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The Road to Adulthood: Maturation of Juvenile Krill

Updated: Apr 11, 2020

Similar to us, there are certain obstacles krill must face when venturing out into the world on the path to adulthood. But instead of learning how to file their taxes, young krill are forced to learn how to adapt to an ever-changing environment if they want to survive.

As the Western Antarctic Peninsula (WAP) warms at a rapid rate, the life history patterns of Antarctic krill are susceptible to change. Antarctic krill spawn in the summer time, when there is sufficient food and light available in the environment to support reproduction. Prior to summer spawning, krill must develop their reproductive organs in the late winter and early spring. Krill typically spawn multiple times throughout the summer season, with longer spawning seasons resulting in higher krill recruitment rates and larger population abundances. The length of the spawning season can be influenced by several factors including food and light availability during the reproductive season and the maturation rate leading up to the reproductive season.

In the WAP, krill like to eat diatoms, which are photosynthetic microorganisms that live in the water and on the ice. However, as the ice retreats more each year, diatom abundances are decreasing, and young krill will likely have to modify their diet in order to survive the food limited winters. Instead of living the vegetarian life, feeding primarily on diatoms, krill may be adopting a more omnivorous lifestyle, incorporating more zooplankton, or microscopic animals, into their diet throughout the winter. I want to know how this climate-induced variation in diet throughout the reproductive development season (winter-spring) affects the maturation rate of juvenile Antarctic krill and what implications that might have for summer spawning in the WAP.

For six months, we fed our krill a diet of either diatoms or copepods (a common type of zooplankton) in an effort to understand how these different prey items affect the physiological growth of Antarctic krill. Each month (May-September), I subsampled our dietary treatment tanks to examine the progression of reproductive organ growth in individual krill when exposed to different types of food. After extracting the krill from their tanks, I measured the length of the krill, took their weight and preserved them in 15 mL falcon tubes for subsequent sexing and reproductive staging analyses.

The first step is to determine if the krill is a male or a female. Using a dissecting microscope, I can examine the external features to figure out what sex the krill is. When maturing, male krill will develop a petasma. These petasmas are located on their first pleopod (swimming leg) and are the male krill’s sexual organ, used to insert sperm packets into the female when spawning. Female krill can be identified by their thelycum. These thelycums are located on the ventral (stomach) side of the krill, behind their gills. Thelycums are the female sexual organ and are what male krill insert their sperm packets into when spawning.

Top: Different stages of a developing petasma for a subadult male Antarctic krill. Taken from Makarov and Denys, 1981.

Bottom: An incomplete (left) and complete (right) thelycum for a female Antarctic krill. Taken from Cuzin-Roudy and Amsler, 1991.

Male krill can be staged based off of the size and structure of their petasma. Similarly, female krill can be staged based off of the external morphology of their thelycum. However, external staging methods are often unable to detect subtle changes in reproductive development. That’s why I decided to stage the female krill using internal staging techniques. By dissecting and extracting the ovaries of female krill, I can look at them under a compound microscope and stage the krill based off of their cellular development. This technique will allow me to accurately assess the relationship between diet and reproductive development.

A key for staging the ovarian development of female Antarctic krill.

I have dissected over six hundred krill so far, and I still have more to dissect! With the North bound ship heading out in less than three weeks, I am working as hard as I can to get these dissections done before departing Palmer Station. Studying the reproductive development of juvenile Antarctic krill has been really interesting and I’m excited to see the results of this experiment. Understanding how diet impacts the reproductive cycle of Antarctic krill will allow us to predict how population dynamics will vary in response to this ecosystem change in food availability.

Taking pictures of the ovarian cells under the microscope.

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