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Life of a green-lipped mussel

Green-lipped mussels are endemic to New Zealand. They make their home on rocks and solid surfaces around New Zealand’s coastline.

Mussel life cycle

During its life cycle, the green-lipped mussel undergoes enormous changes, including fundamental changes in shape. It changes from a free-swimming larval form (which swims in the ocean) to a settled juvenile and adult form (which is anchored to one spot).

Mussels spend up to 6 weeks as larvae and can live for many years as adults. Farmed mussels, however, are harvested after about 18 months in the adult form.

Beginnings: fertilisation

The green-lipped mussel’s life story starts with the release of eggs (from female mussels) and sperm (from male mussels) into the water. A sperm finds an egg and fertilises it, forming a zygote. This process is known as ‘broadcast spawning’.

Once fertilisation has happened, the cells in the mussel zygote start to divide and soon begin to differentiate to form a swimming mussel larva.

Life of a larva

Mussel larvae are free-swimming. They feed on phytoplankton and use small hairs called cilia to move around in the water and to help them collect food. Ocean currents can carry larvae hundreds of kilometres away from their ‘birthplace’.

When larvae reach about half a millimetre in size, they prepare to ‘settle’ (attach to a surface). Larvae settle by secreting strong, stretchy fibres called byssal threads, which anchor the larva to its chosen surface. Larvae usually settle first on flexible filamentous surfaces such as seaweed.

Metamorphosis: from larvae to spat

As soon as larvae have settled, they change (metamorphose) from their larval shape into one that’s more recognisably mussel-like. At this stage, they are commonly known as spat. They also change the way they do things – for instance, they develop gills, which they then start to use to breathe (instead of absorbing oxygen directly through the surface of their bodies) and to gather food.

Resettling

Even though they have settled onto a surface, spat are still very mobile. They can move from site to site by crawling around with their foot or by ‘mucus drifting’ (moving through the water by using threads of their own mucus as parachutes).

As they grow larger, mussels become less mobile and choose a solid surface on which to settle permanently. Their favourite surfaces include rocks, wood (such as wharf posts), ropes (on mussel farms) and other mussels (in mussel beds on the sea floor).

Growing up

Once settled, mussels grow rapidly – from about half a millimetre in length to approximately 24 cm (although farmed mussels are harvested when they reach approximately 10 cm). The colour of their shells deepens over time as they begin to build up their adult shell.

From about 1 year of age, mussels are sexually mature – they can produce and store eggs (orange) and sperm (white) in preparation for spawning. This is why mature female mussels have orange flesh, while mature male mussels have white flesh.

How mussels feed

The main food source for green-lipped mussels is phytoplankton – plant-like microscopic organisms that live in seawater in many millions. Mussels trap phytoplankton by pumping large volumes of seawater over their gills. The phytoplankton gets trapped on the gills and is then transported to the mussel’s mouth and eaten. This process is known as filter feeding.

Farmed mussels in New Zealand have exactly the same food source as wild mussels. Mussel farmers don’t need to feed their mussels, because the coastal waters around New Zealand are very rich in phytoplankton. Run-off of fertilisers from farms into the sea means that our coastal ocean waters contain ample nitrogen and phosphorus to stimulate phytoplankton growth. This is one of the main reasons why coastal areas such as the Hauraki Gulf, which backs onto land that is farmed intensively for dairy purposes, are such good sites for farming mussels.

A home for marine animals – and a meal for others

Because adult mussels are immobile, they are attractive to other marine organisms as a place to live. Barnacles often settle on the outside of mussel shells, and mudworms bore into the shells and cause blisters. Pea crabs take up residence within mussel shells, where they steal the phytoplankton that the mussel has collected on its gills for food.

Being immobile also means mussels are at risk of being eaten. Their hard shell protects them from some would-be predators, but sea stars, crabs and crayfish are all strong enough to access mussel flesh from within the shell. Spat are also at risk of being eaten by fish with strong crushing jaws, such as snapper.

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