# The Effects of Different Substrates in Pond Net Cages on the Succession of Periphyton and the Seedling Protection of Sea Cucumber Apostichopus japonicus

## TL;DR

This study found that curvimurate net substrates in sea cucumber aquaculture ponds support better algal growth and higher seedling survival rates compared to other materials.

## Contribution

The study identifies curvimurate net as the optimal substrate for sea cucumber seedling protection based on periphyton succession and survival data.

## Key findings

- Curvimurate net (CU) substrates had the highest algal species diversity and chlorophyll-a content.
- Sea cucumber seedlings in CU cages showed significantly higher survival rates and yields compared to other substrates.
- Diatoms dominated early stages, while blue and green algae became more prevalent later.

## Abstract

This study set up net cages with different materials in a sea cucumber Apostichopus japonicus aquaculture pond. Experiments were conducted to compare the different types of attachments—curvimurate net (CU), nylon mesh (NM), and ground cage—aiming to understand the occurrence and succession of surface-attached organisms on different types of adhesive substrates and their effects on the cultivation of sea cucumber seedlings. The results indicate that, in the early stage, benthic diatoms dominate, while, in the later stages, blue algae and green algae dominate. However, the net cage with the CU substrate has the highest density, chlorophyll-a content, and number of alga species, resulting in a significantly higher seedling yield and survival rate than the other substrate materials. Therefore, among the different attachments, the CU substrate is the best choice for A. japonicus seedling culture.

With the industry development of sea cucumber Apostichopus japonicus aquaculture, the indoor high cost and low survival rate have become serious problems. Therefore, it is necessary to optimize substrate selection for seedling protection in outdoor pond net cages. This study explores the succession of periphyton on the different substrate surface types, including a curvimurate net (CU), nylon mesh (NM), and ground cages (including a ground cage net (CN) and ground cage plate (CP)), and their effects on the seedling protection of sea cucumbers. In addition, we monitored the substrates’ dry weight, chlorophyll-a, and the community composition of substrates, alongside seedling growth, yield, and survival rate. The results show that a total of 7 phyla, 23 genera, and 31 species were detected on the substrates, with diatoms dominating (19 species) and Chlorophyta (4 species) being the main species. The CU had the highest total number of alga species attached, significantly higher than the other substrates in week 13 (p < 0.05). In week 9, the diatom density dropped to its lowest point, and, after September, it rose with the decrease in water temperature. In terms of dry weight with and without ash, CP increased rapidly in the early stage, with NM, CU, and CP being significantly higher than CN in week 13 (p < 0.05). The chlorophyll-a content showed a decreasing–increasing–decreasing trend, with CU reaching 3.62 ± 0.48 μg/cm2 in the 13th week, significantly higher than other substrates (p < 0.05). Finally, the A. japonicus survival rate and yield in the CU group at week 12 were significantly higher than those in the NM and ground cage groups (p < 0.05). At week 17, the average weight, yield, and survival rate in the CU group were still optimal, with the yield 5.76 times that in the initial dosage. These results suggest that the CU has a suitable mesh size, has good permeability, and may stably support sediment, which is conducive to the growth of benthic diatoms. In addition, it can provide sufficient natural feed and a good habitat environment and is the preferred substrate for A. japonicus seedling protection in outdoor pond net cages.