Striped and yellow-ish UK Isotomidae

I know of four* UK species in the family Isotomidae which fit a similar description: they are roughly yellow-ish as the base colour and then have at least one dark longitudinal stripe running the whole length of the abdomen. There are four species which match this description and are listed on the NBN Atlas. They are:

1. Isotoma riparia
2. Isotomurus palustris
3. Isotomurus plumosus
4. Isotomurus unifasciatus*

*Isotomurus unifasciatus clearly 'fits the bill' in terms of generally resembling the other three species but is of unclear status in the UK (according to here). As such it is probably best to not consider this species an option for ID, although keeping it in mind is worthwhile!

Ignoring Isotomurus unifasciatus, we can make some headway towards determining which species an observation shows by looking at (1) the setae (tiny hairs) and (2) the pigmentation pattern.

Step 1: Is the specimen Isotoma riparia?

As outlined here, Isotoma species as a whole can easily be distinguished from Isotomurus species by looking at the setae on the abdomen. Specifically, Isotoma species have macrosetae (large hairs) on all body segments, whereas on Isotomurus species there are macrosetae BUT they are confined to the last two or three posterior (back) segments, and are absent from the front of the abdomen. To visualise this difference compare some of the photos of Isotomurus here with those of Isotoma here. Following this general rule, it should be clear when a given striped and yellow-ish UK Isotomid is Isotoma riparia. Simply look at where the long hairs are! Generally, we might expect Isotoma riparia to be quite rare because there are only a few records of it in the UK so far (source). Of those, all have been from very wet habitats, including two places along the river Thames and a salt marsh in Dorset (source). If any more records of this species turn up, we might expect them to be from similar places. If there are from very different places (i.e. quite dry habitats), microscopic examination would likely be necessary to make extra sure the ID is correct (N.B. microscopic examination might be warranted anyway, given the species' relative rarity!).

Step 2: Isotomurus palustris vs. Isotomurus plumosus

Having eliminated Isotoma riparia from consideration, and ignoring Isotomurus unifasciatus, the choice is then between Isotomurus palustris and Isotomurus plumosus. If one compares photos of the two species (e.g. here), a slight difference in pigmentation pattern should be clear. Namely, whilst both species have three longitudinal stripes, on Isotomurus palustris all three are clearly visible from the top-down, whereas with Isotomurus plumosus the lateral stripes are only just visible from the top-down and much better seen from a side angle. Isotomurus palustris also seems to have some additional patches of dark pigment as well as the three stripes, whereas Isotomurus plumosus tends not to. Finally, Isotomurus plumosus is described here as being restricted to wet habitats, so habitat description can contribute to an ID too. All that being said, though, being scientifically rigorous, I doubt these differences in pigmentation are truly diagnostic enough to ever be 100% certain of the species' true identity. Therefore, I recommend for these two species putting the iNaturalist ID at genus Isotomurus, then adding a note with 'c.f." and then the species name of choice, to indicate a tentative ID. Having done that, it is then probably the best course of action (provided there are at least two IDs at genus) to have identifiers press 'good as can be' in the data quality assessment (DQA) section of the observation. This will make the observation research grade at genus. To ensure the tentative ID doesn't get lost if the observation data is transformed, it would also be best to add the suggested possible ID as an annotation using the "Possible ID" field.

Hope that helps! :)

Posted on 14 de junho de 2024, 03:38 PM by josscarr