Zooplankton of the Great Lakes

Limnocalanus macrurus

Classification

Kingdom - Animalia
Phylum - Arthropoda
Subphylum - Crustacea
Class - Maxillopoda
Subclass - Copepoda
Order - Calanoida
Family - Centropagidae
Genus - Limnocalanus
Specific epithet - macrurus

Limnocalanus macrurus was first described by G. O. Sars in 1863. It was joined in 1968 by L. grimaldi DeGuerne 1886, which was reported to simply be a subspecies of L. macrurus. Taxonomists decided that the morphological differences between the two subspecies was simply determined by the length of time that individual populations had been separated from the ocean.

Anatomy

Adult (CVI) copepodids of Limnocalanus macrurus range in length from 2.4 - 2.9 mm, with males being slightly smaller than females (Balcer et al. 1984; Czaika 1982). There is a simple median eye and the mouthparts, especially the maxillipeds, are enlarged. Both sexes have elongate caudal rami with five terminal setae (Balcer et al. 1984) (Fig.1). There are few distinguishing sexual characteristics (Fig. 2). Females have a three-segmented urosome with an enlarged genital segment, but they do not carry egg sacs (Carter 1969)(Fig. 3). Males have a five-segmented urosome and a geniculated right first antenna. Unlike many other calanoid copepods, males of this species do not have an enlarged fifth swimming leg for use in mating (Fig. 4).

Distribution

Limnocalanus macrurus is a glacial relict species. It evolved as a marine organism, but now lives in both marine and fresh water environments. In North America it occurs in the Great Lakes, several lakes in Canada, and a few lakes near the west coast (areas once inundated by the ocean). It has a circumpolar distribution, and has been reported in Russia, Norway, Sweden, Poland, Baltic and Caspian Seas, and the Arctic, as well as North America (Balcer et al. 1984).

L. macrurus has been reported in all of the Great Lakes, and average densities range from 600 - 4000 m-3. In Lake Superior, average density is 33 m-3, but densities have been reported in patches of up to 100,000 m-3. L. macrurus is almost absent from western Lake Erie and Southern Lake Michigan. This is thought to be caused by warmer water temperatures, low dissolved oxygen content, or predation by fishes. Carter (1969) reported that L. macrurus was the most abundant deepwater copepod in Parry Sound, Georgian Bay, Lake Huron.

Habitat

Limnocalanus macrurus is a cold-water stenotherm that is usually restricted to the hypolimnion of large, cold, northern lakes. It is rarely found in water temperatures greater than 14 C or with dissolved oxygen content less than 5.1 mgl-1, so it is believed to be a good indicator of eutrophication and pollution (Balcer et al. 1984).

Carter (1969) reported that more mature individuals of L. macrurus occurred lower in the water column than younger stages. As water temperature increased in the summer, age class stratification stayed the same while the entire population moved down in the water column. Under isothermal conditions, L. macrurus begin to move up in the water column at midday, and the first individuals reach the surface at sunset. By midnight, 25% of the population is at the surface, and they start descending at sunrise. When lakes are stratified, the same diurnal migration pattern takes place, but 75% of the population does not swim above the thermocline (Balcer et al. 1984). Their daytime upper temperature limit is approximately 7 oC (Carter 1969).

Feeding Ecology

Limnocalanus macrurus is an omnivorous filter feeder, that feeds on particles from 4 - 24 Ám in size. Their main diet consists of diatoms (Melosira and Asterionella), chrysophytes (Dinobryon), and nannoplankton. Prey items include cladocerans, other copepods, and L. macrurus nauplii (Balcer et al. 1984). L. macrurus was originally thought to be a strict herbivore, but Bowers and Warren (1977) reported carnivorous behavior of the adult copepodids in Lake Michigan. They observed adult L. macrurus preying on Diaptomous ashlandi, and found that L. macrurus exhibited searching behavior when doing so. Warren (1983) reported that predation by L. macrurus was possibly a major source of winter Diaptomous spp. nauplii mortality.

L. macrurus is a very nutritious food for fishes as it has high organic matter and protein contents, as well as low chitin and crude fiber contents. It has been reported in the stomachs of alewife, cisco, emerald shiner, whitefish, herring, and brook silverside. Heavy predation by fishes may have affected the abundance of L. macrurus in Lake Michigan (Balcer et al. 1984).

Life History

Reproduction in Limnocalanus macrurus is sexual, and this species produces one generation every year. Female L. macrurus do not carry eggs sacs, as many other species of copepods do. Mating occurs in the fall when males of the species attach spermatophores to the genital segment of the female. After fertilization, the female releases 10-20 eggs directly into the water. The eggs sink to the bottom, sticking to detritus particles, and hatch after a resting period of varying length (Carter 1969).

The development and hatching of the eggs is temperature dependent, and can take 17 - 38 days. After hatching, the first naupliar stage (N1) remains near the bottom and is difficult to collect with sampling gear. The nauplii grow slowly over the winter and molt to the copepodid form during the plankton bloom in the spring when food is abundant. L. macrurus grow from egg to adult in 6 - 8 months, and the adults live for approximately another 6 months (Balcer et al. 1984).

Figure 1. Limnocalanus macrurus. Note the elongate caudal rami and enlarged maxillipeds.

Figure 2. Dorsal view of male and female Limnocalanus with distinguishing sexual characteristics circled in red.

Figure 3. Female L. macrurus with an enlarged genital segment.

Figure 4. Male L. macrurus with geniculated right 1st antenna circled in red. Note the absence of an enlarged 5th swimming leg common to many male copepods.

Literature cited

Balcer, M. D., N. L. Korda, and S. I. Dodson. 1984. Zooplankton of the Great Lakes. University of Wisconsin Press, Madison, WI.

Bowers , J. A., and G. J. Warren. 1977. Predaceous feeding by Limnocalanus macrurus and Diaptomous ashlandi. J. Great Lakes Res. 3(3):234-237.

Carter, J. C. H. 1969. Life cycles of Limnocalanus macrurus and Senecella calanoides, and seasonal abundance and vertical distribution of various planktonic copepods in Parry Sound, Georgian Bay. J. Fish. Res. Board Can. 26:2543-2560.

Czaika, S. C. 1982. Identification of nauplii N1-N6 and copepodids CI-CIV of the Great Lakes calanoid and cyclopoid copepods (Calanoida, Cyclopoida, Copepoda). J. Great Lakes Res. 8(3):439-469.

Warren, G. J. 1983. Predation by Limnocalanus as a potentially major source of winter naupliar mortality in Lake Michigan. J. Great Lakes Res. 9(3):389-395.