Biology and Ecology of Juvenile Procambarus alleni and Procambarus fallax
from the Everglades, Laboratory Studies
Peggy VanArman
Palm Beach Atlantic University, West Palm Beach, FL and
Nova SE University Oceanographic Center, Dania, FL
Crayfish, fish, and mussels are important members of food webs in most
freshwater aquatic ecosystems in North America, where crayfish often serve as
keystone species, and may dominate energy and nutrient flow. Despite the widespread influences that crayfish have on all trophic levels, they are almost
completely overlooked in conservation efforts. The roles of crayfish as herbivores
and detritivores are well documented, but their importance as carnivorous
predators has received little attention until recently. A growing list of crayfish
prey includes leeches, tadpoles, newts, fish eggs and fry, and molluscs. As
intermediates in food webs, crayfish form part of the diet of more than 36 species
of vertebrates that inhabit the Everglades, including the keystone American
alligator and wading birds. Crayfish burrows provide refugia for other organisms
during times of drought and support an entire burrow ecosystem.
Two species of epigean crayfish, Procambarus alleni (Everglades crayfish) and P.
fallax (swamp crayfish) inhabit the Everglades. Some researchers have
previously confused identification of these species and it was not until 2000 that
P. fallax was officially documented in the southern Everglades. Field collections
indicate that swamp crayfish have been present in the southern Everglades and in
coastal Broward County since 1985. More recent studies suggest that although
these two species occur in sympatric and occasionally syntopic distribution, P.
alleni may prefer shallow water and short hydroperiod conditions, whereas P.
fallax prefer more permanently flooded conditions. Elsewhere in Florida, where
P. fallax was found within the range of P. alleni, P. fallax appeared to be the
more successful competitor. Because of their critical roles in the trophic structure
of Everglades wetlands, shifts in relative abundance of these two species may
have significant effects on wetland communities and overall availability of
crayfish as food for other organisms.
Researchers are now recognizing the critical importance of crayfish in Everglades
wetlands, as bio-indicators to assess effects of hydrology, fire, and nutrient
enrichment; as intermediates in food webs; and as ecosystem components in
community sampling studies. More studies have been conducted on the biology
and ecology of P. alleni than on P. fallax and P. alleni was proposed as an
indicator species to monitor progress of Everglades restoration programs. Only
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two previous studies examined life history and ecology of both P. alleni and P.
fallax. The two species appear to differ in reproductive strategies, growth and
maturation rates, and migration and burrowing behavior patterns. The sparse
information available centers on adults rather than juvenile crayfish.
Current Research. Three experiments were conducted on juvenile P. alleni and
P. fallax under laboratory conditions that simulated conditions in South Florida
wetlands. Adult crayfish of both species were obtained from various areas,
including wetlands that were historically connected to the northern Everglades.
Young crayfish were hatched from berried females captured in the field or bred in
the laboratory. In the first experiment, growth, survival, and development of
hatchlings were monitored to three months of age, under stable conditions. In the
second experiment, effects of biotic and abiotic factors (water levels, food,
density and competition) on growth and survival of hatchlings were examined
during three months. In the third experiment, behavior patterns and substrate
choice were observed in the presence of an adult arthropod predator.
Under stable conditions, juvenile P. fallax had significantly higher survival than
P. alleni. P. alleni had significantly greater weight and total length than P. fallax,
however, at a given length, P. fallax was heavier than P. alleni. More P. fallax
than P. alleni developed gonopods 1 and 2 within three months. In conditions
simulating different water levels, P. alleni survival was impacted most by low
food levels, high density, all three water levels, and intraspecific competition. The
highest survival occurred with high food levels, and low density. Lowest survival
of P. fallax occurred in low food levels, high density, and low water levels, while
high food levels, low density and drying conditions enhanced survival.
P. alleni grew larger than P. fallax in all conditions tested. Growth of P. alleni
was most impacted by low food levels, drying conditions, high density, and
intraspecific competition. The best conditions for growth of Everglades crayfish
included high food levels, low water levels, low density, and interspecific
competition with P. fallax. Growth of P. fallax young was most impacted by low
food levels, high density, and high density interspecific competition. P. fallax
grew largest in conditions of high food levels, low density, and low density
interspecific competition. P. fallax grew to the same size in all three water levels.
With or without an arthropod predator present, both species spent the most time in
Utricularia foliosa and Panicum hemitomon, and the least time in Typha
domingensis and on sand. With a predator present, juvenile Everglades crayfish
were exposed on sand more at night, and more than P. fallax. P. fallax spent
significantly more time secluded in bladderwort than P. alleni. In the absence of
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an arthropod predator, feeding was the primary activity of both species during the
day. P. alleni was more mobile at night, whereas P. fallax spent more time resting
and less time feeding. In the presence of the predator, movement by both species
was reduced and juveniles fed more during the day and rested more at night. P.
fallax spent more time resting then P. alleni during day and night, but P. alleni
wandered around twice as much as P. fallax at night. Both species exhibited
diurnal behavior patterns.
Because of its larger size, P. alleni may have a competitive advantage in resource
holding potential, and by producing more eggs. P. fallax grow more slowly, but
compared to P. alleni are heavier at a given length, reach maturity at a smaller
size, and may reproduce throughout the year. In Everglades wetlands, P. alleni are
more abundant under fluctuating conditions and may survive better during dry
periods. Juvenile P. fallax have higher survival under stable conditions. If
survival and growth strategies of P. fallax are enhanced by long hydroperiods and
appropriate depths, then P. fallax could have a considerable competitive
advantage over P. alleni in survival, growth, and development.
Research Needs. A more holistic approach is needed to understand the role of
crayfish populations in Everglades ecosystems that integrates laboratory, field and
modeling studies. Detailed, long-term information is needed about most aspects of
the life history and ecology of P. alleni and P. fallax, to determine nutritional
needs, life span, reproductive strategies, and predator-prey cycles in Everglades
environments. Studies of water depth, hydroperiod, and water quality are also
needed, to understand how P. alleni and P. fallax can coexist, on a microhabitat
level. Little is known about tolerances of either species to water quality factors
such as oxygen, temperature, salinity, turbidity, or pH for different stages of the
life cycle.
Water managers must consider that even small changes in hydrology may
significantly alter crayfish populations and community structure over large areas
and at multiple trophic levels in Everglades ecosystems. Depth, delivery and
timing of water may favor either species of crayfish, to the detriment of the other.
Conservation strategies should consider the need to provide crayfish habitat that
will allow balanced distributions of P. alleni and P. fallax populations that
maintain floral and faunal biodiversity, and optimize the roles of crayfish as
predators, prey, and habitat managers in southern Florida wetlands.
Peggy, VanArman, Palm Beach Atlantic College, PO Box 24708, West Palm
Beach, FL , 33416, Phone: 561-803-2287, Fax: 561-803-2390,
peggy_vanarman@pba.edu, Ecology and Ecological Modeling
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