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Temperature and light are important factors affecting production in the aquaculture industry, as they can drive behaviour and physiological responses of free‐swimming larval stages. However, the influence of light on crustacean farming has received little consideration. The common spider crab Maja brachydactyla Balss, 1922 has a great potential for aquaculture because of the easy maintenance, high fecundity, and short larval development. In order to optimize larval culture techniques, we quantified the influence of temperature and light on larval survival, development, and elemental (carbon and nitrogen) body composition. Constant darkness resulted in longer developmental time as compared with daily light photoperiod (6–16 light hours). Larvae reared under constant darkness showed also reduced dry mass, carbon and nitrogen content, and C:N ratio. We also found carry‐over effects of light conditions: constant darkness experienced during the zoeal stage led to increased developmental time in the megalopa stage. Temperature and light showed additive effects. We optimized the larval culture of M. brachydactyla requiring around 14 days from hatch to first juvenile by keeping cultures under 21 ± 1°C and light sources simulating the daily light cycle.

We investigated the nutritional vulnerability of Zoeae I of the spider crab Maja brachydactyla in experimental treatments with differential periods of starvation. As response variables, the moulting and survival rates, the point-of-reserve-saturation (PRS), the point-of-no-return (PNR), dry mass, elemental composition (carbon, hydrogen, nitrogen; CHN), hepatopancreas lipid vacuoles and activities of digestive enzymes were measured. Average median time spans when 50% of the larvae reached PNR or PRS (PNR50 and PRS50) values were 2.8 and 1.9 d, respectively. In PNR treatments, complete mortality occurred only after extended initial starvation periods of ≥5 d, and in PRS treatments, Zoeae I were capable of completing the moulting cycle after a short initial feeding period (1 d). In continuously starved control larvae, the moulting cycle was arrested at the onset of apolysis. Concomitantly, considerable amounts of biomass were lost and the C:N ratio decreased, indicating lipid degradation during starvation. This effect was also microscopically visible as a decline of lipid vacuoles in the hepatopancreas. Feeding after previous starvation periods of <7 d resulted in a re-establishment of the lipid vacuoles, indicating successful capture and ingestion of food, but this did not necessarily allow for completing the moulting cycle. In fed Zoea I larvae, digestive enzyme activities increased during the moulting cycle, while a significant decrease of enzyme activities occurred under starvation conditions. In newly moulted Zoeae II, biomass values and enzyme activities were linearly correlated with the duration of the feeding periods in Zoeae I. This study shows that biomass, elemental composition, the occurrence of lipid vacuoles in the hepatopancreas and activities of digestive enzymes are suitable indicators of the nutritional condition of early zoeal stages of M. brachydactyla.

Ontogenetic changes in dry weight, elemental composition (CHN) and activities of digestive enzymes (total protease, amylase) were studied through the moulting cycles of all larval stages (zoea I-II, megalopa; ZI, ZII, M) of the spider crab Maja brachydactyla Balss 1922 reared in the laboratory. All analyses were performed in single larvae to evaluate individual variability. Digestive enzyme activities were analysed by fluorometry. Changes in dry weight were only in premoult M statistically significant, whereas no differences were observed among Z and postmoult M (Fig. 1). No clear pattern was observed in the elemental composition (CHN in % of DW). An increase in enzyme activities throughout development was observed, although no statistically significant differences were found. Amylase (Fig. 2) and total protease activities were different among M moulting stages, being highest at premoult and lowest at intermoult. It seems that high individual variability (high standard errors) masked ontogenical changes in digestive enzyme activities, since previous data obtained from larval pools using spectophotometry analyses indicated that total protease and amylase activities were significantly higher between ZI and M. Despite lacking significance, the present study shows that different moulting stages presented different physiological conditions, and therefore, nutritional requirements may undergo marked changes during larval moult cycles. Hence, moulting stages must be taken into account in future studies to evaluate the nutritional condition of crustacean larvae.

The moulting cycles of all larval stages (zoea I, zoea II, megalopa) of the spider crab Maja brachydactyla Balss 1922 were studied in laboratory-reared individuals. Changes in the epidermis and cuticle were photographically documented in daily intervals and assigned to successive stages of the moulting cycle (based on Drachs classification system). Stage characterizations were mainly based on microscopical examination of integumental modifications in the telson, using epidermal condensation, apolysis, the degree of epidermal retraction, and morphogenesis (setogenesis) as criteria. In the zoea II and megalopa, the formation of new setae was also observed in larval appendages including the antenna, maxillule, maxilla, second maxilliped, pleopods, and uropods. As principal stages within the zoea I moulting cycle, we describe postmoult (A-B), intermoult (C), and three premoult stages (D0, D1, D2). In the zoea II and megalopa, stages stages D1 and D2 had to be combined, because morphogenesis (the main characteristic of stage D1) was unclear in the telson and did not occur synchronically in different appendices. The duration of successive moulting stages within each larval instar and major morphological changes in the telson are summarized in Table 1. The knowledge of this sequence can be used as a tool for the evaluation of the developmental state within larval stages, which may facilitate future studies on crab aquaculture.

The Norway lobster, Nephrops norvegicus, is a commercially exploited decapod which is widely distributed throughout the north-eastern Atlantic and the Mediterranean Sea. Ovigerous females originating from the Mediterranean and the Irish Seas were held in the laboratory until larvae hatched. Biomass and biochemical composition, as well as digestive gland structure, were examined in newly hatched larvae from these two regions. In addition, previously published data from a North Sea population were included in our comparison. Elemental analyses showed that the absolute quantities of dry mass (DM), carbon (C), nitrogen (N) and hydrogen (H) (collectively referred to as CHN) per individual, and the C:N mass ratios, were significantly lower, while the relative CHN, protein and lipid values (in % of DM) were higher in samples from the Irish Sea compared to larvae originating from either the Mediterranean or the North Sea. As in CHN, the absolute level of protein per individual was higher in larvae from the Mediterranean, while no significant differences were observed in the individual lipid contents. Likewise, the digestive gland structure at hatching did not show any differences between study areas. Intraspecific variability in biomass and chemical composition of newly hatched larvae from different regions may be related to differential patterns of reproduction in regions with different climatic conditions. Lobster larvae hatch in the Mediterranean Sea predominantly in winter when both water temperature and planktonic food availability are at a minimum, while hatching in the Irish Sea occurs under more favourable conditions in spring. Hence, significantly higher wet mass, dry mass and protein values in Mediterranean larvae may represent adaptive traits allowing for early posthatching survival and development under food-limited conditions in an oligotrophic environment.

Few studies have been conducted on the culture of Nephrops norvegicus larvae and none of them have permitted significant numbers of postlarvae to be obtained (stage IV). The goal of this study was to determine a suitable diet for rearing N. norvegicus to the juvenile stage V. Survival, moulting periodicity, growth, and elemental composition (CHN) of the young postembryonic stages were used as criteria of physiological condition. Four different diets, fresh enriched nauplii of Artemia (FENA), artificial feed for shrimp (AF), commercial frozen mussels (M), and commercial frozen adult Artemia (FAA) were tested from hatching to stage V at ambient temperature (14-16 °C). Five day old stage I larvae fed with AF, M or FAA showed a similar biomass as freshly hatched larvae, indicating that the energy derived from food allowed them to survive but not to gain length. FENA was the only diet that resulted in development to stage V. Larvae fed with FENA presented the shortest time of development, the highest survival, and the highest growth rates in size and biomass.

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