Inquiry Case 1
The water has become much clearer as the quantity of particulate matter in the water column has
been greatly reduced. This is one of the byproducts of the huge abundance of zebra mussels,
whose filter-feeding activities cycle massive amounts of water every day.
No zebra mussels in sandy areas, as there is no suitable substrate for them to attach to while
developing, ex. Rocks, ballast tanks, large native mussels.
Take shelter from current or other effects which might dislodge them.
Zebra mussel (Dreissena polymorpha) is native to Eastern Eurasia, specifically Black Sea and
Transported across Atlantic in the ballast water tanks of an ocean liner.
First reported in North America in Lake St. Clair in 1988.
Also a second similar invasive specie, the quagga mussel (Dreissena rostriformis)
Life cycle: embryo, glochidium in marsupium, glochidium in host gill, juvenile mussel, adult.
Dioecious (“two houses”) – separate males and females.
Key Points in Unit 1
1. The invasive zebra mussel (Dreissena polymorpha) has had an enormous economic, aesthetic, and
biological impact on the Great Lakes watershed. It originated from eastern Eurasia and is thought to have
been transported by an ocean vessel sometime before 1988.
2. There are 41 native freshwater mussels in Ontario, many of them are endangered. Unionids are obligate
parasites where glochidia (larvae) are released and must attach to the gills of a fish host to complete their
Biodiversity – the variety of genes, species and ecosystems.
different species thrive in environments differently
Shaped by billions of years of evolutionary history of life forms interacting with each other and
Altered by human activity with lasting impacts at large scales that are difficult to predict
Evolution – descent with modification from common ancestors
What factors contribute to threats of native freshwater mussels?
Invasive species (zebra mussels), agricultural practices, decline in fish population affect ability
to mate, drop in water levels, change in natural landscape (rocky beach to sandy), human
impact, water temperature fluctuation (global warming).
Unionids (native mussels) vs. Dreissenids (zebra mussels)
What is the larval Does it need a host? What type of larval
Unionids Glochidia Yes Dispersed by host
Dreissenids Veligers Non-host dependent Dispersed in water
How do freshwater bivalves develop? (Ontogenetic Variation)
By passing early development as a parasitic stage on a host (ex. Unionids)
By producing veliger larvae (ex. Dreissena polymorpha)
By releasing fully developed young mussels (rare)
Unionids life cycles and larval types
Embryo, glochidium in marsupium, glochidium in host gill, juvenile mussel, adult.
Glochidiia: Cannot swim or crawl, must attach to host fish gills
o Strategy for attracting fish host: Conglutinates (Glochidia enclosed in membranous
capsules that mimic host pray – fish thinks it can eat it, however full of parasites). o Strategy for attracting fish host: Lures (Modified mantle tissue mimics fish or
invertebrate prey item to attract fish)
o Host capture: Mussel may physically grip host and pump glochidia over the gills
Dreissena polymorpha life cycles
Embryo, larvae (free swimming) metamorphosis juvenile larvae (settled), adult
Inquiry Case 2
Biologists use the word “diversity” to refer to the number of species in a taxonomic group or a
Some taxonomic groups are very diverse (i.e., contain a lot of species) and others are not.
Some parts of the world have a high level of diversity (e.g., the Amazon) whereas others are
much more limited in the number of species that reside there (e.g., Antarctica).
Factors such as invasive species, agricultural practices, and habitat destruction are
posing significant challenges to the survival of many of the native mussel species.
Species have morphological (physical) traits used to distinguish them
Species are variable, and morphological traits exists along a range of conspecifics
(members of the same species)
Ontogeny – changes in morphology from youth to adult
Variation among individuals must be heritable – passed on to offspring
Sometimes variability caused by environment. Phenotype plasticity – same set of
genes (genotype) can result in different features (phenotype) due to environment
Abundance – number of individuals
Morphological/genetic variation – differences among individuals within a species
Diversity – number of species
Disparity – how physically different those species are from each other
Clade (monophyletic group) VS paraphyletic (non-monophyletic group)
Convergent evolution – feature which has evolved more than one in independent lineages as a
similar adaptation under similar environmental pressures
Homoplasy – having similar traits but evolved independently
Homology – sharing ancestral traits
A well-sampled population approximates a bell curve.
Genetic variation – DNA sequence – genes that are coded (phenotype/genotype differences)
Environmental variation – what something looks like (variation of colour, shape, pattern)
Species evolve at the same time
Speciation – formation of new species
Cladogenesis – splitting or branching into two descendant species
Gene – molecular unit of heredity in a living organism
Allele – form of a gene or group of genes – different alleles may result in different traits (ex. Eye colour)
- (Different alleles producing same trait resulting in genetic variation with little/no variation) Causes of genetic variation:
Agent Definition Effect on Genetic Variation
Mutation A heritable ch