Zooplankton of the Great Lakes

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Copepod: Diacyclops thomasi


Kingdom – Animalia

Phylum – Arthropoda

Subphylum – Crustacea

Class – Maxillopoda

Subclass – Copepoda

Order – Cyclopoida

Family – Cyclopidae

Genus – Diacyclops

Species – thomasi

Diacyclops thomasi was first described by Forbes in 1882 as Cyclops thomasi until the American form was determined to be Cyclops bicuspidatus thomasi and was placed in subgenus Diacyclops which was later elevated to genus.  Diacyclops bicuspidatus thomasi was determined to be a temporary pond species and Diacyclops thomasi was then used to refer to the form found in permanent lakes (Balcer et al., 1984).



     Diacyclops thomasi has several characteristics that are shared with all cyclopoid copepods (Fig. 1).  The antennae are much shorter than the other copepod orders and the mouth parts consist of paired mandibles, maxillules, and maxillae.  Five pairs of swimming legs are present with the fifth pair greatly reduced.  The male first antennae are both geniculate (Fig. 2; Balcer et al., 1984).

     A unique combination of characteristics can be used to distinguish D. thomasi from other cyclopoids.  The caudal rami of D. thomasi are at least three times as long as wide with no fine hairs on the inner margin (Fig. 3).  The inner setae is less than half the length of the rami and not thicker than the outer seta.  D. thomasi has a lateral seta on the outer margin of the rami that is located in the posterior third to middle of the ramus.  The first antennae are longer than the cephalic segment but do not extend to the genital segment (Balcer et al., 1984).



     Diacyclops thomasi is thought to be a very common copepod in North America and can be described as widely dispersed (Balcer et al., 1984).  A possible reason for this is due to the patterns of water movement associated with the advance and retreat of continental glaciers (Stemberger, 1995).  D. thomasi has also been observed to be the dominant copepod species in some river systems (Jeffery & Thorp, 2002)



     Diacyclops thomasi uses specially adapted appendages  in order to physically grab its food items in a raptorial feeding behavior. (Balcer et al., 1984)  D. thomasi was found to ingest some types of dinoflagellates , ciliates, and protozoans in Lake Erie.  Some evidence exists that D. thomasi will eat eggs of loricate rotifers without consuming the female bearing the eggs (LeBlanc et al., 1997).    D. thomasi selected for several small rotifers without a lorica.  D. thomasi did begin to ingest less desirable rotifers when the density of the rotifer populations greatly favored the loricate Keratella species; however, the clearance rate was much lower than the clearance rate for the more desirable aloricate species of rotifers (Stemberger, 1985). 



     D. thomasi males have a shorter lifespan and tend to be less abundant than females which produce 10 to 40 eggs per clutch that take from 28 to 35 days to develop (Balcer et al., 1984).  D. thomasi hatches from egg to a nauplius stage (Balcer et al., 1984) then goes through five immature instars of copepodids and then an adult stage (Reed & Aronson, 1989).  In some lakes D. thomasi copepodids were found to arrest development during times of low algal densities (Hansen & Hairston, 1998).  In other lakes, the density of algal food does not decrease significantly enough to cause D. thomasi to arrest development causing it to be one of the main prey items for other invertebrate predators during times when other zooplankton are in diapause (Spencer et al., 1999)

Figure 4.  Female Diacyclops thomasi with eggs.

Figure 1.  Female Diacyclops thomasi (dorsal view).

Figure 2.  Male Diacyclops thomasi with geniculate antennae.

Figure3.  Diacyclops thomasi. (A) Rami at least 3 times as long as wide. (B) Lateral setae located distal half to distal 1/3 of rami. (C) Inner setae shorter than half the length of the rami.


Works Cited:

Balcer, Mary D., Nancy L. Korda, & Stanley I. Dodson.  1984.  Zooplankton of the Great Lakes: A guide to the identification and ecology of the common Crustacean species.  Madison: The University of Wisconsin Press. 

Hansen, Anne-Mette, & Nelson G. Hairston Jr.  1998.  Food limitation in a wild cyclopoid copepod population: direct and indirect life history responses.  Oecologia.  115:320-330

Jeffery, D. Jack & James H. Thorp.  2002.  Impacts of fish predation on an Ohio river zooplankton community.  Journal of Plankton Research.  24(2): 119-127

LeBlanc, J.S., W.D. Taylor, O.E. Johannsson.  1997.  The feeding ecology of the cyclopoid copepod Diacyclops thomasi in Lake Ontario.  Journal of Great Lakes Research.  23(3): 369-381

Reed, Edward B. & John G. Aronson.  1989.  Seasonal variation in length of copepodids and adults of Diacyclops thomasi (Forbes) in two Colorado montane reservoirs (Copepoda).  Journal of Crustacean Biology.

Spencer, Craig N., David S. Potter, Robert T. Bukantis & Jack A. Stanford.  1999.  Impact of predation by Mysis relicta on zooplankton in Flathead lake, Montana, USA.  Journal of Plankton Research.  21(1): 51-64

Stemberger, Richard S.  1985.  Prey selection by the copepod Diacyclops thomasi.  Oecologia. 65: 492-497

Stemberger, Richard S. 1995.  Pleistocene refuge areas and postglacial dispersal of copepods of the northeastern United States.  Canadian Journal of Fisheries and Aquatic Sciences.  52(10):2197-2210