There aren’t many insect species that live in the intertidal zone. The two you’re most likely to encounter are the springtail called Anurida maritima that scoots across the surface of rock pools on the upper shore and seaweed flies Coelopa frigida that breed in vast numbers in the rotting heaps of seaweed that pile up on the strandline. Even fewer insects complete their life cycle actually submerged in salt water but one is this little chironomid midge called Clunio marinus, that I found amongst the fronds of Cladophora seaweed in rockpools on the beach at Whitburn near Sunderland yesterday.
This specimen was about 5mm. long and living in a tube constructed from fine sand grains amongst the fronds of the seaweed.
It fed by hauling itself through the branches of the seaweed using two short leg-like appendages just behind the head.
The midge larva seemed to be nibbling away at much smaller organisms that encrusting the fronds, using a pair of pointed jaws. As far as I could tell it didn’t eat any of the seaweed – just the organisms that encrusted it.
Chironomid midges are extremely common in fresh water, breeding in vast numbers on ponds and even in small bodies of water like waterbutts in gardens. This species, though, as evolved to survive the rigours of life in salt water rockpools on the middle and upper seashore, where temperature and salinity levels can be extremely variable.
You can find a web site devoted to chironomid midge biology here