13/02/2026
TINY CRITTERS OF SAFETY BAY AND SHOALWATER BEACHES - by Stephen Ambrose
PART 4: TINY FLIES.
This post continues the discussion about the importance of retaining beach-washed seaweed and seagrass (beach wracks) at beaches in Safety Bay, Shoalwater Bay and Point Peron. There is a whole host of creatures that help break down the beach wracks and return nutrients back into the marine environment of the Shoalwater Island Marine Park. Past posts have examined the roles of springtails, sandhoppers and woodlice, the present post takes a look at the types of tiny flies that are found in and around the beach wracks.
Tiny flies are some of the most obvious insect species buzzing around the beach wracks, especially around dusk. A few may be fly species that buzz around your home, but many are found only on beaches where there is an accumulation of decaying seaweed or seagrass. They include kelp flies, gnats (midges and sandflies), shore flies (subfamily Tethininae), and parasitic flies.
Kelp flies (family Coelopidae) are small, bristly flies that swarm around beach wracks and lay their eggs among the kelp. The eggs hatch into maggots which then feed on the kelp, helping to break it down, while the adults feed on the bacteria in the kelp. Fortunately, they do not bite humans, but their swarms can be troublesome when in large numbers. One of the most common kelp fly species on local beaches is the Black Kelp Fly (Coelopa frigida), which is found on beaches at temperate latitudes worldwide.
Although the gnats swarming around the beach wracks are biting midges, they are not the same species that you find in your garden or in other areas inland of the beaches. They belong to the fly family, Ceratopogonidae and have a similar lifecycle to kelp flies. Most species belong to the genus, Culicoides, but I'm not aware of any studies that have identified the specific species that occur on local beaches.
Beach wracks that have high loads of woodlice (isopods) can also attract parasitic flies from the family, Rhinophoridae (woodlouse flies). Eggs are laid in the beach wracks and when they hatch the larvae attach themselves to passing woodlice. A larva burrows into the body, consuming the inside of the woodlouse's body, where it grows and eventually pupates.
All these flies produce lots of maggots (fly larvae). If you lift up a clump of moist seagrass close to the water's edge on Safety Bay Beach, for instance, you will see teems of maggots. They are harmless to people, but they are really important for breaking down the beached seagrass and kelp and, because there are so many of them, they are an important source of food for shorebirds, and the occasional lizard that ventures onto the beach. When wave action retrieves the clumps of decaying seaweed and seagrass, a lot of the maggots are also carried into the water. Shoals of small fish in the shallows then feed on them.
The Coelopidae or kelp flies are a family of Acalyptratae flies (order Diptera), they are sometimes also called seaweed flies, though both terms are used for a number of seashore Diptera. Fewer than 40 species occur worldwide. The family is found in temperate areas, with species occurring in the sou...