Zooplankton of the Great Lakes

Classification

Kingdom - Animalia
Phylum - Arthropoda
Subphylum - Crustacea
Class - Branchiopoda
Order - Cladocera
Sub-Order - Eucladocera
Family - Bosminidae
Genus - Bosmina
Species - longirostris

Bosmina longirostris was identified by O.F. Muller in 1785.

Taxonomic History

Bosmina are notorious for being a confusing genus. Prior to 1957, all organisms were placed into the genus Bosminopsis (De Melo and Hebert, 1994). Since 1957, this genera has been classified into 4 subgenera: Bosmina, Eubosmina, Neobosmina, and Sinobosmina (De Melo and Hebert, 1994). Species recognized under the genus Bosmina range from 2 to 56 worldwide. There is difficulty in identification of Bosmina due to the seasonal and ontogenetic variation in morphology (De Melo and Hebert, 1994).

Anatomy

When looking for Bosmina, females have the edge over males. In the Great Lakes males vary in size from 0.4-0.5mm and females vary in size from 0.4-0.6mm (Balcer,Korda,Dodson, 1984). There are greater numbers of females than males in populations of Bosmina. Key characteristics to males are notched antennules and modified first legs. Females have antennules that are large and fixed to the head, curving backward parallel to each other. Both males and females have a pointed mucro and vary in length depending on where they are found. Bosmina are surrounded by an armor (shell) used for protection from predation. Another feature which aids in protection is the shape of Bosmina. Bosmina are laterally compressed, with a nearly spherical oblate spheroid body form (Zaret and Kerfoot, 1980). When identifying Bosmina you need to look carefully for the mucro. Eubosmina and Bosmina look very similar except for Eubosmina does not have the mucro.

Distribution/Habitat

Bosmina longirostris are found worldwide in both lakes and ponds, mostly in the littoral areas. Bosmina can be found in inland lakes, varying in phenotypic characteristics. Inland lakes will have different numbers depending on predation and morphometry of the lake. Within the Great Lakes, Bosmina longirostris, have been found for over a hundred years, with the earliest recording being found in Lake Erie in 1880 (Balcer, Korda, Dodson, 1984). During the summer, Bosmina longirostris can be found in abundance of 1,000's to 100,000's m -3 in the Great Lakes. This abundance is due to cool, well oxygenated waters.

Feeding

Bosmina longirostris are filter feeders consuming algae and protozoans ranging from 1-3 micrometers (Balcer,Korda,Dodson, 1984). Bosmina are known to have a dual feeding mechanism. They can filter the water using their second and third legs and the first leg will grab the particles to be or not be consumed. The second and third legs have small setules attached to the seta to make a mesh like structure for filtering. They are also enjoyable meals for a number of fish, especially important in the life cycle of young whitefish (Balcer,Korda,Dodson, 1984).

Swimming Ability

The swimming technique of Bosmina is like that of a human breaststroke. Each stroke can be divided into three contiguous phases. The first phase consists of the setae of the antennae sweeping in a broad arc toward the posterioventral margin of the carapce, providing thrust and tilting of the anterior end dorsally (Zaret and Kerfoot, 1980). The second phase consists of the setae collapsing into a tight bundle, pulling the collapsed setae flush along the lateroventral margin of the carapace and head shield (Zaret and Kerfoot, 1980). The third phase consists of the antennae returning slowly to their resting position, a 45 angle nearly perpendicular to the dorsoventral axis of the body (Zaret and Kerfoot, 1980). Each stroke rate in the three phases varies from 5 to 40 Hz (Zaret and Kerfoot, 1980).

Growth and Reproduction

Bosmina longirostris reproduces by parthenogenesis, asexually. They overwinter as resting eggs and when conditions become favorable rapid reproduction commences. These cladocerans are one of the earliest reproducers, with females carrying 9-12 eggs in early June. The population of these cladocerans will increase seven-fold by mid-July (Balcer,Korda,Dodson, 1984). Reproduction response can be initiated by local temperature and food abundance (Balcer,Korda,Dodson, 1984).

Bosmina longirostris

Bosmina showing overall features, antennae, mucro, and gut.

Bosmina inverted image. Showing difference in size.

Bosmina with egg.

Bosmina longirostris showing a full gut tract.

Works Cited

Balcer, Korda, and Dodson. 1984. Zooplankton of the Great Lakes: A Guide to the Identification and Ecology of the Coomon Crustacean Species, p. 66-69.University of Wisconsin Pres. Madison, Wisconsin.

Melo, R.D., and P.D.N. Hebert. 1994. A taxonomic reevaluation of North American Bosminidae. Canadian Journal of Zoology 72:1808-1825.

Zaret, R., and W.C. Kerfoot. 1980. The shape and swimming techniques of Bosmina longirostris. Limnology and Oceanography 25(1),126-133.