is anybody familiar with species identification of Euphausiidae shrimps, also known as krill? A co-worker (W. Langbroek) collected a specimen in a shallow marine bay along our Dutch coast at Katwijk (52°12'39.74"N , 4°24'0.41"E on 24.iii.2018) and I only have the key by Mauchline (1984). In The Netherland, 2 species have ever been reported i.e. Meganyctiphanes norvegica and Nyctiphanes couchi. My specimen is a bit small (about 9 mm) so perhaps not all features characteristic for those species are fully developed. As lateral denticles are missing it seems no M. norvegica, but also a comb-like structure on antenna 2 is missing characteristic for N.couchi. However, I think that it may still be a juvenile specimen of the latter for it has 7 pairs of legs with the last one very much reduced, eyes are undivided, no lateral denticles on the carapax and with reflected lappet on the first antennal segment. Do you agree? Or do you have another suggestion? Are there any other keys for Euphausiid crustaceans which can be used for the NE-Atlantic?
many thanks in advance
Ton
Hi Ton,
Good photographs. We've run it through Mauchline and also we have a copy of Castellani & Edwards, I've attached a scan of the key to genera from that in case it is useful.
We keyed it out to either M. norvegica or N. couchi. Other than the size, it looks more like M. norvegica and has quite a robust lappet which is really small on N. couchi. Also as you point out it is lacking the comb structure, but I don't know if that is present on furcilia stages. As far as we know the carapace denticle should be present on M. norvegica furcilia, have you tried pulling the carapace off and looking at it on a compound microscope? Denticles can be difficult to spot sometimes in fixed specimens. If that reveals a denticle we think it is probably a M. norvegica furcilia, if not we don't know!
Best wishes
Malcolm
Thanks Malcolm. I have studied the carapax with a compund microscope (without dissecting, i do not dare for it is the only specimen) but I could not find anything looking like a spine. What is also bothering me is the lateral spine of the first antennal segment (at the same level of the lappet), which is not depicted in Mauchline. i agree that the size of the lappet is more like norvegica.
I think the lateral spines are OK - they are present in this figure of late furcilia for M. norvegica which I found in this key: http://species-identification.org/species.php?species_group=euphausiids&id=47&menuentry=soorten
I also looked in a copy of Boltovskoy South Atlantic Zooplankton in case this was something exotic that shouldn't be there but nothing matched what you have.
Can you check the telson to see if it is a furcilia or a juvenile?
A juvenile, no terminal spines whatsoever, just a simple telson, although a bit more rounded as in the drawing. I looked at http://species-identification.org/species.php?species_group=euphausiids&menuentry=soorten&id=60&tab=multimedia and saw that the lateral spine is shorter in N. couchi and that the comb on the second antennal segment may be absent.
Still not sure though: on the Marine species ID portal and in Holt & Tattersall (1905) (https://www.biodiversitylibrary.org/pdf4/076604900096254.pdf untill 29 april 2018) both species are separated by the presence or absence of a middorsal spine on the last abdominal segment. As there is a middorsal spine (although not quit large yet) this is supposedly N. couchi. And there is also the matter of the shallow habitat, which is unlike M. norvegica.
Hmm if it is a juvenile then I think it is probably too small to be M. norvegica. I think with no denticle, and a spine on the last abdominal segment that is pushing it more towards N. couchi, I see the lappet can be a bit larger in males as well.
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