Aquaculture of the edible holothuroid Isostichopus fuscus in Ecuador has been partially limited due to occurrence of a highly virulent disease of their planktotrophic larva into the auricularia stage caused by a bacteria-protozoan consortium. The protozoan occurrence (abundance, infection rate and intensity of infection) was evaluated on five length groups of auricularia larva in a trial in April 2016: (1) early auricularia – EA: 250-300 μm; (2) middle auricularia 1 – MA1; 300-600 μm; (3) middle auricularia 2 – MA2: 600-900 μm; (4) auricularia – A: 1.1-1.3 mm and (5) late auricularia – LA: ≈500 μm. Trial lasted 25 days. A total of 14801 parasites were counted in six hundred larva. Of them, 42.5% were parasitized. Infection rate and intensity of infection significantly varied in relation to the host’s body length. Stage A had the highest prevalence and intensity, respectively (62.5±5.7%, 130.5±13.0) than EA (21.7±5.2%, 2.6±0.7, p<0.05). Stage A without parasites were significantly larger (1494.1±52.2 μm) than those larvae with parasites (1237.7±24.4 μm, p < 0.05). Data demonstrates for the first-time dynamic changes of parasite infestation in the developmental stages of auricularia where larvae without parasites grew and developed faster (18-d) than infected ones (25-d).
Nowadays, aquaculturing of the sea cucumber Isostichopus fuscus represent more than their edible part for prized gastronomic seafood industry. Nonetheless, is evident the poor understanding of the diseases that affect the various its life stages during their farming. Our goal is to provide a quantitative data of the occurrence of parasitism disease in the earliest stage of larvae (auricularia as host specificity) of I. fuscus to determine its effects on the growth and survival on its different five length categories at an experimental rearing. This information provides that would help us gain insight into diseases of I. fuscus.
Broodstocks were collected in April 2016 in Santa Elena, Ecuador. Auricularia larvae were reared (stock density: 0.2 larva ml-1) in eight 500-l conical tanks with filtered/UV seawater. Thirty larvae were sampled per replicate (n = 4) at 4 days intervals to monitor larval development, parasite counting and register larval size in length. Larvae fed daily a mix of microalgae. Five length-groups by each larval stage were: early auricularia – EA (250-300 μm); middle auricularia 1 – MA1 (300-600 μm); middle auricularia 2 – MA2 (600-1000 μm); auricularia A (1000-1300 mm) and late auricularia – LA (≈500 μm). Parameters were: temperature: 28.0±1.0°C; salinity: 34‰, pH: 8.2, DO: 5-6 mg O2.l-1 and illumination at 0.5 μE m−2.s−1). The culture trial lasted for 25 days. Abundance, prevalence and infection intensity were determined following Bush et al. (1997). ANOVA tests were performed.
Three clinical signs occurred in the same disease: stomach ulceration, atrophy and rotting-edge. 24540 parasites were counted in 600 auricularia larvae. 53.3% showed between 2 to 279 parasites. Parasite infection risks varied significantly in relation to the five host size categories. The lowest and the highest parasite infection risk values were found in the EA and A, respectively. For abundance (EA: 0.5±0.2, A: 86.7±5.6, p <0.05), infection rate (EA: 21.7±5.2, A: 70.8±3.7, p < 0.05) and intensity of infection (EA: 2.6±0.7, A: 130.5±12.9, p <0.05). The largest auricularia were without parasites (A: 1494.0±52.2 μm in length) and completed their development in 18 days with a high survivorship, but those larva with parasites in 25 days (A: 1238.0±24.4 μm, p <0.05). Our results are comparable to the findings of other authors focused on reviewing the culture density of larvae, water temperature, and quantity and quality of food available against this bacteria-protozoan consortium.