Ontogeny of osmoregulatory functions and structures in cod larvae

The ion and water regulation of teleost fish has been studied in great detail, but only little is known about its ontogenetic development. Survival and establishment in a certain habitat – or even a commercial setting – is often tightly correlated with the ability of each developmental stage to cope with salinity through osmoregulation. Similar to other bony fish that inhabit the sea, Atlantic cod (Gadus morhua) are surrounded by an environment that has a much higher salt content than their inner tissue. This means that the fish constantly loses water to the surroundings through osmosis. Fish cope with this water loss by drinking seawater and actively excreting any excess salt. However, osmoregulation is particularly challenging for fish eggs and small larvae due to their size and the fact that some organ systems involved in salt and water balance are still developing at this point of the lifecycle.

During my thesis, I investigated changes in gene expression and distribution of salt-regulatory cells through early development stages of cod reared on two different diets (i.e. rotifers or copepods). I specifically studied skin and gill construction by applying in situ hybridization (ISH) to larval of different stages (4 – 71 days post hatching). RNA/RNA in-situ hybridization of whole-mount larvae was combined with immunoenzymatic labeling to detect ionocytes with antibodies against the alpha subunit of the antiport enzyme Na⁺/K⁺-ATPase (NKA) and the ATP-bindingcystic fibrosis transmembrane conductance regulator (CFTR). With focus of two osmoregulatory tissues, the expression and localization of nka-α1 and cftr antisense probes revealed the presence of ionocytes in the tegument and around the first gill slits. These epithelial ionocytes are presumed to be fully efficient after hatching. Between 11-31 dph, the expression in the tegument was down-regulated with development and shifted in favor of an increased expression in the evolving gills. Hence, increased osmoregulatory ability was acquired just before metamorphosis.