Caecidotea communis (Eastern Waterslaters) and Caecidotea racovitzai ssp. racovitzai (Great Lakes Waterslaters) are native to the north eastern-central US (from NS to at least Minnisota) and have been introduced across the west, and under previous knowlege have only been distinguishable by male genitalia. However, in the process of making my isopod guide for North America, I’ve noticed a somewhat consistent pattern in the head pigmentation between the two. The head of both species has a dark “mask” with lateral wings extending over and behind the eyes and a lobed rear margin, a clear fairly pigment-free region directly behind the mask and a dusky-dark area in the center of the head. In the individuals I’ve confirmed by genitalia or range (C. r. racovitzai isn’t known to occur in far eastern New England, NS and the MA islands), the pattern in the mask differs specifically in the central lobe: in C. communis the central lobe appears to be sharply triangular and often makes a “bridge” to the central area, while in the individuals of C. r. racovitzai I’ve seen the central lobe is much shorter and rounder. It’d be interesting to test if this difference is consistent between the two species, but I don’t have the ability to access specimens or to go out long distances to find more individuals, so I;m putting a call out to iNatters to try to find these weird little waterbugs and try to get good data on their pigment patterns.
The shots needed for this are:
- a clear shot of the head showing the pigment pattern
- a clear shot of the underside of the telson (the last large oval segment) to show the shape of the first and outer branch of the second pleopods (small flaps near the base of the underside) of the males; this in general is really helpful for any member of Caecidotea
- if you’re using a microscope on a dead specimen, you can also tease out the second pleopod of the males out with a disecting probe to see the inner branch (the pointed one which is usually hidden by the first pleopod) and get a good look at the tip of it. This is the feature that’s used to define species and can help a ton to 100% confirm the species
Looking at the features needed for this is probably best done under a disecting scope (with live or dead specimens) but it’s somewhat achievable in the field with a macro lens. The hardest part of trying this in the field is to get a shot of the pleopods that isn’t blocked by the last two sets of walking legs, which can be done with a bit of patience or a small tub of water to get the isopod to relax in (and therefore spread its legs out more). Males are needed to do this, so grabbing a few individuals from a population to increase the chance of finding a male is preferable, but the pleopods of the females are also somewhat unknown between species and could help if co-observed with males too.
The two species are mostly found in the north and west of North America, but if people could also do this in the southeast (where the diversity is much, MUCH higher) it could help with increasing the number of known photos of live or recently-dead individuals of poorly known species.
Caecidotea r. racovitzai with the macro shots needed:
https://www.inaturalist.org/observations/84187745
Caecidotea communis with the external microscope shots needed (without the inner branch of the second pleopod)
https://www.inaturalist.org/observations/65726099
EDIT: If the first set of claws on the isopods (pereopod 1) could be photographed from the side, it could help find the limits of variation seen in that as well