Seaweed flies

The equinoctial gales washed up lots of nice kelp onto the beaches of Scarborough this year (below).

Freshly washed up kelp. Scarborough South Bay. Photograph by Raymond JC Cannon.

The piles of seaweed were gradually pushed up the beach by the incoming tides, eventually accumulating beyond the strand-line, where it slowly decays and eventually dries out. This gift from the sea is a wonderful bounty which is harvested in some parts of the world. It is also a habitat for a host of invertebrates which have evolved their life-cycles to live on this ephemeral resource. In fact, it is a mini-ecosystem of plants and animals.

Freshly washed up kelp. Photograph by Raymond JC Cannon.

I must confess, I have never paid much attention to piles of seaweed, from an entomological perspective, but this year I decided to have a closer look. In particular, for insects which live on the seaweed. Fortunately, there was not too much human activity on the beach (below) as I went poking around in the lumps of rotting seaweed, although I did get some strange looks. What is that man photographing?

Donkeys on the beach. South Bay, Scarborough on 20th Sept 2020. Photograph by Raymond JC Cannon.

By this time, the kelp had been on the beach for a week or two, and was starting to decay nicely (below).

Rotting Kelp

This was just the sort of substrate that is home to seaweed flies, which lay their eggs in the decaying algae. The larvae feed on the bacteria digesting the seaweed, I think.

There are dozens of different flies which can be seen on and around the seashore of the British Isles, but two types are commonly found on washed up seaweed: kelp flies and seaweed flies. The names are somewhat changeable, but I will refer to kelp flies as those in the family Coelopidae and seaweed flies as members of the genus Fucellia which are in the family Anthomyiidae.

There lots of different Fucellia species, over 20 species, but Fucellia tergina and F. maritima are two species which which occurs around the coasts of the British Isles (and beyond).

Fucellia sp. possibly F. tergina or F. maritima. Photograph by Raymond JC Cannon.

These little anthomyiid flies flick their wings – often just one wing as shown below – as a form of signalling. Although they are very tiny, it is quite easy to watch this wing-flicking behaviour, either with binoculars or a close-up lens (below). 

Fucellia sp. possibly F. tergina or F. maritima showing wing flicking behaviour. Photograph by Raymond JC Cannon.

The wing-flicking is probably a message to both males and females. To other males it may be a form of competition, perhaps saying this is my patch, go away! As a sexual signal, it might be a way for females to chose mates, i.e. on the basis of whether they like that particular males’ little wing flicks! (See Memmott and Briffa, 2015). They also have nice little faces!

Fucellia sp. head on. Photograph by Raymond JC Cannon.

Here is a much more detailed portrait of Fucellia maritima. Very attractive, I’m sure you will agree!

Fucellia maritima, Penmon Point, North Wales, Sept 2019 by janet graham (CC BY 2.0)

The other sort of flies, confusingly called both kelp flies and seaweed flies are in the genus Coelopa. Once again, there are lots of different species, but two – Coelopa frigida and Coelopa pilipes – occur together (i.e. co-exist) on coasts around the North Atlantic. I’m not sure which one I have photographed here; is it Coelopa frigida (the Bristly-legged Seaweed Fly) or Coelopa pilipes (Furry-legged Seaweed Fly)?! The links are to the amazing photographs by Steven Falk.

Bristly-legged seaweed fly (possibly Coelopa frigida) Scarborough. Photograph by Raymond JC Cannon.

Since, Coelopa frigida is the more common species, and likes northerly regions, I’ll go with the Bristly-legged one rather than the Furry-legged one! But if anyone knows for sure, please let me know.

Bristly-legged seaweed flies (possibly Coelopa frigida) Scarborough. Photograph by Raymond JC Cannon.

Both of these families of flies have fascinating behaviour. The Coelopa species are I think, the better studied as they have become a model species for studies on mating, mate choice and sexual selection (e.g. Day and Gilburn, 1997; Shuker and Day, 2002). 

Bristly-legged seaweed flies (possibly Coelopa frigida) Scarborough. Photograph by Raymond JC Cannon.

I would like to try and photograph these little flies again sometime. But, I think I will try and find a better location, where there are more flies and less people; and hopefully better weather!

Seaweed spider. Photograph by Raymond JC Cannon.

Finally, there were lots of tiny little spiders running around on and in the decaying kelp. They had strung masses of web-lines around, so the appeared to move magically from one spot to the other! How wonderful to get a glimpse into their tiny little, other worlds! The grains of sand in the following picture, show how small the spiders are!

Seaweed spider. Photograph by Raymond JC Cannon.


Day, T. H., & Gilburn, A. S. (1997). Sexual selection in seaweed flies. Advances in the Study of Behaviour26, 1-58.

Memmott, R., & Briffa, M. (2015). Exaggerated displays do not improve mounting success in male seaweed flies Fucellia tergina (Diptera: Anthomyiidae). Behavioural processes120, 73-79.

Shuker, D. M., & Day, T. H. (2002). Mate sampling and the sexual conflict over mating in seaweed flies. Behavioral Ecology13(1), 83-86.

Yerbury, J. W. (1919). “Seashore diptera.” Journal of the Marine Biological Association of the United Kingdom 12.1, 141-145.


  1. Hi Ray, I am just identifying flies caught on sticky boards in a prawn processing plant in Kilkeel and what i thought were Kelp flies are actually your seaweed fly – Fucellia maritima. I was looking for good wing photos and came upon your website. I’m not sure if Janet’s photo is Fucellia maritima. I would like to keep in touch. Remember NZ flatworms ? Please send me a message.

  2. Hi Ray – I think the middle picture of your Coelopa series is almost certainly C. pilipes – the fuzz around the fore femur looks right, though I find it much easier to be sure from a side view of the face where the bristles below the eye are either hair-like (pilipes) or bristley (frigida) – both species should occur in Scarborough. Do you use iRecord – if you put the image on there I can approve it and get it entered into the database – or if you give me the grid ref and date I could do it for you. Have you been back to find more flies – there are a few more species to look out for …

    Donald Smith Kelp Fly Recording Scheme –

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