Risk'INTRODUCTION T e r r e s t r i a l ecologists have accumulated data relating bird species diversity to foliage height diversity (MacArthur andMacArthur, 1961;MacArthur, 1964;Recher, 1969); complexity of foliage pattern s e e m s to be a better predictor of bird species diversity than i s the t r e e species diversity. There i s some indication that a s i m i l a r relationship exists on coral reefs, between fish species diversity and substrate complexity. Talbot (1965, p. 453j, in comparing fish collections from a r e a s with either few o r many species of corals present, s t a t e s "It i s probable that the g r e a t e r variety of coral s p e c i e s . . .allows for a m o r e varied fish population than the single species," and, "It is probable that the m o r e complex coral population i n the mixed stand provides more ecological niches than a r e available in the single species stand." Hiatt and Strasburg (1960, p. 118) found more species and individuals on "ramose" than on "glomerate" coral heads: "Because glomerate coral headsaregenerally devoid of interstices in w!lich small o r g a n i s n ? -an hide, such heads a r e visited m l y by fish species intent on browfjins o r grazing coral polyps." I11 a study on Conus, Kohn (1967, p. 257) found "Type 111 habitats a r e topographically the most complex.. .and these support the mnst divers? a s s e mblages of Conus." The author attempted t o m e a s u r e the relationship between fish species diversity and the physical makeup of the coral substrate, following the work done on bird species diversity. PROCEDURE ( a j Study aveaThe a r e a selected was a portion of a low-lying, shallow patch reef located in the eastern p a r t of Greater Lameshur Bay, St. John, Virgin Islands. Substrate types withiti the a r e a were live coral (Montastvea annulavis. Millepova a l c i c o d s , I'oorites jiwcata, and Acavicia agavicitesl, dead coral, sponges (mostly I~c i n i a spp.) and carbonate sand.Two transects were laid out in the form of a "T", with the upright perpendicular to the shore, and the crosspiece further offshore and parallel to the shoreline.The "upright" transect began about 50 m e t e r s f r o m shore, in 4 m e t e r s 31' water, and extended 16 m e t e r s seaward, to a depth of 4.5 meters. The "crosspiece" transect was also 16 m e t e r s long, and 4.5 m e t e r s deep throughout its length.'~e p a r t m a n t of Biological Sciences, University of Southern California, Los Angeles, California 90007 (Manuscript r e c e i v e d Feb. 1971--Eds.) ( C ) D e l e~m i n a l i o n of fish species divelmity, " I I Y Only territorial fishes were considered in t h i s study. Several days were spent diving and observing in the a r e a , in o r d e r to determ'ne which species were territorial. When doubt existed about a particular fish, i t was frightened into i t s hom? t e r r i t o r y s e v e r a l t i m e s over the course of 2 o r 3 days. Only f i s h e s consistently returning to the s a m e location were counted; appropriate m e m b e r s w e r e thre...

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Abstract. A marked increase in the biomass of Cliona delitrix infesting Montastrea cavernosa substrate occurred in a portion of the Grand Cayman fringing reef affected by the discharge of untreated fecal sewage. It is suggested that the six‐fold increase in bacteria biomass (both “coliforms” and natural marine bacterioplankton) in reef waters receiving the sewage effluent is linked to a five‐fold increase in sponge biomass at the polluted site relative to a control site. The elevated density of C. delitrix biomass signifies a similar increase in the amount of M. cavernosa skeleton that has been eroded by this sponge and reduced to silt‐sized sediment. Thus, the proliferation of a bioeroding organism in the sewage‐stressed environment has caused a shift in the carbonate balance on the reef.

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