannochloropsis oculata supplementation during the late zoeal period (from the third to fourth zoeal stages). After moulting to megalopae, all larvae were fed newly hatched Artemia. The somatic sizes of the last stage (fourth stage) zoeae and megalopae were large, and developmental velocity was faster in the Artemia-fed groups than that in the rotifer-fed groups. However, Artemia feeding induced unviable and morphologically abnormal first-stage crabs with close-set eyes, regardless of Nannochloropsis supplementation. In experiment 2, third stage zoeae were reared to first stage crabs with variously treated Artemia such as newly hatched, 48h starved, and Artemia enriched with commercially available digestible Nannochloropsis with physically broken cell walls to confirm the negative effects of feeding Artemia. Larvae reared with the newly hatched and starved Artemia containing lower eicosapentaenoic acid (EPA) content exhibited low survival (1.9–3.5%) from the megalopal to the first crab stage, whereas larvae reared with Artemia enriched with digestible Nannochloropsis containing higher EPA content achieved higher survival (40.7%). In experiment 3, newly hatched Artemia were cultured under various conditions such as supplementation with untreated Nannochloropsis, digestible Nannochloropsis, or without phytoplankton to investigate what caused the Artemia to starve. The treatments supplemented with untreated Nannochloropsis and without phytoplankton suppressed Artemia growth and resulted in total mortality 4days after hatch. Supplementing with digestible Nannochloropsis maintained Artemia growth and resulted in higher survival, suggesting that early Artemia nauplii could not digest Nannochloropsis, which resulted in starvation. Our results indicate that Artemia starve in larval rearing water regardless of supplementation with Nannochloropsis; however, Nannochloropsis are usually added to seed production tanks. Artemia starvation during the late zoeal period exerted a negative carry-over effect during the post-larval period. We recommend nutritionally enriching Artemia with EPA and supplementing larval rearing water with digestible Nannochloropsis during the late zoeal stage.Improving larval rearing techniques for producing crablets.