To date there have been about 2000 species described in the copepod order Calanoida, which is probably just a fraction of what is left to be discovered. Calanoid copepods are characterized by their long antennae, which are at least half the length of their torpedo shaped body. Typically, calanoids have a body length of 0.5 to 2.0mm, however species as large as 18.0mm and as small as 0.25mm have been measured. Compared to other copepod orders, the overall morphology of calanoids is uniform; a calanoid will generally look like another calanoid.
Unlike the other copepod orders which lack circulatory systems, calanoids possess a heart, but just the heart, not the complex pathways of the other vessels one typically associates with a whole circulatory system. Having a heart may assist calanoids in their active lifestyles.
Calanoids are pelagic for the most part, spending their lives in the water column. Roughly 75% of calanoid species are marine, while the remaining 25% are found in fresh water. Being mainly herbivorous, calanoid copepods are extremely important in marine environments, providing a bridge between phytoplankton and fish and even whales.
Many calanoid species make a daily journey called diurnal vertical migration or DVM, in which they ascend to the water’s surface at night to feed on phytoplankton and descend again with the rising of the sun. By migrating to the surface at night, calanoids are able to effectively avoid being eaten by predators which rely on the sun’s light to see, but the depths to which they return aren’t without predators. Many calanoid species deal with these predators in the deep with bioluminescence.
Light produced by organisms is called bioluminescence. The light is made by a class of enzymes called luciferases which catalyze a reaction between a compound called luciferin and oxygen. An example of bioluminescence that most of us are probably familiar with is the light produced by fireflies on summer evenings. However, there are many other organisms besides the firefly that are capable of bioluminescence; within the ocean it is actually quite common, being displayed by fish, crustaceans, jellyfish, cephalopods and dinoflagellates.
Within the copepods, most bioluminescent species belong to the order Calanoida. Bioluminescent calanoids are different from other light producing organisms in that they have glands which produce and shoot the light producing substances from their bodies to confuse and distract potential predators. This adaptation allows these calanoid species to protect themselves in depths filled with predators where they spend the day before ascending to the surface at night to feed.
Copepods of the Heterorhabdus genus are slower swimmers and live in low densities at deeper, darker depths than other calanoids, and are characterized by weak bioluminescence. The Heterorhabdus species are carnivorous, having made the switch from suspension feeding which is common in other copepods. Being carnivorous, they have adapted to this lifestyle by evolving a specialized tooth with which they are able to inject venom into their prey, reducing their need for movement while hunting. Because ofHeterorhabdus’ low amount of movement, motion-dependant predators do not eat them as often and thus there is less evolutionary selection towards having strong bioluminescence to bewilder these predators compared to their more active kin.
Well, that’s it for now, hopefully you learned a thing or two about the Calanoida, which are just one of the many copepod groups, each displaying their own unique adaptations to be discovered and discussed.
Takenaka, Y., Yamaguchi, A., Tsuruoka, N., Torimura, N., Gojobori, T., Shigeri, Y., 2012. Evolution of bioluminescense in marine planktonic copepods.Molecular Biology and Evolution. 29(6):1669-1681
Mauchline, J., 1998. The Biology of Calanoid Copepods. Advances in Marine Biology. 33
Eduiaptomus Vulgaris.jpg By Tigerente (Berger Harry) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons