120 THE BIOLOGY OF MARINE ANIMALS 



salt solutions, especially when an energy source (glucose) is added. Some 

 of the perfusion fluids described in the appendix have been devised for 

 hearts and tested on them. For many marine invertebrates, in which the 

 salt composition of the blood closely approximates sea water, the latter 

 provides a satisfactory medium for sustained cardiac activity. Where a 

 high degree of ionic regulation takes place, e.g. in decapod Crustacea, the 

 perfusion fluid must closely resemble the blood in salt composition and 

 alkalinity for optimal heart activity. The heart of Maia, for example, will 

 beat in sea water, but the amplitude is much stronger in a perfusate re- 

 sembling normal plasma in heightened levels of potassium and calcium, 

 and reduced magnesium. Only in a few invertebrates, e.g. in certain brachy- 

 urans, is the osmotic pressure different from that of the external medium. 

 Fish regulate ionically and osmotically. 



The ionic composition of invertebrate heart tissue is not known, but it 

 probably resembles that of somatic muscles, which are high in K + , and 

 low in Na + , Ca ++ and Mg ++ , compared with blood and sea water. These 

 ionic differences are maintained by a polarized bounding membrane. Any 

 radical changes in concentrations and ratios of cations in the surrounding 

 medium bring about depolarization and deterioration of conduction and 

 contractility in the myocardium. In addition, when control is neurogenic, 

 altered ionic conditions affect the neural pacemakers. It is apparently 

 only cations which influence cardiac activity to any perceptible degree. 

 The nature of the anion (save that it be non-toxic) is of small significance, 

 and chlorides are usually employed in artificial media. Sodium is the 

 predominant cation; small and definite amounts of potassium and calcium 

 must also be present for normal functioning; magnesium is also necessary 

 for some, but not all, invertebrate hearts. Rather than list the manifold 

 effects of altered ionic ratios on the many invertebrate hearts which have 

 been tested, some representative examples have been selected from the 

 major phyla. Tables summarizing ionic effects on hearts are available in 

 Prosser (108). 



Mollusca. Excess sodium has a stimulatory effect on the oyster heart 

 {Ostrea), arresting it in systole. Potassium in excess increases rate and 

 tonus, and at a concentration six times normal arrests the heart in systole. 

 The effects of these cations are offset by calcium, high levels of which slow 

 the heart and stop it in diastole. In the absence of calcium the cardiac rate 

 increases (pacemaker-stimulation), but the heart comes to rest in diastole, 

 owing to interference with contractility. Magnesium is essential for normal 

 cardiac functioning in Ostrea: absence of magnesium results in cardiac 

 acceleration and arrest in systole; high magnesium inhibits the pacemaker 

 and arrests the heart in diastole (100, 130). 



Ionic effects on the cephalopod heart (Loligo, Octopus) are probably 

 somewhat similar. Raised potassium accelerates the heart and increases 

 the force of the beat. High calcium stops the heart in diastole; low calcium 

 increases rate and arrests in systole. The heart will beat in the absence of 

 magnesium, but addition of magnesium increases the amplitude of the beat. 



