Elsevier

Aquaculture

Volume 496, 1 November 2018, Pages 10-18
Aquaculture

DHA enrichment of the red earthworm Eisenia fetida for improving its potential as dietary source for aquaculture

https://doi.org/10.1016/j.aquaculture.2018.07.005Get rights and content

Highlights

  • This study aimed at assessing, for the first time, the potentials for omega-3 fatty acids enrichment (particularly DHA content) in the red earthworm Eisenia fetida.

  • The levels of some of the n-3 PUFA, important for cultured marine species, including 18:2n-6, 18:3n-3 and 20:5n-3 (EPA), were found to be high, while in contrast, 22:6n-3 (DHA) was quite low in the earthworms.

  • In the bedding technique, DHA increased linearly for up to 9.6 fold, while this increase was fluctuating in the bed-free technique, reaching a maximum of 8.7 fold over the controls.

  • The suggested optimal dose of enrichment is 25 g ES inclusion into 100 g compost, for 12-h enrichment time for a fast boosting of DHA in the earthworms in bed-free technique, whereas in bedding technique a longer period of up to 96 h is recommended.

  • The present study showed that earthworm (E.fetida) can be successfully enriched in its DHA content; making it a novel and easily producible feed resource, particularly as broodstock feed, in aquatic hatcheries.

Abstract

This study aimed at assessing the potential of omega-3 fatty acids enrichment in the red earthworm Eisenia fetida. Specifically, the investigation focused on the effects of a commercial enrichment emulsion (ES) supplied at graded doses (0, 10, 25 and 50 g ES/100 g of compost) and at different enrichment times (0, 12, 24, 48 and 96 h) on the resulting fatty acids composition of red earthworm reared with or without additional bedding materials. In the first experiment (bed-free technique), inclusion of ES, in the highest tested dose (50 g/100 g compost), exerted stress on the earthworms resulting in high mortality, while the use of bedding in the second experiment led to safer but slightly reduced enrichment. Dominant fatty acids in earthworms were found to be PUFA (51.08–53.04 mol%), followed by MUFA (25.95–26.90 mol%) and SFA (21.16–22 mol%). Non-enriched earthworms contained some of the PUFA, important for cultured marine species, including 18:2n-6, 18:3n-3 and 20:5n-3 (EPA), whereas in contrast, 22:6n-3 (DHA) is very low. Overall the results of this study have, for the first time, showed that LC-PUFA, and especially DHA, contents of E. fetida can be enriched to levels much higher than those of the control (non-enriched) groups. In bed-free technique, DHA accumulation in the worms increased from 0.31 mol% (0 h) to 14.38–14.63 mol% at 12 h, 9.65–9.81 mol% at 24 h, 10.65–15.41 mol% at 48 h and finally 15.81–18.31 mol% at 96 h. In the bedding technique, DHA also rose from the base level to 5.27–5.97 mol% at 12 h, 5.25–8.60 mol% at 24 h, 11.06–13.01 mol% at 48 h and finally 14.05–17.14 mol% at 96 h. In the bedding technique, DHA increased linearly for up to 9.6 fold, while this increase was fluctuating in the bed-free technique, reaching 8.7 fold over the non-enriched controls. Throughout the experiments, n-3/n-6 ratios rose from 1.35–1.56 in the controls to 3.38–3.72 in the enriched-groups. As a result, 12-h enrichment time is suggested to be suitable for a fast boosting of DHA in the earthworms with bed-free technique, while a longer period of up to 96 h is recommended with bedding technique at optimal dose of 25 g ES inclusion into 100 g compost. The present study showed that earthworm (E.fetida) can be successfully enriched in its DHA content; making it a novel and easily producible feed resource, particularly as broodstock feed, in aquatic hatcheries.

Introduction

In aquaculture, fishmeal and fish oil are considered to be crucial dietary sources of proteins and lipids, respectively, to efficiently meet the essential amino acid and fatty acid requirements for cultured fish and crustaceans (Medale and Kaushik, 2009; Turchini et al., 2009; Jobling, 2016). When it comes to dietary lipids, it is well known that farmed marine animals, as with all vertebrates, cannot de novo synthesize polyunsaturated fatty acids (PUFA). It is also known that if PUFA are provided in their diets in the form of C18 PUFA (PUFA with 18 atoms of carbon, such as those found in terrestrial vegetable oils) most of the farmed marine animals cannot, or have very limited capacity to, bioconvert them into long chain PUFA (LC-PUFA), such as EPA (eicosapentaenoic acid; 20:5n-3) and DHA (docosahexaenoic acid; 22:6n-3) (Castro et al., 2016). Therefore, for cultured marine species, n-3 LC-PUFA are considered to be essential nutrients, as they are very important for various physiological processes, and therefore must be present in dietary meaningful quantities (Turchini et al., 2009; Tocher, 2010). However, n-3 LC-PUFA containing raw materials available for the aquaculture feed (aquafeed) industry are currently limited to marine derived products such as fish oils, which are restricted in their supply, under increasing demand and thus resulting in constantly increasing prices (Turchini, 2013). Reducing production costs, improving overall system efficiency and increasing environmental sustainability have always been priorities in the aquaculture and aquafeed sector. Thus, the search for alternative protein and lipid sources, to substitute and/or reduce the dependence upon marine resources, has been one of the main targets of the aquaculture sector since its global expansion 50 or so years ago (Jobling, 2016).

In this context, earthworms, though increasingly cultivated for vermicompost production in many countries, may well be considered to be an underutilised, and yet understudied, but potentially promising resource. These invertebrate animals can be efficiently cultured in great quantities, at cheap production costs and with very simple technologies. They are reared on decaying/fermenting organic matters of almost any kind, such as cow, sheep, goat, poultry, pig and horse manures, as well as all type of plant and animal matters, food and feed wastes and various agricultural by-products (Chauhan et al., 2010; Fairchild et al., 2017). Among many species, the red Californian earthworm (Eisenia fetida) is the most suitable for captive production and currently the most commonly farmed species worldwide. Earthworms have been reported be rich in protein, essential amino acids, n-3 PUFA as well as a suite of minerals and trace elements (Stafford and Tacon, 1985; Paoletti et al., 2003; Istiqomah et al., 2009). The quality and quantity of amino acid contents of earthworms have been reported to be comparable to that of fish meal (Tacon et al., 1983; Tacon and Metian, 2009; NRC (National Research Council), 2011), but their amino acid composition has also been reported to be variable from one species to another and relative to the food type they have been feeding upon (Tacon et al., 1983; Dong et al., 2010).

Earthworms (E. fetida) have been reported to be a good source of n-3 PUFA and EPA in particular, but limited in their DHA content (Liu et al., 2008; Beksari, 2017). Thus, earthworms' DHA content has been suggested to require attention before their possible use in feed for marine cultured species, particularly if to be used as broodstock feed (Liu, 2006). Darmawiyanti (2013) has been the first researcher to report that an earthworm species (Pheretima sp.) could be enriched in their nutritional composition. Specifically, in that study, it was reported that phospholipid, cholesterol and beta-carotene content of the earthworm can be improved, and that the enriched-worms could be successfully used as feed for shrimp (P. vannamei) broodstock, instead of marine worms (Nereis sp.). To the best of authors' knowledge, so far there has been no attempt to study the possible enrichment of the earthworm E. fetida for n-3 LC-PUFA, and especially for DHA. If DHA enrichment is proven doable, earthworms could have good potentials to be utilized as live, fresh or in dried forms, instead of more expensive marine counterparts, particularity for broodstock in marine hatcheries.

The aim of this study was therefore to evaluate the potential effects of an enrichment solution on the n-3 LC-PUFA, and especially DHA, content in the earthworm E. fetida. To achieve this, the response to the enrichment solution was tested at different, graded doses (0–50%), for different periods of time (0–96 h), and with or without the presence of bedding material, in two different experiments. Ultimately, the objective of this study was that of assessing the potential DHA fortification of earthworms, to provide the basis for their possible utilisation in marine hatcheries, as well as for other uses in the aquafeed industry.

Section snippets

General culture conditions

This study was carried out in the Mariculture R & D Centre of the Faculty of Fisheries of Çukurova University, Adana - Turkey. The red earthworms (Eisenia fetida) specimens used for the in vivo experiments were obtained from a commercial producer (Argesol Tarım Hayvancılık San. Tic. Ltd., Balıkesir - Turkey). After transport, earthworms were cultured using standard procedures in three plastic tanks (0.6 × 3 × 0.5 m) for several weeks before being used in the experiments. The compost material

Results

The fatty acid results showed that the non-enriched earthworms used in this study contained 20.16–22.00 mol% SFA, 25.95–26.90 mol% MUFA and 51.08–53.00 mol% PUFA (Table 1, Table 2, Table 3). In the initial and control (non-enriched) earthworms, the levels of some of the main PUFA, for example 18:2n-6 (10.30–11.35 mol%), 18:3n-3 (10.17–12.22 mol%) and eicosapentaenoic acid, EPA, 20:5n-3 (8.44–11.02 mol%), were relatively high, whereas in contrast, docosahexaenoic acid, DHA, 22:6n-3, was found to

Discussion

The present study demonstrated that: i) although the red earthworm E. fetida contains relatively high levels of PUFA, their natural DHA content is very limited, as previously observed by Liu et al. (2008) and Beksari (2017); but ii) the DHA content of the earthworm can be dramatically improved by specific dietary enrichment, and this, to the best of authors' knowledge, is the first report of such potential. The objective of this study was indeed to assess if earthworms nutritional composition

Conclusions

This study has demonstrated that n-3 PUFA and specifically DHA content of the red earthworm E. fetida can be elevated to levels up to 8–9 folds over the control worms by simply adding a commercial enrichment solution into their culturing compost. In order to effectively boost DHA level of the worms, a 12 h short enrichment period can be recommended when the bed-free enrichment technique is used, while longer periods of preferably 96 h should be chosen if the bedding technique is preferred. For

Acknowledgment

This study was supported by the Scientific Research Fund of Cukurova University (Grant No: FDK-2016-6272).

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