Functional development of lumpfish Cyclopterus lumpus larvae in relation to growth and diet.

The lumpfish (Cyclopterus lumpus) is one of the most important cleaner-fish used to combat salmon lice infections in Atlantic salmon cages. They are cultivated commercially, most often with formulated diets as first feeding. However, larval growth and survival is very variable, and very little is known about the functional development of these larvae. Our aim was to describe the functional development of the lumpfish larvae, by studying the digestive and bone skeletal systems and possible allometric growth patterns. We fed the larvae different diets commonly used in commercial juvenile production of marine fish. Three larval groups were fed either a commercial formulated diet, cultivated copepods (Acartia tonsa), or enriched Artemia nauplii, during the first 20 days post hatching (dph), before weaning to a formulated diet until the end of the experiment 50 dph. The larvae were reared in nine 150 litre tanks at salinity 34 ppt, 10+0.5 ºC, and a light regime of 20 hours light and four hours darkness. Larvae were embedded in the plastic “Technovit® 7100” for histology, and stained with Alizarin Red for bone development. Feeding with enriched Artemia resulted in significantly faster growth, more rapid functional development and higher survival than with either of the other diets. At 51 dph, the mean dry weight of Artemia larvae was approximately the double of the weight in the other groups. The liver in newly hatched larvae contained large lipid stores. Some developing stomach crypts were apparent at 10 dph (6.6 – 6.8 mm standard length/SL). The stomach and pyloric caeca seemed functional at 34 dph (8.04 – 9.12 mm SL) in Artemia fed larvae, and at 42 dph in larvae from the other groups. The liver of larvae start-fed with a formulated diet or live copepods showed reduced nutritional status before weaning, but no pathology was found in either larval group. No size-difference in functional development was found between larvae that were fed different diets. Skeletal ossification was present at hatching, and all bony elements were well ossified in the largest larvae by 50 dph (>12 mm SL). Larval deformities were observed in 7 - 17% of the larvae at the end of the experiment, with no significant difference between groups. A suggested optimal larval period/size for startfeeding with live prey in relation to a functional juvenile state will be discussed.