Introduction
Polyculture of mussel and sea cucumber species is highly attractive since both these organisms have important economical and environmental value. Mussel farming provides more than 1/3 of EU aquaculture products, being one of the most spatial-extended industrial production of mollusk in Europe (Tamburini et al., 2020). On the other hand, the sea cucumbers are considered luxury seafood, especially in Asian countries
(Purcell et al., 2016). Moreover, mussel and sea cucumber co-culture potentially doesn’t need external food input resulting an environmentally friendly alternative as a high-quality protein source. Mussels, in fact, are filter-feeders able to feed on natural primary production from the water column, without other food supplies (Tamburini et al., 2020). They represent a fundamental organic material connection between the bottom and the water column , by settling organic matter on the seabed. This organic biodeposit could be an attractive food source for deposit feeder sea cucumber s, which are able to ingest sediment and organic material, playing an important role in cycling and energy flow in benthic ecosystems (Purcell et al., 2016).
Although the suitability of mussel and sea cucumber polyculture have been investigated in Indo-Pacific ( Slater et al., 2007; Zamora et al., 2016), there is no investigation for Mediterranean species. Hence, for the first time in the present study, an Integrated Multi-trophic Aquaculture among Mediterranean species of sea cucumbers (Holothuria
tubulosa and Holothuria
polii ) and mussels (Mytilus galloprovincialis) was investigated.
Materials and Methods
During the Spring of 2020, a IMTA experiment was carried out at Ittimar Soc. Coop. mussel farm, a Mytilus galloprovincialis long-line submerged productive system (South of Adriatic Sea, Italy). Specimens of H. tubulosa and H. polii were collected along Apulian coast and sorted in two size classes (small size: 40-100 g; large size: 100-200 g). For each species, representative specimens of both size classes were placed in 20 seabed cages beneath the long-line mussel farm at a density of 670 g/m2. The suitability of H. tubulosa and H. polii to use M. galloprovincialis waste as the sole food source was evaluated in terms of survivorship and somatic growth performances over four-month experiment.
Results
At the end of the experiment, both H. tubulosa and H. polii showed high survival, respectively of 94 % and 92 %. Instead, in terms of growth performances, high differences emerged between the two species and the two size classes tested. H. tubulosa showed higher growth than H. polii, respectively of 6.07 % and -25.37 %. When comparing growth performances among size classes, the highest SGR % was achieved in small H. tubulosa specimens (0.09 % day-1), which resulted significantly higher than the value obtained in large ones (0.01 % day-1) (Fig. 1). Both these values, however, were higher than those achieved in H. polii, which gained negative somatic growth in both size classes (SGR of -0.26 % day-1 and -0.45 % day-1 respectively for small and large specimens).
Discussion
The present study evaluated for the first time on European species the potential of integrated aquaculture between sea cucumbers and mussels. This polyculture is potentially able to increase energy-use efficiency inside mussel farms, promoting productivity increase and rearing diversification with new commercially important species. Furthermore, these results clearly indicate the different compatibility of H. tubulosa and H. polii in polyculture with mussels, highlighting that these two sea cucumbers have different feeding behaviors and culture requirements. In fact, Boncagni et al. (2019) observed that H. tubulosa and H. polii
present species-specific use of the same resources that would allow their co-existence in the same benthic ecosystems. This partial degree of niche segregation, which could be related to the differences in digestive physiology (e.g in terms of intestinal microbiota) and feeding specialization in microhabitat (different sediment layers), potentially explained the better suitability of H. tubulosa for polyculture with Mediterranean mussel M. galloprovincialis . However, we hypothesize that the high rearing density of H. tubulosa in the present study could have mitigated its growth performances. Hence future investigations are needed to define the optimal rearing density of this sea cucumber species, to be adopted
as extractive species in association with commercial farms of M. galloprovincialis.
References
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