Physiology of Finfish and Shellfish (2+1)

2.1.17. Fish blood as a gas carrier

Fish blood consists of blood cells and plasma similar to other vertebrate animals and the blood cells are composed erythrocytes, leukocytes and thrombocytes. Erythrocytes (red blood cells) are a round or ellipse shaped cell and contains dense haemoglobin.

Hemoglobin (also spelled haemoglobin and abbreviated Hb or Hgb) has the iron -containing oxygen -transport and Hg is the metalloprotein in the red blood cells of all vertebrates (except the fish family Channichthyidae) and the tissues of some invertebrates. Hemoglobin in the blood transports oxygen from the gills to the rest of the body (i.e. the tissues) where, it releases the oxygen for cell use. Hemoglobin is a large protein with four polypeptide chains and four heme groups. Each heme group has an iron atom attached at the centre form the pigment called heme which gives blood its red colour and its ability to combine with oxygen to be carried to cells and tissues. Each heme group is enfolded in one of 2 or 4 chains of amino acid units that collectively constitute the protein part of the molecule called globin. The haemoglobin of most vertebrates has molecular weights of approximately 65,000.

(The crocodile icefish or white-blooded fish (Channichthyidae) are a family of perciform fish found in the cold waters around Antarctica and southern South America. Their blood is transparent because it contains less than 10% hemoglobin and/or only defunct erythrocytes. Oxygen is absorbed directly through their scaleless skin from the water. It is then dissolved in the plasma and transported throughout the body without the hemoglobin protein. The loss of hemoglobin is not fatal because of the cold environment in which channichthyids live. Cold water has much higher dissolved oxygen content than warmer water)