Introduction
Historically, jellyfish (JF) products are not new. These gelatinous organisms have been used for centuries as human food in Asian countries, while modern exploitations are applied in medicine and cosmetics. Nevertheless, due to their high water content, JF are often presumed to be a poor food source and a trophic dead end1. However, in addition to the vertebrate predators that extensively consume gelatinous species, at least 124 fish species, some of them with economic value like the seabream Sparus aurata, the piked dogfish Squalus acanthias and the Atlantic saury Scomberesox saurus, are reported to prey on JF in natural waters2.
On a dry-mass basis, JF contain a higher proportion of polar lipids than what is found in standard aquafeed pellets, including n−3 highly unsaturated fatty acids, especially eicosapentaenoic acid, and n−6 HUFAs, such as arachidonic acid3. In addition, some jellyfish species are important amino acids, such as glycine and taurine, that dominates the free amino acids (FAAs)4. Therefore, several studies have investigated the potential of using JF as feed for aquaculture fish5,6.
In the framework of the Horizon 2020 ‘Gojelly’ project, three JF species - A. solida, Cotylorhiza tuberculata and Catostylus tagi - were selected for product evaluation as aquafeed, based upon the following criteria: 1) species need to have been recorded in the geographic area to avoid the incidental introduction of non-indigenous species in the ecosystem, in the case of trials with live JF; 2) species need to present biochemical composition potential to be used as feed; 3) biomass availability.
Materials and methods
The first trial, conducted at the facilities of the Mariculture Centre of Calheta, Madeira, evaluated the potential of using live jellyfish to feed juvenile’s seabream, using all the life stages benthic polyps (< 1 cm), and the free-swimming ephyrae (≈ 2 m) and medusa (≈ 1cm) of reared A. solida (one of the most worldwide distributed jellyfish genera that fulfil the first criteria) versus standard seabream aquafeeds. For two days, seabream were fed with the different items for two hours before removal of the remaining food to estimate the ingestion rate.
The second trial aimed to determine the potential of incorporating jellyfish biomass into a compound diet for seabream at the juvenile stage. C. tuberculata was collected in Mar Menor, Spain, immediately frozen (-20ºC) and transported to “Sparos Lda.” facilities (Portugal). Based on the composition of the jellyfish biomass (dry and fresh forms) determined upon arrival and taking into account the feasibility of its use in industrial feed extrusion processes, the trial comprised six dietary treatments: the control diet (CTRL) that mimicked a commercial feed for gilthead seabream juveniles and four additional diets incorporating at mixing (before extrusion) either dry or fresh jellyfish biomass at 2.5 and 5.0%. A sixth diet contained 2.5% of the fresh JF biomass but incorporated post-extrusion by vacuum coating to potentiate the jellyfish biomass bioactivity role. Seabream juveniles (initial weight of approximately 24g) were fed with the six diets (three replicates per treatment) for two months in a flow-through system. Survival, growth, and biochemical composition were determined at the end of the feeding trial that lasted two months.
Finally, a third trial was conducted to determine the acceptability of freeze-dried JF (C. tagi) by the goldfish Carassius auratus, one of the most common freshwater fish found in pet shops, versus a diet based on commercial fish flakes. Fish were fed four dietary treatments: one exclusively commercial feed and three others with different percentages of JF (25, 50, 75%) plus commercial feed, once a day, for 45 days. The remaining food was weighted, and feeding was video recorded (30 s before feeding, 60 s during feeding and 60 s after feeding). At the end of the trial, biochemical composition and food consumption were analyzed.
Results and Discussion
The first trial showed that seabream displayed predatory activity over benthic (polyps) and pelagic stages (ephyra and medusa). Although standard dry pellets were the most preyed item, within JF, the highest ingestion rates were observed for polyps, followed by ephyrae, and last by medusa. Results of the ingestion rate on polyps and ephyrae highly exceed the values found by Marques et al. (2016), which might be explained by the fact that seabream used in that study were larger (min 70g) and likely less interested in small prey.
Results of the second trial showed no differences in growth, biochemical composition, nor in most immunological parameters, except for peroxidase (p 0.025), showing that inclusion of JF, tended to increase peroxidase levels. Further studies need to investigate the potential of jellyfeeds to increase this enzyme activity, as peroxidase activities in fish erythrocytes suggest the predominant role of this enzyme in the protection of polyunsaturated acids against uncontrolled oxidative processes possibly indicating an important positive role of including jellyfish biomass in aquafeeds.
Results of the goldfish trial indicated that jellyflakes were accepted. As soon as jellyflakes were delivered into the aquaria, fish exhibited a “chasing behavior” (defined as increased swimming speed towards the flakes) and increased biting frequency (refers to the number of times that each fish was seen biting on the flakes deposited at the bottom of the aquarium per meal). Biochemical results indicated that the proximate composition of goldfish fed different percentages of JF had significantly higher protein content and lower lipid content.
Fish generally accepted all JF aquafeeds products in all trials, and no adverse effects were found. Both freeze-dried JF and compound diets with JF as an ingredient present the advantage of availability throughout the year, easier to store and manipulated by users.
References
1Sommer U et al. (2002) Pelagic food web configurations at different levels of nutrient richness and their implications for the ratio fish production: primary production. Hydrobiologia 484: 11–20.
2 Pauly D, Graham W, Libralato S, et al (2009) Jellyfish in ecosystems, online databases, and ecosystem models. In: Hydrobiologia. pp 67–85.
3Stenvers V., et al. (2020). Seasonal Variability of the Fatty Acid Composition in Aurelia aurita (Cnidaria: Scyphozoa): Implications for Gelativorous Food Web Studies. J Plankton Res 13;42(4):440-452.
4Miyajima-Taga Y, et al. (2018) Efficacy of feeding tiger puffer Takifugu rubripes on moon jellyfish with respect to nutritional composition and behavioural traits. Aquac Nutr 24:504–514.
5Marques R, et al. (2016) Jellyfish as an alternative source of food for opportunistic fishes. J Exp Mar Bio Ecol 485:1–7.
6Miyajima-Taga Y, et al. (2015) Effect of giant jellyfish Nemopilema nomurai as supplemental feed on threadsail filefish Stephanolepis cirrhifer . Nippon Suisan Gakkaishi (Japanese Ed 81:701–714.