Ions first: Na+ uptake shifts from the skin to the gills before O2 uptake in developing rainbow trout, Oncorhynchus mykiss.

This is the first direct physiological evidence in support of the ionoregulatory hypothesis, challenging the long-held assumption that teleost gills develop initially for gas exchange. Resting unidirectional sodium (Na(+)) uptake and oxygen (O(2)) uptake across the skin and gills were measured simultaneously in larval rainbow trout, Oncorhynchus mykiss, during development. In soft and hard water, Na(+) uptake shifted to the gills by 15 and 16 days post-hatch (dph) while O(2) uptake took 50-80% longer and shifted by 23 and 28 dph, respectively. This suggests that gills are required for ionoregulation prior to gas exchange in developing rainbow trout. The age of transition for Na(+) uptake, gill Na(+), K(+)-ATPase (NKA) alpha-subunit protein expression and gill NKA enzyme activity were not significantly different between soft and hard water-reared groups, which suggests a lack of plasticity in gill ionoregulatory development. In rainbow trout, the gills assume a dominant role in ionoregulation before gas exchange, suggesting that ionoregulation may be the initial driving force for gill development. Further investigation is required to determine whether this pattern is consistent with other teleosts and more basal fishes during early development to gain insight into the role of ionoregulation in vertebrate gill evolution.

Ontogeny and paleophysiology of the gill: new insights from larval and air-breathing fish.

There are large changes in gill function during development associated with ionoregulation and gas exchange in both larval and air-breathing fish. Physiological studies of larvae indicate that, contrary to accepted dogma but consistent with morphology, the initial function of the gill is primarily ionoregulatory and only secondarily respiratory. In air-breathing fish, as the gill becomes progressively less important in terms of O(2) uptake with expansion of the air-breathing organ, it retains its roles in CO(2) excretion, ion exchange and acid-base balance. The observation that gill morphology and function is strongly influenced by ionoregulatory needs in both larval and air-breathing fish may have evolutionary implications. In particular, it suggests that the inability of the skin to maintain ion and acid-base balance as protovertebrates increased in size and became more active may have been more important in driving gill development than O(2) insufficiency.

Ontogenetic changes in the toxicity and efficacy of the anaesthetic MS222 (tricaine methanesulfonate) in zebrafish (Danio rerio) larvae.

Median lethal (LC(50)) and effective (EC(50)) concentrations for 1-h and 24-h exposures to the anaesthetic MS222 (tricaine methanesulfonate) were determined for zebrafish Danio rerio larvae ranging in age from 3 days postfertilization (dpf) to 9 dpf. Cessation of heart beat was used as the indicator of death (LC(50)) while failure to respond to direct mechanical stimulation of the head region was taken as an indication of deep anaesthesia (EC(50)). 1-h LC(50)s, 1-h EC(50)s and 24-h EC(50)s all decreased gradually but significantly (all P<0.01) with age. Mean values for 1-h LC(50)s were 1633 mg L(-1) and 730 mg L(-1), respectively, for 3 dpf and 9 dpf larvae. Mean value for 1-h and 24-h EC(50)s were 106 mg L(-1) and 100 mg L(-1), respectively, at 3 dpf and 65 mg L(-1) and 31 mg L(-1), respectively, at 9 dpf. The gradual increase with age in sensitivity to the anaesthetic implied by these indicators is probably a reflection of ontogenetic changes in the activity of detoxification pathways. Mean values for the 24-h LC(50) also decreased significantly (P<0.001) with age, from 566 mg L(-1) at 3 dpf to 64 mg L(-1) at 9 dpf. However, unlike the other indicators, the decrease was not gradual but occurred in a step-like fashion with virtually all of the change occurring between 4 dpf and 7 dpf. This sharp increase in sensitivity coincides with the shift in the major site of systemic ionoregulatory activity from the skin to the gills. The implications of these ontogenetic changes in lethal and effective levels for researchers or others intending to use the anaesthetic with fish larvae are discussed.