3. Ecosystem health

Particle feeding is undertaken by a wide range of organisms that feed on plankton and dead organic matter from either the water column or sea floor. Particle feeders include worms and sea squirts that use mucous nets to adhere to particles; reef building corals, sea fans, feather stars, worms and some jellyfish that passively feed from passing water currents; bivalves, sponges and some barnacles that actively create currents to feed from; and baleen whales, manta rays and some fishes that are active particle feeders.⁷⁸⁷ 

Particle feeding octocorals (gorgonians, sea whips, sea pens and soft corals) are the second most common group of macrobenthic animals on the Reef after hard corals.³¹⁸ Sponges have shown potential as a natural collection point for sampling environmental DNA.⁷⁸⁸ On a reef scale, it is estimated that sea cucumbers may bioturbate (turn over) 64,000 tonnes of sediment per year,⁷⁸⁹ which may partially buffer the impacts of ocean acidification through its contributions to reef biogeochemistry.⁷⁷² A single oyster can filter more than 200 litres of sea water per day, and oyster beds play a vital role in coastal ecosystems.⁷⁹⁰ Particle feeding can assist corals under heat stress or in turbid waters. ⁷⁷²

Despite increasing research on particle-feeding marine invertebrates, gaps in knowledge remain, such as of their tolerance to changing environmental conditions.⁷⁹⁰ Threats vary between species, but they include climate change-related ocean warming and acidification, pollution and targeted extraction.⁷⁹⁰ The Queensland saucer scallop stock has seen declines in recent years as a result of overfishing compounded more recently by elevated water temperatures.^791,792 Current estimates show the stock to be 15 per cent of unfished biomass,⁷⁹³ and it is classified as depleted.⁷⁹⁴ Increasing water temperature is expected to lower net primary production and result in declines in shallow benthic filter‑feeding communities.⁷⁹⁵ The larval stages of many invertebrates rely on ocean currents for transport and settlement of their offspring, and changing currents in the future may affect recruitment and distribution of some species.⁷⁹⁰

Microplastics, especially those that have accumulated biofilm, are ingested by many particle feeders from shellfish to baleen whales.^{796,797,798,799} Evidence suggests transfer to muscular tissue is low for Reef species.⁷⁹⁶

A range of pressures affect particle feeders, and condition varies between species and species groups. No consistent trend has been identified since 2019. Since 2019, cover of some fast-growing species of corals has increased, but condition of corals in some areas remains poor. Long-term declines have occurred in the commercially harvested saucer scallop population, and the stock is currently classified as depleted.