Grazing has a key influence on periphyton biomass. Here we investigated the effects of intra- and interspecific competition on periphyton biomass using varying density combinations of sandfish (SC) Holothuria scabra juveniles (~5 mm) and the sea slug (SS) Stylocheilus striatus in a 28-day experiment in 0.28 m-2 circular hapa nets containing; SC15, SS15, SC15/SS15, SC50, SC100 and SC0/SS0 (control). Periphyton chlorophyll a and ash free dry weight (AFDW) contents were measured weekly. Food consumption was estimated from the faecal production of SC and SS after 24 h and weekly thereafter. Growth performance and survival of sandfish juveniles were measured on days 14 and 28.
Chlorophyll a and phaeopigment were significantly higher by day 28 and lowest by day 7. Periphyton AFDW was significantly higher in the SC15 treatment and was lowest in the SC15/SS15 treatment. Notably, AFDW in the control was lower than in the SC15 treatment, although differences were not significant. Sandfish juveniles reared in the SC15 treatment were significantly larger and absolute growth rate was higher (Table 1) compared to that in all other treatments on days 14 and 28. Lowest growth rate occurred in the SC100 treatment. Coefficient of variations (length) was significantly higher in the SC100 treatment on days 14 and 28; but was not significantly different to those of SC15/SS15 and SC50 juveniles by day 28. Survival was significantly higher in the SC15 treatment on days 14 and 28; however, differences were not significant compared to juveniles reared in the SC15/SS15 and SC50 treatments on day 14. Differences in food consumption among treatments was significant only at 24 h and on day 21 with greatest consumption in the SS15 treatment.
Sea slugs in the SS15 treatment consumed a higher amount of periphyton (262.3 ± 45 mg) than those in all other treatments after 24 h and on day 21. Lowest food consumption was observed in the SC100 treatment (0.05 ± 0.01 mg) on day 21. Those in the SS15 consumed 17.5 mg periphyton ind-1 day-1 (dry weight) compared to 4.5 mg ind-1 day-1 by sandfish juveniles reared in the SC15 treatment. The higher grazing rate of sea slugs may limit the growth performance of sandfish juveniles.