Metals in Crawfish - Aquaculture and the Environment: A Shared Destiny - page 197

2.2 Biology
Both red swamp and white river crawfish are considered a temperate species of crustaceans
that are able to tolerate moderately cold winters. There are some differences of note in these
two species. White river crawfish spawn in the fall of the year and produce larger but less
numerous eggs than do the red river crawfish which spawn year around. Red river crawfish
have higher feeding rates at temperatures above 30° C. Additionally, the red swamp
crawfish are more tolerant to the low dissolved oxygen content that sometimes occurs in
crawfish ponds. This may be the reason that the red swamp crawfish dominate most
crawfish production ponds. The two species are very similar in appearance but can be
differentiated between by an experienced person. Red swamp crawfish is the preferred
species because of this species greater reproductive potential and a more prolonged
reproductive season. There is no evidence of natural hybrids between the two species.
2.3 Life cycle
Both species live about 2 years, have high juvenile survival rates and have reproductively
active and inactive periods. After mating in open water, the female crawfish then stores the
sperm in an internal receptacle. Egg development in the females takes place internally.
Developing eggs in the ovary increase in size and change from a light color to black. More
than 500 mature eggs are expelled and attached to the swimmerets under the tail of the
crawfish. Fertilization of the eggs takes place externally when the female releases the stored
sperm. During egg production the female builds a burrow and spawning takes place in the
burrow. The incubation period for the eggs is approximately three weeks. The hatchlings
remain attached to the female's swimmerets through the first two molting phases. After
detaching from the mother, the young crawfish resemble adult crawfish and begin to feed.
The female and young crawfish remain in the burrow for several weeks but then must leave
because little food is available in the burrows. Pond flooding or heavy rainfall will
encourage the crawfish to emerge sooner rather than later. Reproduction is somewhat
synchronized in pond-raised crawfish because rice fields are flooded in the fall after the rice
is harvested to coincide with the optimum time for reproduction. While red river crawfish
spawn at anytime, autumn is the best time for white river crawfish to spawn. Peak
production can best be attained when reproduction is properly timed in autumn.
Crawfish, like all crustaceans, must molt periodically to increase in size. Growth rate is
dependent on variables such as: water temperature, oxygen levels, population density and
food quality and quantity. Optimum harvest size can be reached in as little as 7 to 9 weeks,
but usually takes 3 to 5 months after hatching. Crawfish molt approximately 11 times before
reaching maturity. Though there are five major stages in the molt cycle, it is a continuous
process. The inter-molt stage is the phase in which the exoskeleton is hard and fully formed.
During this stage the crawfish feed to increase their energy and tissue reserves. In the pre-
molt stage an underlying soft exoskeleton forms and begins re-absorbing calcium for the
older exterior. During molting feeding stops and temporary shelter is sought. Molting takes
place in a matter of minutes as the old exoskeleton splits between the carapace and the
abdomen on the dorsal surface. The crawfish then withdraws by flipping its tail several
times. After emerging from the old exoskeleton the crawfish is in the “soft” phase during
which the new exoskeleton expands to a larger size than the previous exoskeleton.
Calcification of the new exoskeleton takes place in two phases. During the first phase the