Ontogenetic Development and Nutritional Requirements in Early Life Stages of the European Lobster (Homarus gammarus, L.)

The European lobster, Homarus gammarus, is an ecological and economically important species. Historically, it has been subjected to intense fishing pressure that caused the decline of several wild populations. To counteract the decrease in annual landings various stock enhancement programs have been launched around Europe. These initiatives are supported by hatchery-reared juvenile lobsters that are released into natural habitats. The commercial cultivation of the species is also attracting considerable attention and H. gammarus is currently considered an emerging aquaculture species. However, the economic viability of European lobster hatcheries is hampered by several drawbacks including low survival and growth rates, the strong cannibalistic nature of the species, and the dependence on live, fresh, and frozen diets.

The main purpose of this Ph.D. project was to increase knowledge on the metabolism and nutritional requirements of H. gammarus early stages and thus provide solutions to nutritional challenges faced by hatchery production units. Specific goals were: (1) to identify ontogenetic changes in the digestive capacity and potential nutritional requirements of larvae and postlarvae; (2) screen macronutrient levels in the design of formulated feeds for early juvenile stages; (3) assess the potential of alternative and more sustainable ingredients in formulated feeds for juvenile lobsters; and (4) evaluate the effect of dietary composition on the resilience of juvenile H. gammarus to environmental variation.

For that, five studies were conducted. In the first study, the digestive enzymatic activity (trypsin, amylase, and lipase) and biochemical indices (RNA:DNA ratios, proximate and lipid class composition) were measured in stages I, II, and III larvae, and stage IV postlarvae. Main results showed that lipase activity increased from stage I to stage III but not further, suggesting the increasing importance of lipids during the first three larval stages. Amylase activity increased significantly at stage IV pointing to a shift towards a carbohydrate-richer diet after metamorphosis. Cholesterol and phospholipids (phosphatidylcholine and phosphatidylethanolamine) were the most abundant lipid classes in larvae and postlarvae body tissues suggesting high dietary requirements for these compounds.

The second study examined the impact of formulated feeds on the metabolic cost of feeding (specific dynamic action, SDA), basal metabolism (standard metabolic rate, SMR), and nitrogen retention in juvenile H. gammarus. The metabolic cost of digesting and assimilating extruded feeds was comparable to that of digesting and assimilating a standard diet composed of thawed Antarctic krill. However, the basal metabolism and nitrogen retention decreased significantly in juveniles fed low protein (40%) and medium (1:2) to high (1:1) lipid : carbohydrate ratios likely caused by the low intake of these feeds. The higher level of carbohydrates (1:3 ratio) in low protein feeds (40%) enhanced feed intake, and consequently, the metabolic capacity and nitrogen retention. The basal metabolism and nitrogen retention in juveniles fed feeds with high protein content (50%) was comparable to that of lobsters fed Antarctic krill, regardless of the lipid : carbohydrate ratio.

In study 3, the performance of juvenile H. gammarus fed formulated feeds was compared to the performance of lobsters fed Antarctic krill during 8-weeks. Growth, survival, and nutritional condition (evaluated through RNA:DNA ratio) were assessed and results showed that the most suitable experimental formulated feed was the 500MED with a macronutrient combination of 50% protein, 24% carbohydrates, and 12% lipids. Juveniles reared on this feed performed as well as those reared on Antarctic krill in all the performance indices. Additionally, among the experimental formulated feeds, the 500MED yielded the most efficient feed conversion ratio (FCR).

Study 4 tested the inclusion of shrimp waste meal (SWM) as an alternative protein source in formulated feeds for early juvenile H. gammarus. Growth, survival, nitrogen metabolism, and exoskeleton colouration were evaluated. The highest inclusion level of SWM (28% of dietary protein) did not affect growth nor nitrogen metabolism and enhanced survival of juvenile lobsters demonstrating its potential as an alternative ingredient. The highest SWM inclusion level was insufficient to improve exoskeleton colouration which was hampered by the lower, than initially projected, astaxanthin level in the alternative ingredient.

In the last study (study 5), the interaction effect between temperature and dietary composition on the energy metabolism and performance of juvenile lobster was evaluated. Juveniles fed with extruded feeds with either a high-protein (HP) or a high-carbohydrate (HC) content and thawed Antarctic krill (AK) were exposed to low temperatures of 13°C. Corresponding control groups for each diet were maintained at optimal rearing temperatures of 19°C. Lower temperatures induced a decrease in feed intake in all dietary groups but results showed that H. gammarus juveniles fed the HC feed were the least resilient to temperature variation. Results also disclosed that juvenile lobsters adapted their energy metabolism according to the diet received.
In conclusion, the results from this thesis provide new insights into the metabolism and nutritional requirements of the early stages of H. gammarus. Results demonstrate that protein is a key nutrient for all stages examined, while lipids are of particular importance during larval development and dietary carbohydrate requirements increase after metamorphosis. The potential protein-sparing effect of carbohydrates is of great interest and should be further studied. Lipid sources of richer phospholipid content might improve H. gammarus lipid digestion and assimilation and, therefore, its inclusion in feeds for H. gammarus early stages deserves further investigation.