Zooplankton of the Great Lakes

Skistodiaptomus oregonensis(Lilljeborg 1889)

Overview of the Skistodiaptomus oregonensis

Classification

Phylum - Arthropoda

Subphylum - Crustacea

Class - Maxillopoda

Subclass - Copepoda

Order - Calanoida

Family - Diaptomidae

Genus - Skistodiaptomus

Species - Oregonensis

 

In the Great Lakes zooplankton literature, this sepcies was consistently referred to as Diaptomus oregonensis until the subgenus name skisodiaptomus was elevated to genetric level.

 

Anatomy picture of Skistodiaptomus oregonensis

Skistodiaptomus oregonensis, female

 

Anatomy

In lake Superior the females range from 1.2-1.4mm and average 1.33mm. Males were somewhat smaller, ranging from 1.1-1.3mm and averaging 1.19mm.

Adult females of Skistodiaptomus oregonensis have three-segmented urosomes and small rounded metasomal wings with minute sensilla. The genital segment is only slightly expanded and the two lateral setae on the terminal claw of leg 5 do not reach the middle of the claw.

The right antennule of adult male calanoids is geniculate and may possess a lateral projection on the antepenultimate segment. Lateral spine on terminal segment of exopode of right 5th leg is subterminal in position.

 

Skistodiaptomus oregonensis, male

Female metasomal wing

Male metasomal wing

Lateral spine on terminal segment of exopode of right 5th leg

Life History

S.oregonensis is present year round in the Great Lakes(Jahoda 1948; Carter 1969;Wilson and Roff 1973; Gannon 1975). It is most abundant in the summer and fall but peaks have been observed from April to December.Gannon(1972a,1974) reported abundance peaks as early as February in Green Bay.

This species produces two generations each year in Lake Michigan. Adults overwinter and produce a slow-growing spring generation that coincides with the phytoplankton maximum. This generation reproduces in the summer, and the offspring grow rapidly, maturing by fall overturn. Most animals wait until spring to reproduce although Gannon(1972a) reported some winter reproduction in Green Bay. In Lake Superior, where abundance is low, S. oregonensis only produces one generation each year and overwinters as resting eggs (Selgeby 1974)

Sex rations of 1:1 are most common (Jahoda 1984;Davis 1962,1968; Torke 1975), but females may outnumber males (Stewart 1974). Clutch size appears to be positively correlated with size of the females( Davis 1961)

Reproduction and Growth

Reproduction in the S.oregonensis is sexual. The male clasps the female with his modified first antennae and/or fifth legs, then transfers a packet of sperm, the spermatophore, from his genital pore to her genital segment. the sperms are stored in a seminal receptacle located in the female's genital segment. When the female releases eggs from her genital tract, they are fertilized by the stored sperm. Female brood the eggs in one egg sac attached to the genital segment. After the first clutch of eggs has hatched, some females fertilize a second and third clutch utilizing more of the sperm stored from their first mating.

Its egg hatch into small, active larva known as nauplii. The first nauplius stage (NI) is characterized by three pairs of appendages. The animals grow rapidly and molt to the second nauplius stage (NII), which is slightly longer. The animals continue to grow and add appendages as they pass through six naupliar stages. The next molt is to the first copepodid stage(CI). At this period the young species have the general body shape of the adult but are smaller and lack several of the swimming legs. Growth, addition of swimming legs, and modification of the limbs continue at each molt until the adult(CVI)stage is reached. The animals then mate and produce the next generation

 

                

nauplii stage of S.oregonensis

 

Distribution

Skistodiaptomus oregonensis is one of the most widely distributed diaptomid species, occuring throughout the northern United States and southern Ontario, Canada( Marsh 1893, 1929; Carl 1940).  This species has been reported in all the Great Lakes, especially abundant in the warmer, southerly portions of the Great Lakes and is of decreasing importance in the cooler, northerly regions( Robertson 1966). Now this species is rare in Lake Superior(Selgeby, 1975) and Lake Ontario, only appearing in fall and most common in Lake Erie (Davis 1961, Wright 1955) which peaks in abundance occur in September.

Habitat

S.oregonensis always inhabits permanent water bodies, but occurs in lakes of  different morphometric and tropic characteristics. It is common in Lake Michigan, but also occurs in quite shallow inland lakes and ponds. It frequently occurs with other calanoids, Leptodiaptomus minutusis, L.ashlandi and L.sicilis, and occasionally with S.pallidus.

S. oregonensis is more abundant offshore than in the littoral zone (Stewart 1974). During periods of thermal stratification, this species is usually found in the meta- and epilimnia, migrating toward the surface at night, but most densely concentrated near the thermocline during the day (Juday, 1903; Langford, 1938; Wells 1960). In unstratified water masses and upwelling areas, distribution throughout the water column is more uniform ( Wilson and Roff 1973). In Teapot Lake, Ontario, Cooley(1970) found that day-night differences in mean depth distribution were not significant for naupliar stages, but were significant for copepodid and adult stage, although these day-night distribution means only involved a depth difference of 1m or less, and some animals apparently did not migrate. Birge(1895) observed no vertical movement for this species in Lake Mendota, Dane Co. Diurnal migratory behavior is apparently weakly developed in this species, and the extent of migration may be influenced by age and individual behavior, and perhaps by temperature, light level, time of year and thermocline depth

 

Food and Feeding Behavior

After S. oregonensis molts to the first copepodid stage, it can filter feed on the same size class of particles( 100-800Ám│) as the adults(McQueen, 1970; Comita and McNett 1976).The copepod prefers larger phytoplankon(McQueen, 1970). Richman(1966) showed that S. oregonensis exhibited it maximum ingestion rate in food concentrations of 25 000-50 000 cells ml‾╣, ingestion rate declined dramatically. Skistodiaptomus oregonensis may be adaped to feeding on large cells in  high concentration, and would be at a feedin disadvange in lakes of very low productivity.

S.oregonensis has been found in the stomach of severl species of fish including ass, crappie, carp, suckers, freshwater drum, trout-perch, yellow perch, and whitefish(Ewers 1933; Wilson 1960). Comparing only a small percentage of the diet of some fish, it was found to be an important food item for small bloater( Wells and Beeton 1963).

 

Work Cited

Balcer, Korda, and Dodson. 1984. Zooplankton of the Great Lakes: General Morphology and Ecology of the Crustacean Zooplankton, p.8-11.University of Wisconsin Pres. Madison, Wisconsin.

Balcer, Korda, and Dodson. 1984. Zooplankton of the Great Lakes: Life History and Ecology of the Major Crustacean Species, p.91-93.University of Wisconsin Pres. Madison, Wisconsin.

Byron Torke. 2001. The distribution of calanoid copepods in the plankton of Wisconsin Lakes. Hydrobiologia 453/454: 351-365, 2001.

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