Zooplankton of the Great Lakes
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Organism: Daphnia parvula
Kingdom - Animalia
Phylum - Arthropoda
Subphylum - Crustacea
Class - Branchiopoda
Order - Diplostraca
Suborder - Cladocera
Family - Daphniidae
Genus - Daphnia
Species - Daphnia parvula
(Myers et al., 2016)
Figure 1: An adult female Daphnia parvula
Systematic studies of the genus daphnia are primitive. There are over 100 species within the genus Daphnia and current research shows that 33 of these species are present in North America. Identification of Daphnia species is revealed through morphological differences which may be difficult to see (Colbourne & Hebert 1996).
Cladoceran are characterized by their body being enclosed in a bivalve carapace and they have five appendages adapted for gas exchange and filtering. The head contains a large, dark compound eye (Balcer et al., 1984). D. parvula can be identified by the absence of an ocellus and a small and concave head (figure 3). The first antennae are attached to the ventral side of the head and they have small setae (figure 3). The second antennae are large and are used for swimming (figure 4). A small rostrum projects from the head (Balcer et al., 1984). D. parvula have a widely rounded head and their posterior spine is less than one-quarter of the valve length (Ward et al., 1959) (figure 5). D. parvula contain a posterior claw (figure 6).
Figure 3: A close up of the head of a female D. parvula. Note the large, dark compound eye, concave head, and the absence of an ocellus. The arrow points out the first antennae.
Figure 4: The second antennae of D. parvula. This appendage is used for swimming.
Figure 5: The posterior spine of D. parvula.
Figure 6: The claw of a dissected D. parvula.
Geographic Distribution and Vertical Migration
Daphnia generally reside in the open-water zone of lakes and oceans (Peņalva-Arana et al., 2007, Winder & Mooij, 2004). Daphnia are a very common species and they are found throughout the Americas, Europe, and Australia. There are 33 known species of daphnia in North America (figure 7), this continent has the greatest species richness of daphniids (Colbourne & Hebert 1996).
Both juvenile and adult Daphnia are daily diel vertical migrators. Like many cladoceran, daphniids avoid the surface during daylight and migrate up the water column at night (Rose et al., 2012). Research suggests that the migrations are a mechanism to avoid predation by planktivorous fish during the day (Zaret & Suffern 1976). It is confirmed that avoiding fish is an important factor however, UV exposure has been shown to be a more important driver of Daphnia vertical migration. Damage received from UV exposure will elicit a more significant downward migration than when Daphnia are in the presence of fish alone (Rose et al., 2012).
D. parvula reach maximum density in the fall and remain high in numbers through spring (Taipale et al., 2009, Pace et al., 1984). Population densities can be compared to biotic and abiotic factors in the water column. Densities are high in winter when algal biomass is lower and populations decrease in warmer months when algal biomass and temperature increases. Populations are also affected by the increased population of D. parvula predator Chaoborus (Pace et al., 1984).
Figure 7: This map shows the North American distribution of D. retrocurva and D. parvula. D. parvula is shown in blue and is widely distributed throughout North America (Constanzo & Taylor, 2010).
Figure 2: Two adult female daphnia with eggs in their brood pouches.
D. parvula is an herbivorous species which consumes through filter feeding (Peņalva-Arana et al., 2007, Balcer et al., 1984). Within their carapace, Daphnia contain four ventral thoracic feeding appendages (figure 8, 9) which create a feeding current to filter the water for food particles nonstop starting at birth at a rate of all algae in 4ml of water in one hour (Peņalva-Arana et al., 2007). Their diet consists of mainly phytoplankton and methane-oxidizing bacteria (Taipale et al., 2009). Daphnia biomass is highest in autumn and research suggests that this is because mixing during this season results in methanotrophic bacteria which can sustain high populations of Daphnia (Taipale et al., 2009).
Figure 8: The gut of a dissected D. parvula. The arrow is showing the filtering teeth.
Figure 9: A picture of the filtering teeth in an intact D. parvula.
Daphnia growth is highly dependent on the environment. Availability and quality of food as well as abiotic factors such as temperature and pH are factors that determine Daphnia growth. Some studies suggest that primarily phosphorus in the environment assists with Daphnia growth and survival (Acharya et al., 2004), other studies suggest that nitrogen and carbon influence growth (Mueller-Navarra, 1995). A study of Daphnia growth done by Lampert & Trubetskova showed that ultimately, concentration of food has the greatest influence on growth (1996).
In order to grow, Daphnia must molt their exoskeleton. After each molt, they take in water to rapidly increase their volume before their new molt hardens. They typically molt two to five times to reach maturity and they can molt up to 25 times after (Balcer et al., 1984).
For the majority of the year, Daphnia reproduce through cyclic parthenogenesis using mitosis. During favorable conditions, the mother Daphnia deposits 2 to 20 2N eggs in her brood pouch (figure 2, 10, 11) and the juveniles are released during the next maternal molt cycle. These juveniles are identical to the mother (Balcer et al., 1984, Fink et al., 2011). When environmental conditions become unfavorable, Daphnia produce resting eggs covered by an ephippium, a protective covering, which undergo diapause. This switch is caused by three environmental stimuli including food limitation (starvation), crowding, and the amount of illumination received (length of day) (Kleiven et al., 1992). Resting egg production requires meiosis and a male Daphnia. During these stressful times, females produce haploid (N) eggs and diploid (2N) eggs which become males. The diploid male Daphnia produce haploid sperm which fertilize the haploid eggs. These now diploid eggs will become a fertilized resting egg and when favorable conditions return, they will hatch into females. This process results in genetic recombination and helps maintain diversity in the population.
Figure 10: Eggs in the brood pouch of a pregnant D. parvula.
Figure 11: A D. parvula egg.
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