Despi te 

 non-scl 

 z o a n t h i 

 s i v e 1 y 

 dominan 

 Buss , 1 

 aJL, 198 

 Green , 



Here 

 competi 

 compone 

 I si ands 

 s i g n i f i 

 of such 

 aspects 

 c o 1 o n i a 



this situation, it is becoming increasingly evident that 

 eractinian invertebrates such as encrusting sponges, 

 ds, ascidians, gorgonians and soft corals also compete aggres- 

 for space in these open-reef habitats, and may often be the 

 t space occupiers (Rutzler, 1970; Reiswig, 1973; Jackson & 

 975; Karlson, 1980, 1983; Benayahu & Loya, 1981; Birkeland, et 

 1; Bunt, et ah, 1982; Sebens, 1982; Sheppard, 1982; Suchanek & 

 1982; Sammarco, e_t aj_., 1983). 



we present a set of preliminary results from a study of such 

 tive interactions between scl eractinian and non-scl eracti nian 

 nts of an open-reef habitat at a site in the U. S. Virgin 



from depths of 10' to 120'. Specifically, we focus a) on the 

 cant contribution made by demosponges to the species diversity 



an assemblage and b) on the quantitative and qualitative 



of competition between demosponges and other encrusting 

 1 invertebrates, especially scleractinian corals. 



METHODS 



Dat 

 compet 

 obtai n 

 M i s s i o 

 (17°47 

 M i s s i o 

 a 1 oca 

 N i k o n o 

 used t 

 substr 

 for th 

 photo- 

 for ea 

 analys 



u 1 1 i n 



1 inked 

 ca . 0. 

 some q 

 analys 

 I : 29 



Qua 

 betwee 

 60' , 9 

 for th 

 c o n t i g 

 depth 

 intera 



a on dep 

 i t i v e in 

 e d with 

 ns at Sa 

 ■N;64°45 

 n II (=N 

 tion map 

 s IV cam 

 o photog 

 ate at 6 

 e b e g i n n 

 quadrats 

 ch depth 

 ed for p 

 e of eac 

 to an A 

 5cm of t 

 uadrat-p 

 ed repli 

 @ 60' (= 

 n t i t a t i v 

 n encrus 

 0* and 1 

 e start 

 uously a 

 = 30 @ 1 

 ction wa 



th di 

 terac 

 the a 

 It Ri 

 'W): 

 0AA# 

 . A 

 era a 

 raph 

 0" , 9 

 ing o 

 in e 

 cont 

 ercen 

 h spe 

 PPLE 

 he or 

 hotos 

 cate 

 14.5m 

 e and 

 ting 

 20' u 

 at ea 

 1 ong 

 0' , 3 



S SCO 



s t r i b u 

 t i o n s 

 id of 

 ver Ca 

 M i s s i o 

 82-12) 

 fixed 

 nd 15m 

 0.5m 2 

 0' and 

 f thre 

 ach tr 

 our. 

 t cove 

 c i e s u 

 1 1 com 

 i g i n a 1 



were 

 guadra 

 2), 28 



q u a 1 i 

 c o 1 o n i 

 sing a 

 ch dep 

 that d 

 @ 30 

 red as 



tion, species richness, percent cover and 

 for encrusting colonial invertebrates were 

 SCUBA during two N0AA NULS-1 Hydrolab 

 nyon , St. Croix, U.S. Virgin Islands 

 n I (=N0AA# 81-12) in Oct/Nov 1981 and 



in Aug/Sept 1982. See Suchanek (1983) for 

 camera frame (quadrapod), outfitted with a 

 m lens (fully corrected for parallax), was 

 areas (85.75cm X 58.31cm) of coral reef 



120' depths. Random locations were chosen 

 e contiguous transects (with 10 replicated 

 ansect) , yielding 30 possible photo-quadrats 

 Resulting color photographs (8" X 12") were 

 r of sessile organisms by tracing the 

 sing a Houston Instuments HIPAD digitizer 

 puter; resolution using this technique was 



substrate. Because of equipment failures 

 lost, resulting in the following number of 

 ts (and associated total areas) for Mission 



@ 90' (-14.0m 2 ), 27 @ 120' (=13. 5m*). 

 tative data on competitive interactions 

 al fauna and flora were taken at 10', 30', 



1.0m 2 quadrat. A random point was chosen 

 th and replicate quadrats were then placed 

 epth contour (number of replicates at each 

 ', 20 60', 14 @ 90' and 17 @ 120'). Each 



a) an overgrowth encounter or b) a standoff 



RESULTS 



Digitized photographs from Mission I demonstrate that sponges 

 comprise the most diverse taxonomic group, representing nearly half 

 (30/63) of all species recorded, as well as dominating the total 

 species richness (SR) values at each depth (Fig. 1). The mean SR of 

 sponges and corals in the photo-quadrats was not significantly 

 different at 60', but sponges were significantly more diverse on the 



56 



