Introduction

The heart is a pump composed largely of one particular kind of tissue, cardiac muscle. The hearts of fish (Figure 3 A, B, C) consist of three or four chambers arranged in a single series. All fish have a sinus venosus into which the returning blood flows; it may have a layer of cardiac muscle, but in many teleosts (Figure 3C) this is reduced to a few scattered fibres and in others, muscle is lacking altogether. The atrium is a capacious, thinly-muscled sac which generates just sufficient pressure to fill the thickly-muscled ventricle. This is most commonly sac-like, but may be pyramidal, as in the Clupeidae, or tubular as in the hake (Merluccius merluccius) (Santer et al 1983). It is the chief pressure-raising chamber of the heart and comprises 58–85% of its weight. The fourth chamber, in elasmobranchs and some primitive bony fish, is the conus arteriosus (Figure 3B); it is barrel-shaped and invested with contractile cardiac muscle. In most teleosts it is replaced by an elastic chamber, the bulbus arteriosus (Figure 3C), the wall of which contains much elastic tissue and some smooth muscle.

The efficient operation of such a heart depends on the sequential activation of its chambers. At the start some tissue must have the ability to generate a heart beat, for the fish heart, like that of other vertebrates, is myogenic. It is not activated by motor neurones like the heart of a crab. Furthermore, blood has mass and takes time to flow from one chamber to the next. Some mechanism to delay the forward march of excitation must be provided whilst flow occurs.