156 THE BIOLOGY OF MARINE ANIMALS 



tivity of young salmon. Other experiments relate to hormonal control 

 of metabolism in decapod Crustacea. Removal of both eyestalks produces 

 a significant rise in 2 consumption, which is opposed by injection of eye- 

 stalk extracts (Uca, Gecarcinus). An increase of respiratory rate is charac- 

 teristic of the premoult period, and moult is attended by an even sharper 

 shift in metabolism. The eyestalks of decapod crustaceans contain endo- 

 crine organs, the removal of which deprives the animal of moult-inhi- 

 biting and other hormones. Among the functions of the latter may be 

 included regulation of respiration; or the effects produced on respiratory 

 metabolism by interference with the eyestalk-glands may be indirect, con- 

 sequent upon changes in some other activity (see Chapter 15) (12, 13, 17, 

 40,51, 77a, 95a, 111, 112). 



Variation during the Life Cycle. Metabolism changes during the life 

 cycle, in correspondence with sequential events of development, differentia- 

 tion, growth, maturity and senescence. During embryonic life there are 

 often measurable changes in respiratory rates correlated with recognizably 

 distinct developmental stages, namely fertilization, cleavage, vasculariza- 

 tion, hatching, etc. In eggs of Arbacia (sea-urchin), Fundulus (teleost), and 

 some other forms which have been examined, there is a marked increase 

 in oxygen consumption at fertilization. Starfish eggs (Asterias), however, 

 show no rise in oxygen consumption at this time, and in Chaetopterus 

 (polychaete) and Cumingia (lamellibranch) there is a decrease. Following 

 fertilization the metabolic rate increases progressively. Slight fluctuations 

 in oxygen consumption of fertilized eggs (Psammechinus, Urechis) during 

 cleavage are associated with changes in mitotic activity. 



Oxygen consumption in killifish eggs (Fundulus) rises progressively dur- 

 ing embryogenesis. Subsequent to the increase at fertilization, a rise in 2 

 uptake accompanies the establishment of embryonic circulation (third day). 

 Another peak occurs on the ninth day, followed by a decline and subsequent 

 rise at hatching (twelfth day) (Fig. 4.9). These later changes have not been 

 correlated with any noticeable ontogenetic events. Metabolism of develop- 

 ing stages has been investigated in the sea-horse Hippocampus. Respira- 

 tion of embryos in the brood pouch rises gradually and reaches a maximum 

 at birth. Generally, respiration is maximal at time of hatching, and declines 

 in post-natal life. In planktonic stages of benthic animals there may be a 

 further increase in respiratory rate of free-swimming larvae after hatching, 

 but metabolism declines once the animals have settled on the bottom, 

 e.g. Arbacia, Mytilus, Teredo (99, 101, 133, 141, 171, 172, 173, 174). 



Observations on a wide variety of animals show that young individuals 

 possess higher respiratory rates than older animals of the same species. 

 This conforms to the relationship between size and metabolism discussed 

 above. Once maturity is reached there may be a decrease in metabolic 

 activity with age, as senescence sets in. Studies on isolated tissues are in 

 agreement with observations made on the whole animal. Thus, tissues 

 from young specimens of the bivalve Mercenaria mercenaria have higher 

 respiratory rates than those from older animals (79, 163). 



