Counting Copepod Eggs

A Calanus pacificus copepod (top right) escapes the grasp of a large ctenophore (photo credit: Mike Stukel)

One of the experiments I am working on during our cruise is measuring the reproductive health of a copepod species, Calanus pacificus, which is one of the dominant zooplankton in our local California Current waters. Copepods are small crustacean zooplankton, which means they have segmented bodies that undergo molting like crabs and lobsters do. They are also very numerous in the ocean (over 13,000 species have been identified!) and generally dominate zooplankton communities.

Calanus pacificus eggs (small white dots).

One overarching question of our CCE-LTER research is how much zooplankton populations change from year to year and between different areas in the California Current (in this case, nearshore newly-upwelled water versus older water farther offshore). One way to quantify zooplankton in the ocean is to measure their biomass, or the total amounts of various species within a sample. Our Bongo and MOCNESS net tows help answer this. A second way to determine zooplankton health and viability is to measure reproductive success. To answer this, I am measuring egg production rates in Calanus pacificus, or the number of eggs that a female copepod produces in a 24 hour period. I also count how many of those eggs successfully hatch into copepod nauplii (larvae).

Egg production rate experiments use a special Bongo net with sealed collecting jars to keep animals alive as we haul them up. We then sort through the samples for mature female Calanus pacificus, placing them in dishes of seawater and incubating them in a cool, dark chamber overnight to mimic in situ ocean conditions. I check the dishes the next day to see if the females have laid eggs, and if they have I incubate the eggs for another day to see if they hatch.

Calanus pacificus nauplii (the newly-hatched stage).

Copepod egg production during this cruise has been quite interesting, likely reflecting the influence of the filament and its evolution over time. During the first two cycles of the cruise (close to shore, in the newly-upwelled filament waters), the copepods were cranking out large numbers of eggs (including one record high of 109!), sometimes laying two clutches a day. The third cycle showed much more variability, perhaps due to older, dying phytoplankton (food) availability, and the fourth cycle showed healthy female copepods but almost zero egg-laying. Further analysis, including quantifying how many nauplii hatched from the eggs, will tell us even more about how reproductively successful female copepods are this year and in relation to the filament.

We have officially ended Cycle 4 (our final cycle of the cruise) and are on to a three-day survey of the filament using our trusty SeaSoar. It will be interesting to see how the filament has evolved since we first surveyed it three weeks ago. While we’re at it, a few more days of sunshine and blue water!

Mark educates about copepods to a microscoping crowd in the main lab


Posted by: Laura Lilly, SIO

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