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THE BIOLOGY OF MARINE ANIMALS 



and megalospheres, which reproduce asexually and sexually. The large 

 asexual bodies or amoebulae which arise by multiple fission of the parent 

 carry away some of the symbionts, but the gametes are free of algae and 

 the microspheres must be reinfected from without. 



A few marine ciliates and flagellates also contain symbiotic algae. The 

 peritrich Spatostyla sertulariarum encloses a small number of zooxanthellae 

 which are passed to the daughter cells during cell division. Of particular 

 interest is the marine ciliate Mesodinium rubrum which contains small red 

 algal symbionts. The young protozoans are colourless, but as they mature 

 the cytosome becomes filled with algal cells and the animal ceases to take 

 in preformed nourishment. Gohar (33) figures a ciliate that feeds upon 

 xeniid alcyonarians and acquires zooxanthellae from the latter. Noctiluca 



Fig. 14.18. Zooxanthellae in Radiolarians 



(a) Acanthometra pellucida in which the algae are intracapsular in position; (b) the 

 colonial species Sphaerozoum acuferum, with extracapsular zooxanthellae. (Redrawn 

 from Buchner (12).) 



miliaris (Dinoflagellata) is sometimes infected with zoochlorellae, which 

 may be very numerous and fill the cell. 



Porifera. Many instances of algal symbiosis in sponges have been re- 

 corded {see Buchner (12)). The plant symbionts involve red algae (Rhodo- 

 phyta) with Reniera, filamentous green algae (Chlorophyta) with Hali- 

 chondria, Chrysophyta in Grantia, and blue-green algae (Cyanophyta) in 

 Hircinia. Unicellular algae occur in the chambers of the sponge, or in the 

 mesogloea; more striking is the case of certain red algae which have 

 branches and thallus entirely enclosed in sponge tissue. 



Coelenterates. Symbiosis with algae is of widespread occurrence in 

 these animals and may be of considerable importance in their economy. 

 Zooxanthellae are characteristic of numerous littoral coelenterates occurr- 

 ing in warm inter-tropical waters, namely Hydrozoa, Alcyonaria and 

 Zoantharia. They are almost invariably present in reef-building madre- 



