Kūtai/mussels improve water quality
As filter feeders, kūtai/ mussels are the ‘kidneys of the sea’ removing heavy metals, harmful bacteria, clearing the water and stabilising the seafloor.
Equipped with a powerful pump, a mussel can filter vast amounts of water – up to 1–2 bathtubs (150 to 350 litres depending on the size of the mussel) per day – as they feed on plankton.
In the process they ingest free floating sediment and other impurities (e.g. heavy metals), binding these up as mucus balls (pseudofaeces) that drop to the seafloor.
The reduction of sediment improves water quality and light penetration, which benefits fish, other shellfish, seagrass and seaweeds.
Just as plants stabilise the ground, kūtai bind together using their anchor threads creating a carpet of mussels that stabilises the seafloor and prevents sediment from becoming re-suspended.
Scientists estimate that the 500 sq.km of mussel beds once found in the Firth of Thames filtered that entire body of water in a single day.
Kūtai reefs act as nurseries and habitat for marine creatures
Kūtai reefs provide shelter for species like shrimps and crabs, and are a nursery for juvenile fish. By re-establishing kūtai reefs you’re creating a housing boom for marine life!
Once dropped onto the sea-floor, kūtai crawl around and attach to each other with anchor threads creating diverse 3D structures filled with numerous hidey holes.
Small sea creatures quickly flock to these underwater life rafts as they provide safety in numbers, physical refuges, all with a handy snack bar!
Without these reefs, many marine creatures, especially the young and vulnerable ones, are left exposed to the roaming predators that are looking for a quick meal.
Kūtai create a food source for marine communities
Kūtai/mussels not only filter the water and create habitat, they also directly provide an important food source for a number of creatures large and small.
They provide an easy meal for larger species like whai / rays, tāmure / snapper, whēke / octopus, and their waste provides nutrient-rich meals for seafloor creatures such as worms and crabs.
Unsurprisingly, kūtai reefs have shown to have 4 times the invertebrate density, 7 times the weight of living creatures and 10 times the amount of fish compared with soft-sediment seafloor communities (McLeod 2009).
Recent modelling shows that these reefs are particularly beneficial for worms, small crustaceans, sea cucumbers and sea snails, as well as larger species such as kahawai, snapper, triplefin, mackerel, mullet, trevally and tarakihi (Fleming, 2021, Sea, 2022).
At some sites, the abundance of fish was 20 times higher and the abundance of invertebrates 100 times higher when compared to soft sediment locations without mussels.
Kūtai provide a vital link in the food chain
As well as creating habitat, kūtai play an important role in cycling nutrients and energy in the underwater food web.
They eat microscopic organisms by filtering them from the water column, in so doing, capturing the nutrients at the seafloor where they are used by the kūtai and made available for many of the other inhabitants living in these kūtai beds.
In doing so they feed a whole community of sea-bed creatures, such as starfish, shrimps, hoppers, snails and worms.
Small fish hiding in the kūtai bed feed off these seafloor animals and ultimately become larger fish, or will be eaten by predators like kahawai and kingfish, extending the food web.
Kūtai act as a nitrogen sink
Auckland University scientists have recently discovered the community of organisms in a kūtai bed can process up to 200% more nitrogen than exposed sediment!
Nitrates are used by microscopic algae to grow, and kūtai feed on algae, which transfers these nutrients into the sea-floor through waste, which are consumed by bacteria – creating a natural nitrogen sink.
Already nitrogen offsetting is in place overseas and initial modelling has put the value of these restored beds at $US19 million per hectare.
With nitrogen levels increasing year-on-year in the Hauraki Gulf – Tīkapa Moana, these beds will help – in part – to reverse the nitrogen load coming into the gulf from land run -off.
Scientists at Auckland University are now looking at whether kūtai beds are carbon sinks too.
Find out more about our work in restoring the mussel reefs.