The intertidal flies of the Helford River

The shores of estuaries are exposed to more extreme environmental changes, particularly chemical, than shores bordering the open sea. Fresh water from rivers, streams and run off from the land meet and mix the tidal flow from the sea twice daily in a comparatively restricted area. The salinity of estuarine water varies widely and continuously not only according to the distance from the sea but also vertically from low water mark to the upper shore. The Helford River complex is subject to these variations in salinity as well as other chemical changes from agricultural and industrial operations. As part of an ongoing survey of the distribution of intertidal flies of West Cornwall and the Isles of Scilly, the Helford is being monitored to discover if the species found along the adjacent strictly marine shores, occur in the estuary.

A surprising variety of flies, (True Flies or Diptera, with a single pair of wings) are resident on a typical marine beach with strand-line debris and rocky outcrops. Twenty or more fly species from a range of families are regular inhabitants of the intertidal zone but unless the strandline is disturbed or the seaweed and barnacle-covered rocks examined closely, they usually remain unnoticed. All species are black, brown, grey or a mixture of all three merging with their microhabitat of seaweed, debris, sand and pebble beach or rocky outcrop, and are invariably reluctant to fly more than a few centimeters. Those occupying the strandline of decomposing seaweed range from the minute ephydrids, some less than 2 mm in length, to the large predatory helcomyzid, Helcomyza ustulata which up to 10 mm long is more conspicuous than most as it darts foreword for a metre or more to the next hiding place. Associated with colonies of barnacles on rocks, stone walls and jetties are several species of the genus Aphrosylus at least one of whose larvae prey on living barnacles. The adults prey on small midge and other larvae found in the surface algal film between barnacles and on surrounding substrates.

The current survey of intertidal flies is concentrating on the distribution of ten target species all of which can be accurately identified with a hand lens or microscope without dissection of genitalia. Only one of the target species has a widely used common name so these species are identified by their scientific name as is the case for most Diptera.

Coelopa frigida (Kelp or Wrack Fly) The Kelp Fly is found on most marine beaches in West Cornwall even where there are only small amounts of decomposing seaweed, and in late summer and autumn emerging adults from strandline kelp deposits can reach astronomical numbers causing mass movements of flies searching for further breeding sites. Coelopa frigida is found on both sides of the North Atlantic, extending to the Arctic Circle above Hudson Bay on the western side and north to Iceland along the European coastline. In North America this hardy fly is known as the Arctic Seaweed Fly, and also as the Flat-backed Kelp Fly because of its flattened body that enables it to complete most of its life-cycle between layers of decomposing kelp. Shore flies are forced to move up the beach to escape the incoming tide and remain above the splash zone until the ebb. Unlike most other intertidal species the Kelp Fly visits flowers for pollen at high water but is not restricted to this period for it can be found on flowers at any state of the tide. Occasionally it is found as a vagrant up to two kilometres inland perhaps windblown, or even emerging from spread seaweed deposits on farmland!

Kelp Fly Coelopa frigida

Surprisingly the Kelp Fly has not been located within the Helford River area during recent surveys. Large deposits of decomposing kelp were not encountered during these visits but deposits of brown seaweed were examined on both sides of the river. Although the larvae of this marine fly can survive increased levels of salinity there appears to be no information for this species on the effect of decreased salinity such as in estuaries.

Fucellia (Anthomyiidae) There is no descriptive common name to label this genus of flies three of which are intertidal – Fucellia fucorum, maritima, and tergina. These are small greyish black flies superficially resembling the House Fly, and are found in a range of microhabitats including scanty strandlines of seaweed and debris, stone and pebble beaches with attached Fucus, and shallow rocky outcrops with pools. Decomposing fish and crab refuse, and particularly decaying mussels, can attract large numbers of F.tergina especially during the summer. Fucellia tergina is the commonest of the three species on local marine beaches and this is the case in the Helford with records extending up river as far as Helford Passage. Records so far for F.fucorum are limited to St Anthony-in-Meneage and Gillan Creek and the only record for F.maritima is from Treath.

Aphrosylus (Dolichopodidae) Except on very exposed sea coasts one or more species of Aphrosylus can be found exploring barnacle-covered rocks. Four species occur in the British Isles and all have been reported from the Helford River estuary – Aphrosylus celtiber, raptor, ferox and mitis. The first two are the largest reaching 5-6 mm in length whilst the other two are minute at 1.5 mm or less making them difficult to spot on rocky surfaces. Sweeping with a fine net at suitable sites is the usual method of collecting these tiny flies. The larger species can be recognized in the field as Aphrosylus by their enlarged silvery palps on each side of the head reflecting the light. Aphrosylus celtiber and ferox occur in rocky coves or outcrops on both sides of river as far inland as Helford Passage; the uncommon raptor at Polgwidden Cove and mitis at six localities. Prior to 1990, Aphrosylus mitis was known from only from seven localities in the British Isles and six in northern France, and since then a further 20 sites have been located in West Cornwall and one in the Isles of Scilly. It prefers sheltered sites in estuaries especially where granite quays or jetties are partially covered in a film of algal slime and there are colonies of the Australian Barnacle Elminius modestus. However no direct association with this barnacle has been shown so far.

Scathophagidae This family of flies include the familiar golden coloured Dung Fly but the two marine scathophagids are very drab indeed – both conforming to the dull greys and blacks of intertidal flies. Ceratinostoma ostiorum, about 6 mm length, is a hardy shore fly found even on thin strandlines where it is a predator of other flies particularly the small ephydrids. In the Helford it appears to be widespread in small numbers wherever there are tidal deposits. The other scathophagid on the target list, Scathophaga litorea, once common along the shores and coastal marshes of West Cornwall and Scilly, has almost disappeared and the only site recently discovered is in the Hayle Estuary. In April,1904 it was reported by a visiting entomologist to the Isles of Scilly as the commonest fly on St Marys ‘sitting on the roads, on the seaweed and was a positive nuisance when sweeping freshwater marshes’. Thirty years later it was reported by an entomologist resident in West Cornwall as ‘common on the coast everywhere’ and as late as 1961 it was found on St Marys and Bryher of the seven islands visited in Scilly. Recent surveys of St Marys and Tresco has failed to find litorea, and the last record on the mainland, before the recent discovery of a colony in the Hayle Estuary, was a single specimen at Cadgwith in 1984. Disappointedly it has not been found in the recent surveys of the Helford River despite some beaches being very similar in profile and vegetation content to the Hayle Estuary site.

Provisional findings Nine of the ten target species of shore flies selected occur in the estuary of the Helford River but, except for one colony of Aphrosylus mitis, are restricted to the marine dominated lower region extending to a line roughly from Helford to Helford Passage. Scathophaga litorea, once common around the shores of West Cornwall and the Isles of Scilly, was not located. Aphrosylus mitis, now classified as Nationally Scarce, has been identified at six sites in the Helford and these are possibly the largest concentration of colonies of this rare western European species.

As shown in the Collins Ray Poulding
Guide to Insects 1986 p211

Ray Poulding

Extract from HVMCA newsletter No.32 Spring 2006