Apart from the special case of the channichthyid icefishes, most oxygen in the blood is carried by red cell hemoglobin, oxygenated when Po2 is high and deoxygenated when it falls, according to the oxygen dissociation curve (blood Po2 plotted against the amount of O2 bound to the hemoglobin). Oxygen dissociation …

Different fishes have very different lifestyles, so it is not surprising that the properties of their blood vary according to metabolic demands, and the way that the fish acquires O2 and excretes CO2. For example, blood in active fishes such as scombroids must have a much higher O2 capacity than …

A remarkable variety of accessory pumps occurs in the venous circulation of different fishes. These range from the portal heart of hagfishes behind the liver which has cardiac-type muscle and resembles the atrium of the main heart, with an ECG with P and T waves; to the caudal hearts of …

Fish hearts are S-shaped and four-chambered with, from behind forwards, sinus venosus, atrium, ventricle and either a bulbus or conus leading to the ventral aorta. We have already seen that some airbreathing fishes have a double circulation (the Japanese mudfish, Channa argus, even having a double ventral aorta), but only …

The oxygen acquired (from water or air), and the carbon dioxide excreted at the gills, have to be transported around the body by the circulation of the blood. In fishes using the gills as a gas exchanger, the primary circulation is single, blood leaves the heart to pass first through …

The Origin of Respiratory Gills In ascidian and doliolid tunicates, and in amphioxus, the gills are ciliated food collecting devices, trapping particles on mucous nets produced by the endostyle; blood flowing through them is probably de-oxygenated since the ciliary tracts of the gill bars must use more oxygen than is …

Hagfish In hagfishes, unidirectional water flow through the serial muscular gill pouches is chiefly brought about by rolling and unrolling of velar folds. These lie in a chamber developed from the naso-hypophyseal tract and are operated by a complex set of muscles inserting onto cartilages of the neurocranium. Peristaltic contractions …

The variety of freshwater fishes is striking, for they show radiations into different habitats with some of the most extraordinary adaptations of all fishes. The African mormyrids and the South American gymnotids, for example, have independently evolved amazingly specialized electrical signaling systems for use in crowded and turbid waters, and …

Diversity of Freshwater Fishes Despite representing only a miniscule (0.0093% according to Horn, 1972) part of the world’s aquatic habitat, freshwaters contain a disproportionate number of species (over 40%, again according to Horn, 1972). Unlike the oceans, which constitute broad, uninterrupted expanses of water, freshwater habitats tend to be much …

Epipelagic Fishes The open ocean beyond the continental shelf covers nearly two-thirds of the surface of the Earth, and some 2500 species are found there, distributed vertically from the uppermost waters to the greatest depths, about half being benthic, half pelagic. Near the surface is the euphotic zone, where light …