620 THE BIOLOGY OF MARINE ANIMALS 



Another functional role ascribed to algal symbionts is the automatic 

 removal of waste products, namely carbon dioxide, and nitrogen and 

 phosphorus end-products. The algae would thus be acting as adventitious 

 excretory organs. This is a generalization which is very difficult to evaluate, 

 but until evidence is presented to the contrary there seems to be no particu- 

 lar reason for assuming that marine invertebrates have any difficulty in 

 getting rid of nitrogenous wastes. In this connexion Yonge (107) notes 

 that the kidneys of Tridacna are enlarged, in conjunction with the necessity 

 for disposing of the remnants of the zooxanthellae digested by the blood 

 phagocytes (109). 



Symbiosis with Heterotrophic Algae 



A few instances are known of animals containing algal associates which, 

 although pigmented, lack chlorophyll, and no doubt many more such 

 cases remain to be discovered. In the two associations now to be described 

 the plant cells are sufficiently abundant and coloured to tint the soft parts 

 of the animals in which they occur. 



The first concerns a deep-water ctenophore Beroe abyssicola, occurring 

 below 180 metres. In this animal the stomodaeum is coloured red due to 

 the presence of numerous red flagellates in the vacuolated cellular tissue 

 lying underneath the epithelial lining. Other species of ctenophores are 

 suspected of harbouring similar organisms (6). 



The green colour of Marenne oysters is due to infection by a green 

 diatom, Navicula ostreaha, located in gills and palps. Again, the pigment, 

 in this case known as "marennine," is produced by the algal symbiont; 

 chemically it appears to be a carotiprotein (63, 78). 



There is little information about the interrelations between these algal 

 associates and their animal hosts, and it is premature to characterize 

 them as symbionts or parasites. 



The green colour of the intestine of the polychaete Chaetopterus has 

 been the subject of much enquiry and speculation. The green pigment, 

 known as chaetopterin, is a mixture of phaeophorbides, and occurs in 

 peculiar small green bodies lying in the epithelial cells of the gut (Figs. 

 11.2, 13.2) (7, 56a, 63). Earlier suggestions that these green bodies are 

 non-motile stages of a flagellate have not been substantiated (p. 476). 



Fungal Symbionts in Excretory Organs 



A single instance of this kind of association is known in the Molgulidae 

 (colonial ascidians). These animals possess a small closed excretory vesicle 

 or storage kidney containing renal concretions (urates), and in this vesicle 

 flourish peculiar fungal bodies. These are said to propagate by sexual and 

 asexual means, and infection of new hosts takes place by means of zoo- 

 spores. The significance of the association remains obscure, but it is 

 assumed that the fungal hyphae make use of the excretory material of 

 Molgula (12). 



