Stefan,

I would recommend looking at the setation and spination on the P4, particularly on the coupler and coxa, as the species show some pretty clear differences here. It is difficult to describe in words, but if you have a copy of Einsle's 1996 guide, these differences are clear. However, for a quick ID, if you look at the terminal segment of the P4 endopod, this should have 3 spines and 2 seta on A. robustus, but 2 spines and 3 seta on M. viridis. Of course I am making the assumption that you really do have these two species, and not other species of Megacyclops or Acanthocyclops!

Ian,

thanks a lot for the answer.The features are right as you described matching the A. robustus. I will look for Einsle's guide, now I am using a copy of  Rybak & Bledzki 2005 guide and the only thing that confuse me is the rounded shape of the first segment of the cephalotorax which is drown rounded for the Megacyclops, while in Acanthocyclops is drouwn relatively conical. Anyway i suppose as it is not described as determinative feature it could vary.

The speciment have A1=17 segments, but not 12 like A. venustus(which have similar terminal segment of P4 endpodit),  the basipodite of P4 have pair of well expressed spines - one placed on the inner side of the endopodite and second between the endo and exopodite, Also the angle between the two setae of P5 is very sharp, thеy are almost parallel and not very long if the last have matter.

Thanks again!

Stefan,

the shape of the cephalotorax is not a reliable character to identify these species. As suggested by Ian, you should focus on legs and furcal branchs ornementation.

• Megacyclops viridis : First article of P5 wider than long. Subapical spinula at second article not extended beyond the end of the article. Ti longer than the furcal branch. Last article of enp.P4 with 2 apical spines, shorter than the article.
• Acanthocyclops robustus : First article of P5 as long as wide. Subapical spinula at second article extended beyond the end of the article. Ti shorter than the furcal branch. Last article of enp.P4 with 3 apical spines, shorter than the article.

Note that the taxonomy of Acanthocyclops robustus is not up to date in both Rybak & Bledzki 2005 and Einsle’s 1996 guides. Most of the time, specimens identified as A. robustus are in fact A. trajani.

See :

• Mirabdullayev, I. M. & D. Defaye, 2002. On the taxonomy of Acanthocyclops robustus species complex (Copepoda, Cyclopidae). 1. Acanthocyclops robustus (G.O. Sars, 1863) and Acanthocyclops trajani n. sp., Selevinia 1-4: 7-19.
• Mirabdullayev, I. M. & D. Defaye, 2004. On the taxonomy of the Acanthocyclops robustus species complex (Copepoda, Cyclopidae): Acanthocyclops brevispinosus and A. einslei sp. n., Vestnik Zoologii 38: 27-37.

Frédéric, thank you for the guidance.