WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2016 33 Atlantic halibut Hippoglossus hippoglossus is the largest existing flatfish and reportedly can grow to more than 4.7 m in length and weigh 300 kg over a lifespan of over 50 years. This species traditionally has been fished for centuries and remains a highly valued seafood source today. Originally Atlantic halibut was distributed throughout the northern part of the North Atlantic Ocean and in parts of the Barents Sea. However, overfishing has led to substantial stock decline without any sign of recovery and it is now listed as an endangered species. Today, most Atlantic halibut fisheries have been closed, although there is by-catch mortality through demersal fishing gear. Thus, Atlantic halibut has become a scarce commodity while demand for it has remained high, a combination that has drawn the interest of the aquaculture industry and resulted in slowly emerging aquaculture production, initiated by Norway and Iceland in the mid-1980s. The production of Atlantic halibut has shown some signs of expansion in recent years, and in Norway there is currently a small but steady production of the species. Although there are persistent challenges in the production cycle, particularly during first feeding stages, some important issues, such as abnormal development and mal-pigmentation during metamorphosis, have been reduced in Atlantic halibut production after a diurnal cycle with a period of 7 h of darkness was introduced (Harboe et al. 2009). Although it remains to be verified, this period of darkness may permit more complete digestion and absorption of ingested feeds. However, it is important to build basic knowledge of the production biology of Atlantic halibut, inasmuch as this will permit further improvement of feeding protocols, diets and larval quality, which will ultimately help to increase production efficiency and profitability. Atlantic Halibut Metamorphosis Similar to other flatfish, Atlantic halibut larvae (Fig. 1) go through a very striking metamorphosis compared to other fishes. Metamorphosis is associated mainly with the morphological transformation of an organism, but looking at the bigger picture, this process signifies a wide range of changes in behavior, habitat and physiology. The common knowledge of the public connects this complex process almost exclusively with biology textbook examples of amphibians (the frog) or insects (butterflies), yet a fascinating case of metamorphosis can be found in flatfish species, including Atlantic halibut. Metamorphosis also occurs in most marine fish species, but the process is much more visible in flatfish. Metamorphosis means “changing form” (from the Greek, meta meaning change and morphe meaning form) and classically it is considered as a dramatic and abrupt transformation of a larva into a juvenile. Since the dawn of natural history studies, this fascinating process has captivated scientists and artists alike. Despite all the scientific progress since the first documented experiments (Gudernatsch 1912), one century later many questions remain unanswered. In most species, environmental conditions play an important role in providing signals that trigger transformation of a larva into its adult form (Laudet 2011). The hormonal system has a key role in this process, translating environmental cues into a coordinated program that remodels the organism. In addition, larva and adult do not live in the same habitats or at least do not share the same resources, thus the metamorphosis is also an ecological transformation. Moreover, metamorphosis is an extensive transformation of the animal and is never just a morphological change; it includes physiological, biochemical and histological remodeling that affects a range of tissues and organs. Therefore, metamorphosis is a change in form and function that allows the organism to transit between life-stages to use new resources in a different habitat. It is exactly this advantage that offers an explanation of why an energy-intensive process such as metamorphosis evolved in the first place. By allowing larvae and adults to occupy entirely different ecological niches, metamorphosis eliminates competition between them and elevates overall chances of survival. Metamorphosis is a crucial event in the ecology of Atlantic halibut and coincides with the final recruitment of larvae into its adult ecological niche. During their transformation, the Atlantic halibut drastically changes in form and function, enabling it to transition from pelagic to benthic habitats and change feeding behavior entirely. From first feeding through metamorphosis to settlement, Atlantic halibut larvae can be classified according to their external morphology and this permits identification of a sequence of distinct larval stages: 5: premetamorphosis; 6-7: prometamorphosis; 8: proclimax metamorphosis; 9: climax metamorphosis; and 10: post-metamorphosis (Sæle et al. 2004). Atlantic halibut larvae of stages 5-7 are bilaterally symmetrical and transparent, but with pigmented eyes. The transition to asymmetry starts during stage 8, in which larvae are still pelagic, but have a relatively large size and frequently have started to tilt to one side. At Metamorphic Challenges Rearing Atlantic Halibut Larvae Ana S. Gomes, Torstein Harboe and Ivar Rønnestad FIGURE 1. Atlantic halibut fish larvae after being fed Artemia (Photo: Torstein Harboe). (CONTINUED ON PAGE 34)
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