Caenidae Family - ID Tips
Class: Insecta; Order: Ephemoptera; Family: Caenidae
|
Vital Stats |
Rating |
Scale |
|
BMWP Score |
7/10 |
1 = Filthy to 10 = pristine |
|
LIFE Flow Group |
4/6 |
1= Rapid velocity (>100cm/sec) to 6= drought resistant |
|
Family Level Difficulty Rating |
3/10 |
1= Easy to 10= very difficult |
|
Species Level Difficulty Rating |
6/10 |
1= Easy to 10= very difficult |
There are nine species belonging to the Caenidae family in the UK. In comparison to most other Ephemeroptera they are quite easily overlooked due to their small size. They are unable to swim but instead cling to aquatic plants or on silt substrates and feed on detritus. They are fairly intolerant of organic pollution.
Identification
Caenis luctuosa/macrura and C. rivulorum are generally the most frequently encountered in routine sampling of rivers. Characters are provided in the FBA key to separate Caenis luctuosa and C. macrura but as stated in the key these are not entirely satisfactory so normally these two species are recorded lumped together as a group. They can be separated from all other Caenidae by noting the presence of the deep notch on the last abdominal segment, which is observed on the ventral side
Figure 1. Posterior abdominal segments of Caenis luctuosa gp.
Small specimens may have to be scrutinized carefully to observe this feature.
Caenis rivulorum are often found with C. luctuosa group; the last abdominal segment is truncate but specimens may be more quickly separated by virtue of its ‘pied’ appearance caused by the contrast between the dark brown gill covers and pale abdominal segments between the gill covers and thorax
Figure 2. Caenis rivulorum (left), C. luctuosa gp (right)
Another species, Caenis pusilla, has been recorded from a small number of sites in Southern Britain, although it has likely been overlooked at further sites. The lateral margins of the abdominal segments of all other Caenis spp have a fringe of fine hairs, Caenis pusilla, however, has instead a fringe of short blunt spines
Figure 3. Caenis pusilla, showing lateral spines
Caenis horaria and C. robusta are two species more associated with slack water habitats, such as lower reaches of rivers, ponds and canals. The shape of the pronotum of these two species are noticeably different to the other Caenis species. The lateral margin of the pronotum of Caenis horaria is concave in outline and diverges outward towards the anterior margin, forming a rounded corner to the pronotum
Figure 4. Caenis horaria
In contrast, although the lateral margin of the pronotum of Caenis robusta can also be slightly concave, it meets the front margin to form a distinct sharp rather than rounded corner
Figure 5. Caenis robusta
As stated in the FBA key, the tarsal claws are bent to a more obtuse angle in Caenis robusta than C. horaria but this may be bit tricky to establish when both species aren’t available for comparison.
There are two further Caenis species which were discovered in Britain in the late 1990’s, C. beskidensis which was found on the River Lugg in Wales, and C. pseudorivulorum, which was found in the River Derwent in Yorkshire, and also more recently at sites in north-east Scotland and north Wales (John Blackburn, pers. comm.) Both these species have a slight notch on the underside of the last abdominal segment , however much less pronounced than the notch present in Caenis luctuosa gp.
The final species in the British list is Brachycercus harrisella, which has a sporadic distribution in Britain, mainly found in mud and silt in rivers. It is probably the most immediately recognizable of the Caenidae by virtue of the three prominent tubercules on the head and the backward pointing projections on abdominal segments 3-7 (see diagram in the FBA key).
Ref: J.M. Elliott, U.H. Humpesch and Macan T.T. 2010.Mayfly Larvae (Ephemeroptera) of Britain and Ireland: Keys and a Review of Their Ecology - FBA Scientific Publication No. 66
Caenis pusilla found in River Test samples
APEM staff recently undertook some macroinvertebrate sampling on the lower River Test near Testwood. The stretch of river sampled was about 1m in depth and slow flowing. When these samples were processed back at the Dorset laboratory larvae of the mayfly Caenis pusilla were found to be present in the samples. According to the most recent edition of the FBA key to the Ephemeroptera of Britain and Ireland Caenis pusilla has only been recorded from three locations in the British Isles, these being Candover Brook (a tributary of the River Itchen in Hampshire), and in the rivers Wye (Wales) and Frome (Dorset). It is probable that the species is actually more widespread than this but has been misidentified as the much more common Caenis luctuosa gp which is similar in overall appearance and occurs in the same habitat; both taxa were present in the River Test samples. Confirmation of these specimens as Caenis pusilla probably represents a new record for the River Test. The rivers Test and Itchen are geographically close and similar in terms of water chemistry and physical characteristics so it is not surprising that the species occurs in the catchments of both of these rivers and probably occurs in the River Itchen itself.
