Female blue crabs mate only once in their lives when they become sexually mature. The females hold their fertilized eggs in a large mass that remains attached to fine hairs beneath their abdomen until the eggs hatch. This is a dangerous time for the parent and offspring. It takes many weeks for the larvae to develop and young crabs have to travel great distances through different types of water for nearly a year before they become sexually mature. Only those that are able to survive in unprotected waters reach the age of maturity to reproduce. Egg masses consist of between 750,000 to eight million eggs depending on the size of the female.
(adapted from www.blue-crab.org/lifecycle1.htm; Etherington & Eggleston, 2000; Eggleston, personal communication 8/14/03).
Factors that affect blue crab populations are described below:
Dissolved oxygen/water quality. Blue crabs tend to move out of deep waters affected by low dissolved oxygen and into shallow areas. Low dissolved oxygen may also interfere with the dispersal of juvenile crabs into the estuary and with the migration of adult females into the mouth of the Chesapeake Bay to spawn.
Habitat loss. Juvenile crabs find refuge from predators in underwater grass beds. The loss of these grasses is a loss of nursery habitat.
Predator abundance. Predators affects blue crab population dynamics, survival and reproduction.
Prey availability. Blue crabs feed on benthic prey (live on the bottom of the ocean), whose abundance can be limited by long periods of severe hypoxia (low oxygen).
Disease. Blue crabs harbor a range of fatal pathogens.
Climate change. Changes in water temperature could affect predator abundance, prey availability and winter mortality rates. Sea level rise could affect the habitats that serve as nursery and foraging areas. Changes in precipitation could increase areas of low dissolved oxygen, which could reduce foraging resources and limit the amount of available suitable habitat for blue crabs.
Environmental variability. The survival of blue crab larvae in coastal waters and their settlement in grass beds and other habitats depend upon oceanic flow, storm events and weather patterns. Tides, salinity and other chemical cues also impact larval settlement, while seasonal fluctuations in temperature affect the timing of other life history events. Extremely cold winters can cause significant blue crab mortality in the Bay, especially in low-salinity waters.
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Recall that we can mathematically model population growth. Populations change in size and composition over time. Some of these models represent growth without environmental constraints, while others include limited resources. Mathematical models of populations can be used to accurately describe changes occurring in a population and, importantly, to predict future changes.
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Explain how limiting factors - including living and non-living components in their environment - affect the carrying capacity of a blue crab population. Use evidence
