Because you know it’s all about the krill

I am currently at sea  with NOAA-AMLR on their winter oceanographic survey cruise,  also known as the krill cruise.  We are interested  in the entire zooplankton (animals that can swim up and down but  not horizontally against a current) current,  but mostly we are interested in krill.   The data we get from this cruise is used as part of an ecosystem based management strategy for the Antarctic krill fishery.    We are trying to learn all we can about krill and krill associated zooplankton,  but mostly krill.

We are trying to describe population dynamics for krill. Essentially we want to know how the krill population changes in terms of abundance (how much krill there is),  distribution  (where we find krill), and size stratification (what size are the krill and how many of each size class are there), and what is the sex ratio (how many boys and girls are in the population), and finally what is the sexual maturity or stage of the population (how ready are they to spawn and make more krill.

Let me walk you through our process for getting all this information.  Krill form large swarms.  When we catch krill, we tend to catch A LOT of krill.  We can’t possibly count, measure, and sex and stage them all.  So the first thing we do is prepare to sub-sample.  We mix the krill with sea water until all of it reaches a known volume.  In the example below that volume is 32 liters.  Then we  give the krill a good stir,  get it all mixed up  so that the krill are less likely to be segregated by sex or size.  Then we dip in a known volume container and take a random sample.  We’ll take multiple samples to get our numbers high enough.


Next we count every single krill in the random sample.  Since we know what percentage of the total sample it was,  we can multiply our count to estimate the total number of krill caught.

In this tow we counted just over 1,900 krill and estimated that there were 13,680 krill in the total catch.  From the counted krill.  We take another random sample that gets measured, sexed and staged.

A subset of the measured krill  get used  for a number of science projects.  Measuring krill is fairly straight forward.  You measure, in millimeters from the tip of it’s rostrum to the tip of its tail.

The krill above are going too get frozen and stored at  -80C  until scientists in Oregon can study their guts.  This is just one of 6  projects are are collecting measured, sexed and staged krill for this year.

Determining the sex and stage of the krill is a little less intuitive.  Krill don’t develop  observable sexual characteristics until they get larger than about 25 or 26 mm.    To figure out  what sex a krill is you  need to look in two places,  on its abdomen near the gills and on its first swimming leg.

If you put a krill on its back,  female krill will have a thylecum roughly where the square is.  early stage males and juveniles will have nothing in that region.  If you look carefully in the longer rectangle on the picture above you will see the first swimming leg.  In males, this is modified into a petasma- or the male sexual appendage.  Very mature males will also have obvious ejaculatory ducts and sperm packets visible on their abdomen.

In small  females,  the thylecum is transparent and very hard to see!!  During spawning females store sperm packets near their thylecum.

In larger females, the thylecum becomes a bit more opaque  and picks up an organish reddish color.  When the female is ready for spawning, the thylecum will turn a bright, fire engine, red.  This won’t happen until spring,  so I don’t have any photos of that.

The petasma, in males, evolves from a little nubbin to a rather complicated gnarly claw in individuals that are ready to spawn. 

At this time of year we get  2 stages of males, Male 2 and Male 3.  Male 2s are starting to develop their petasma and nothing is visible on their abdomen.  Pictured above is a Male 2b.  His petasma has started  to  split into two distinctive regions, but has not yet developed that claw like look of males ready to spawn.

Above is a fully developed petasma from a male 3.   See how it looks  like a series of hooks?  Pretty gnarly huh?

When a male is ready to spawn it is a  stage 3B.  That means that ejaculatory ducts are visible on its abdomen, they are filled with sperm packets, and the sperm packets can be easy released by gently pushing on the ducts. Spawning starts in spring, and we have caught a number of males who are raring to go.

In the photo above, the you can see the ejaculatory ducts inside the yellow circle.  The duct on the left has had its sperm packet expressed.  The empty duct looks a little like a “t” outline in orange.  The loose sperm packet is in the center of the abdomen between the two ducts.

In the center of the zoomed in photo, the arrow points to the sperm packet.  The empty duct can be seen on the left and a full duct can be seen on the right.

And there you have it.  You are now ready to sex and stage krill.  Maybe you could come help me out so that it doesn’t take me more two hours to goo through 70 krill!  We have two very experienced  female scientists on board who can each work up  150 krill in the time it takes me to do 70!   Krill goals right there,  krill goals.

It looks like were are going to be socked in with krill for a while.  One day I hope to leave zoo lab during daylight so that I can get more scenic photos!