grunts over the grass beds of Tague Bay, 

 St. Croix, is similar to those reported in 

 the Florida Keys. The French grunt, 

 Haenulon flavol ineatum , was most abundant 

 over sparse grass or bare sand bottom, 

 while the v/hite grunt H. plumeri was usu- 

 ally observed in dense grass. Numbers of 

 coral reef fishes (grunts and squirrel- 

 fishes) feeding nocturnally over seagrass 

 were positively correlated with a measure 

 of habitat complexity. This correlation 

 implies organization of the fish assem- 

 blage while feeding (M.B. Robblee, in pre- 

 paration). Lutjanids were not found in 

 significant numbers either on the reef or 

 in the grass beds. 



These observations on the distribu- 

 tion of fishes over the feeding ground 

 suggest that the nature and quality of 

 grass bed and sand flat habitat adjacent 

 to a coral reef nay influence both the 

 composition and abundance of these noctur- 

 nal fishes on a reef. Randall (1963) 

 stated that whenever well-developed reefs 

 lie adjacent to flats and these flats are 

 not shared by many other nearby reefs, the 

 grunts and snappers on the reef may be 

 expected to be abundant. Starck and Davis 

 (1966) and Robins (1971) also noted that 

 it is understandable, given the require- 

 ment of most pomadasyids and several 

 lutjanid species for back-reef forage 

 area, that these fishes are almost com- 

 pletely absent from certain islands in the 

 Caribbean which have fringing reefs with 

 only narrow shelf and very limited back- 

 reef habitat. Conversely, grunts and 

 snappers form resting schools over char- 

 acteristic coral heads, most commonly 

 Acropora palamata and Porites porities 

 (Ehrl ich and Ehrlich 1973; Ogden and 

 Ehrlich 1977), which also influences their 

 population size. Starck and Davis (196G) 

 commented that these species are excluded 

 from many suitable forage areas by the 

 absence of sheltered locations for diurnal 

 resting sites. When artificial reefs were 

 established in the Virgin Islands (Randall 

 1963; Ogden, personal communication), 

 rapid colonization by juvenile grunts 

 occurred, indicating the importance of 

 shelter to these fishes near their poten- 

 tial feeding grounds. 



Much of the interpretation given 

 above is speculative, but in light of 



current hypotheses, the structuring of 

 coral reef fish communities is probably 

 largely controlled by their physical 

 requirements for living space. Sale 

 (1978) speaks of a lottery for living 

 space among coral reef fish communities 

 composed of groups of fishes with similar 

 requirements (the representatives on any 

 one particular reef being determined by 

 chance recruitment). Alternatively, Smith 

 (1978) advocated the ordered view that 

 recource-sharing adaptations determine 

 which species can live together. Resources 

 external to the reef influence the species 

 composition and abundances of at least 

 nocturnally feeding, supra-benthic species 

 (grunts and snappers), and perhaps several 

 of the holocentrids. 



It has been hypothesized that the 

 die! activity patterns exhibited by these 

 fishes contribute to the energy budqet of 

 the coral reef. Billings and Munro |1974) 

 and Ogden and Zieman (1977) suggested, as 

 originally proposed by Johannes (personal 

 communication), that migrating pomadasyids 

 may import significant quantities of 

 organic matter (feces) to the reef. 

 Thayer and Engel (in preparation) have 

 also postulated a similar mechanism for 

 green turtles, whose contribution to reef 

 nutrient budgets may also be important. 

 These assertions are open to investiga- 

 tion. 



Temporary visitors from the coral 

 reefs are not limited to fishes. The 

 urchin Diadema antillarum moves off patch 

 reefs at night into the turtle grass- 

 dominated grass bed immediately adjacent 

 in Tague Bay, St. Croix (Ogden et al . 

 1973). The prominent halo feature asso- 

 ciated with many patch reefs is attributed 

 to the nocturnal feeding forays of these 

 longspine urchins. Of areater signifi- 

 cance, the spiny lobster ( Panulirus 

 arqus ) , is known to move onto offshore 

 reefs as adults in the Florida Keys, seek- 

 ing shelter in caves and crevices (Simmons 

 1980). Lobsters remain in their dens dur- 

 ing daylight; at or after sunset they move 

 onto adjacent grass beds to feed solitar- 

 ily, returning to the reef before dawn 

 (Hernkind et al . 1975). While farther 

 from the reef, the spiny lobster ranges 

 over considerable distances, typically 

 several hundred meters. 



77 



