T. Mac and Friends

We have entered the part of the cruise that the Chief Scientist refers to as “mowing the lawn”.  We are in a region where the sampling stations are relatively close together and there is very little  Antarctic krill (Euphuasia superba).  Instead we get small euphausiids (relatives of krill). The catches in this region are often dominated (in terms of number and biomass) by the charismatic small euphausiid  known as T. mac  (Thysanoessa macrura).

I love T. mac.   T mac is the absolute best!  It is the only euphausiid we get that has bi-lobed (kidney bean shaped) eyes.  That means that we can pick them out of the catch without needing a microscope.  There is no way that we could confuse T. mac with another critter.  T. mac is so courteous that even after it loses its eyeballs, we can still identify it at a glance.

T. mac has a  “racing stripe”,  a solid stripe of  bright red from the tip of their head to the tip of their tails. No other euphausiid has such a cool stripe.  T. Mac also has a ginormous leg that only critters in the genus Thysanoessa have


Considering how  abundant T. mac are,  and how easy they are to identify   you would think that heaps of people study them.  But you would be wrong.  These little shrimp like guys are everywhere,  but no one reports them in diet studies.   Seriously,  stuff must eat T. mac.  They look like they would be a great snack- or you know if you snack on them all day you might not need dinner.  But sadly when folks find pink mush in something’s stomach down here they  call it   “krill” and  poor T. mac’s ecological role remains undescribed

Some people get distracted by T. mac’s flashy cousin  Euphausia triacantha,  which I must admit is gorgeous  and looks a bit like a krill made from sugar.

Triacantha has a lovely  read spot that makes the adults easy to identify.  We find them to be  so pretty that we often make neat rows of triacantha in our sorting dishes.  Triancantha’s beauty demands that sort of thing.  Every other species just gets counted and piled. But triancatha is ordered and appreciated.

But triacantha has round eyes,  and little triacantha may not have an obvious spot.  Actually little triacantha can be easily confused with other round eyed euphausiids without the aid of the microscope.  Under the scope we are looking mostly at differences in the rostrum, the pointy bit above the eyes.   Here let me show you.


Ok on the left we have Euphausia frigida another small euphausiid that we don’t get in large numbers.  It has a rather blunt rostrum.  There is no pointy bit that goes between its eyes.  Next to frigida is T. mac. You can see the bilobed eyes and racing stripe.  You’ll notice that its rostrum has a crease down the middle,  and extends to the midline of those enormous eyes.  Next to T. mac is Euphausia superba (krill).  Krills rostrum is  intermediate between frigida and T. mac, and doesn’t have an obvious crease.  Finally on the right is triacantha.  You can see the red spot.  The pointy bit of the rostrum extends further than krill’s, there is an obvious crease,  it has a different overall shape.   Yes these are subtle differences.  I need a reference picture in front of me every time I ‘scope out small euphausiids.  It takes me forever…which is why I love T. mac so much (hardly any scoping required).

Well this has been fun,  but now I must get back to counting a boatload (actually an accurate term in this instance) of small euphausiids