I have read that females of Asellus aquaticus develop their marsupium 45-90 days (depending on conditions) after hatching, so females that are the same age may be in different stages of their sexual development. Hence I am looking for another visual (no dissection) method to determine the sex of mature Asellus aquaticus.
Related to that: Is there a way to tell whether male A. aquaticus are sexually mature?
It has been a long time since I sexed them for my PhD but as I recollect, the body shape and width of the thorax. In mature females, even without eggs, the upper thorax was always wider; males always tended to narrow towards the head. It took a while to recognize the difference reliably but after a while it was quite accurate.
Hey Moritz! Next to the habitus shape, safe-to-use morphological differences relatively easy to see with a stereo microscope are to be found in the second pleopods, which are modified in the male for copulation and simply leaf-shaped in females, as well as the palm of the first pereopod claw which differs quite a bit between the sexes: it is stronger in the males with the propodus broader and more angular on the ventral side. Check out the following paper which has very detailed illustrations:
Verovnik, R., Prevorcnik, S. & Jugovic, J. (2009) Description of a neotype for Asellus aquaticus Linne´, 1758 (Crustacea: Isopoda: Asellidae), with description of a new subterranean Asellus species from Europe. Zoologischer Anzeiger, 248, 101–118.
Hi Moritz,
As Torben has previously stated you can happily use morphological differences to separate sexes. Males have a pair of backward pointing copulatory styles on the ventral surface at the hind end of the thorax, whilst this is absent in females. If you have access to the following publication there is a good figure depicting this (pg. 70): Gledhill T, Sutcliffe DW & Williams WD (1993). British Freshwater Crustacea Malacostraca: A Key with Ecological Notes, FBA SP No. 52.
Kind regards - Drew.
Thanks everybody, eye-baling bodyshape and checking the second pleopods for specimens where I was unsure worked quite well.
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