Introduction
More than 50% of the marine and coastal aquaculture products cultivated worldwide are macroalgae, with brown and red species dominating the sector and green algae contributing only to a small fraction
. Among the green algae of economic interest is Caulerpa lentillifera , commonly known as "sea grapes" or "green caviar". This edible seaweed is popular in the Indo-Pacific region for its unique texture and its high nutritional value
. In Viet Nam, the cultivation of C. lentillifera takes place mainly in earthen tidal ponds, where the algae are either planted directly into the sediment (sowing method) or grown on perforated plastic trays (tray method), depending on the characteristics of the pond’s substrate
. Also of economic interest in the region and used for human consumption is the spotted babylon snail Babylonia areolata . Co-cultivation of extractive species like seaweeds and fed species has been shown to effectively reduce negative impacts of aquaculture such as eutrophic wastewater
. The objective of this study was to assess whether the integration of Babylonia areolata into existing sea grape ponds could potentially increase the economic profit of sea grape farmers without reducing the yield of Caulerpa lentillifera . The factors of (1) sea grape culture method (tray vs. sowing), (2) co-culture approach (spatially separated vs. common bottom space), and (3) B. areolata stocking density were investigated.
Material and Methods
The study consists of two experiments that were conducted at the Institute of Oceanography (IO) in Nha Trang, Khánh Hòa Province, Viet Nam: an outdoor terrace (six weeks) and an indoor laboratory experiment (four weeks) . The outdoor experiment investigated how the culture method of C. lentillifera (in the following referred to as Caulerpa) and the co-culture approach with B. areolata (in the following referred to as Babylonia ) affect the growth and physiology of C aulerpa. Three different treatments were assigned to nine 400L tanks (n=3): Only Caulerpa ; Caulerpa and Babylonia together; Caulerpa and Babylonia spatially separated. Each tank contained Caulerpa in two different culture methods , on perforated plastic trays and directly in the sediment, respectively . The second experiment focused on different Babylonia stocking densities. Four different snail densities were set up in 10L aquaria: Control, low, medium, and high with 0, 2, 4, and 8 snails, respectively (n=5). The response parameters weight of both organisms, harvestable biomass, photosynthetic efficiency (maximum quantum yield of photosystem II, Fv/Fm), antioxidant activity ( AOA, ABTS assay, following
, and total phenolic content ( TPC, Folin-Ciocalteu assay , following
of the algae as well as NOx and PO4 in the water were measured. AOA and TPC will not be considered further in the following (article in progress) .
Results and Discussion
Overall, the presence of Babylonia had a positive effect on algal growth and physiology. The tray method resulted in higher weight gain and harvestable biomass of sea grapes compared to the sowing method over the course of the experiment, but only when co-cultured with Babylonia (Fig. 1A, B) . Algae in the monocultures exhibited lowest growth rates and even decreased in weight. In the monocultures, the sowing method resulted in better growth of the sea grapes in the first weeks due to the availability of nutrients in the soil
. The poor growth of Caulerpa in the monocultures in the tray cultures was probably due to nutrient deficiency since NOx and PO4 concentrations in the monocultures (0.06±0.08 µmol L-1 and 0.48±0.13 µmol L-1, respectively) were significantly lower than in the snail treatments (NOx: 7.7-33.68 µmol L-1; PO4: 0.9-2.53 µmol L-1). Limitations of nutrients are known to inhibit metabolic activities of seaweeds, negatively affecting photosynthesis and growth
, which is why nutrient deficiency would also explain the decreased Fv/Fm values in the monocultures. Babylonia growth and survival were not affected by spatial separation from Caulerpa . Babylonia growth was consistent in both treatments and the snails nearly doubled in weight over the course of this study. Survival of the snails was 100% in both treatments. The indoor laboratory experiment showed that Caulerpa weight gain was positively correlated with snail density. The high increase in algal biomass in the high density- snail treatment indicates that snails can be implemented at high quantities without negative implications for the seaweed. The study showed the high potential of integrating Babylonia into existing sea grape ponds. Co-culture of these two organisms could increase the economic profit of sea grape farmers without reducing the yield of Caulerpa .
References