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The effects of sea cucumber (Holothuria scabra) juvenile density and interspecific competition from the opisthobranch Stylocheilus striatus (SS) on periphyton biomass and growth performance of H. scabra (HS) were assessed. The number of H. scabra juveniles (15, 50, 100) in net enclosures (treatments HS15, HS50, and HS100) was varied and feeding activity of S. striatus was assessed at a density of 15 individuals per net enclosure (treatment SS15), and in co-culture with H. scabra (treatment HS15/SS15). A control treatment without H. scabra or S. striatus was included in the 28-d experiment. Highest and lowest growth rates of H. scabra occurred in the lowest (HS15) and highest (HS100) stocking density treatments, respectively, on days 14 and 28. Growth of H. scabra juveniles in the HS15 treatment was more than double that of those in the HS15/SS15 treatment. Juvenile survival was significantly higher in the HS15 treatment on days 14 and 28, but did not differ significantly from that of juveniles in the HS15/SS15 and HS50 treatments on day 14. Grazing activity of sea cucumber juveniles and S. striatus resulted in a decline in periphyton biomass (AFDW) after day 7. Differences in food consumption (based on faecal production) among treatments were significant only on days 1 and 21 with the greatest consumption (17.5 mg periphyton ind−1 d−1) by S. striatus in the SS15 treatment. Holothuria scabra juveniles in the HS15 treatment consumed 4.9 mg ind−1 d−1, while those in the HS100 treatment consumed only 1.8 mg ind−1 d−1 after 24 hrs.

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