grow rapidly, molting on average every 3–4 weeks
depending on water temperature (Smith and Chang,
2007), making them important prey for a variety of fish
and birds, but they are also important predators on other
small invertebrates (Lipcius et al., 2007). Therefore,
young juveniles (<20 mm carapace width) occupy shallow, structured habitats that also contain food sources
including seagrass beds, macroalgae, and oyster reefs
(Lipcius et al., 2007). During the 12–18 months required
to reach sexual maturity (at 90–100 mm carapace width),
habitat use expands based on size and density-dependent
factors (Hines, 2007; Lipcius et al., 2007). Smaller juveniles (20–30 mm carapace width) move to alternative
nursery habitats including marsh creeks and marshfringed mud flats (Lipcius et al., 2007). Larger juveniles
(>20 mm carapace width) begin venturing into unstructured habitats and, as they grow, inhabit deeper areas
where they continue to be important predators but are prey
to fewer organisms (Hines, 2007).
Unlike females that exhibit a final molt to reach maturity, adult males grow throughout their lives (reaching
sizes of >200 mm carapace width), molting every
30–40 days depending on temperature, and they typically
return to more protective habitats during molting, as they
are particularly vulnerable to predators. As a result of
ontogenetic shifts in habitat as well as movement into
lower salinity areas, blue crabs can be found in a wide
array of habitats, throughout the estuarine-to-ocean salinity gradient (e.g., 5–35 ppt) (Hines, 2007; Lipcius et al.,
2007). Because they represent both predator (contrary to
popular belief, they are not scavengers) and prey in these
habitats, blue crabs are a critical component of the estuarine food web both within and between estuarine habitats.
Blue crabs have been an important food item for
humans since the early 1700s and have supported
a commercial and recreational fishery since the 1800s
(Kennedy et al., 2007). As a result, blue crabs are part of
the historic, economic, and social fabric of communities
along the Atlantic and Gulf coasts of the United States.
Blue crabs are consumed as “hard crabs” (crabs with
a hard carapace, typically in the intermolt stage) and as
“soft crabs” (crabs with a soft carapace as a result of recent
molting) (Kennedy et al., 2007). The predominant fishing
techniques vary with the season and with the sex of the
harvested crabs. During the warmer seasons (late
spring-late fall), blue crabs are typically harvested with
a trap or “pot,” and the catch is predominantly males.
During the winter, particularly in the mid-Atlantic region,
blue crabs are harvested by a dredge, and the catch is
predominantly females.
Bibliography
Epifanio, C. E., 2007. Biology of larvae. In Kennedy, V. S., and
Cronin, L. E. (eds.), The Blue Crab: Callinectes sapidus.
College Park, MD: Maryland Sea Grant College, pp. 513–528.
Hines, A. H., 2007. Ecology of juvenile and adult blue crabs. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 565–630.
Kennedy, V. S., Oesterling, M., and Van Engel, W. A., 2007. History
of blue crab fisheries on the U.S. Atlantic and Gulf coasts. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 655–705.
Lipcius, R. N., Eggleston, D. B., Heck, K. L. J., Seitz, R. D., and van
Montfrans, J., 2007. Ecology of postlarval and young juvenile
blue crabs. In Kennedy, V. S., and Cronin, L. E. (eds.), The Blue
Crab: Callinectes sapidus. College Park, MD: Maryland Sea
Grant College, pp. 535–559.
Millikin, M. R., and Williams, A. B., 1980. Synopsis of
Biological Data on the Blue Crab, Callinectes sapidus
Rathbun. Washington, DC: National Oceanic and Atmospheric
Administration, p. 39.
Smith, S. G., and Chang, E. S., 2007. Molting and growth. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 197–245.
Cross-references
Soldier Crabs (Mictyridae)
BULKHEADS
Harry C. Friebel
U.S. Army Corps of Engineers, Philadelphia District,
CENAP-EC-EH, Philadelphia, PA, USA
Synonyms
Retaining walls; Revetments; Seawalls
Definition
Vertical structures or partitions that hold or prevent soil
from sliding seaward and reduce land erosion. A secondary purpose of these structures is to provide protection
to the upland from light-to-moderate wave action
(CHL, 2013).
Bulkheads protect bluffs and cliffs by retaining soil
from eroding at the toe, thereby increasing stability. Bulkheads may cause increased erosion immediately seaward
and adjacent to the structure (flanking) due to wave reflection, and they offer no protection to adjacent areas. Bulkheads may be cantilevers, anchored (e.g., sheet pile), or
gravity structures (e.g., stone) (USACE, 1981; USACE,
1984; USACE, 2002).
Cantilever bulkheads require adequate ground embedment to retain soil and prevent overturning and are typically used where lower structures are needed. Scour at
the toe of the structure can effectively reduce the embedment length and cause failure (USACE, 1981).
110
BULKHEADS
depending on water temperature (Smith and Chang,
2007), making them important prey for a variety of fish
and birds, but they are also important predators on other
small invertebrates (Lipcius et al., 2007). Therefore,
young juveniles (<20 mm carapace width) occupy shallow, structured habitats that also contain food sources
including seagrass beds, macroalgae, and oyster reefs
(Lipcius et al., 2007). During the 12–18 months required
to reach sexual maturity (at 90–100 mm carapace width),
habitat use expands based on size and density-dependent
factors (Hines, 2007; Lipcius et al., 2007). Smaller juveniles (20–30 mm carapace width) move to alternative
nursery habitats including marsh creeks and marshfringed mud flats (Lipcius et al., 2007). Larger juveniles
(>20 mm carapace width) begin venturing into unstructured habitats and, as they grow, inhabit deeper areas
where they continue to be important predators but are prey
to fewer organisms (Hines, 2007).
Unlike females that exhibit a final molt to reach maturity, adult males grow throughout their lives (reaching
sizes of >200 mm carapace width), molting every
30–40 days depending on temperature, and they typically
return to more protective habitats during molting, as they
are particularly vulnerable to predators. As a result of
ontogenetic shifts in habitat as well as movement into
lower salinity areas, blue crabs can be found in a wide
array of habitats, throughout the estuarine-to-ocean salinity gradient (e.g., 5–35 ppt) (Hines, 2007; Lipcius et al.,
2007). Because they represent both predator (contrary to
popular belief, they are not scavengers) and prey in these
habitats, blue crabs are a critical component of the estuarine food web both within and between estuarine habitats.
Blue crabs have been an important food item for
humans since the early 1700s and have supported
a commercial and recreational fishery since the 1800s
(Kennedy et al., 2007). As a result, blue crabs are part of
the historic, economic, and social fabric of communities
along the Atlantic and Gulf coasts of the United States.
Blue crabs are consumed as “hard crabs” (crabs with
a hard carapace, typically in the intermolt stage) and as
“soft crabs” (crabs with a soft carapace as a result of recent
molting) (Kennedy et al., 2007). The predominant fishing
techniques vary with the season and with the sex of the
harvested crabs. During the warmer seasons (late
spring-late fall), blue crabs are typically harvested with
a trap or “pot,” and the catch is predominantly males.
During the winter, particularly in the mid-Atlantic region,
blue crabs are harvested by a dredge, and the catch is
predominantly females.
Bibliography
Epifanio, C. E., 2007. Biology of larvae. In Kennedy, V. S., and
Cronin, L. E. (eds.), The Blue Crab: Callinectes sapidus.
College Park, MD: Maryland Sea Grant College, pp. 513–528.
Hines, A. H., 2007. Ecology of juvenile and adult blue crabs. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 565–630.
Kennedy, V. S., Oesterling, M., and Van Engel, W. A., 2007. History
of blue crab fisheries on the U.S. Atlantic and Gulf coasts. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 655–705.
Lipcius, R. N., Eggleston, D. B., Heck, K. L. J., Seitz, R. D., and van
Montfrans, J., 2007. Ecology of postlarval and young juvenile
blue crabs. In Kennedy, V. S., and Cronin, L. E. (eds.), The Blue
Crab: Callinectes sapidus. College Park, MD: Maryland Sea
Grant College, pp. 535–559.
Millikin, M. R., and Williams, A. B., 1980. Synopsis of
Biological Data on the Blue Crab, Callinectes sapidus
Rathbun. Washington, DC: National Oceanic and Atmospheric
Administration, p. 39.
Smith, S. G., and Chang, E. S., 2007. Molting and growth. In
Kennedy, V. S., and Cronin, L. E. (eds.), The Blue Crab:
Callinectes sapidus. College Park, MD: Maryland Sea Grant
College, pp. 197–245.
Cross-references
Soldier Crabs (Mictyridae)
BULKHEADS
Harry C. Friebel
U.S. Army Corps of Engineers, Philadelphia District,
CENAP-EC-EH, Philadelphia, PA, USA
Synonyms
Retaining walls; Revetments; Seawalls
Definition
Vertical structures or partitions that hold or prevent soil
from sliding seaward and reduce land erosion. A secondary purpose of these structures is to provide protection
to the upland from light-to-moderate wave action
(CHL, 2013).
Bulkheads protect bluffs and cliffs by retaining soil
from eroding at the toe, thereby increasing stability. Bulkheads may cause increased erosion immediately seaward
and adjacent to the structure (flanking) due to wave reflection, and they offer no protection to adjacent areas. Bulkheads may be cantilevers, anchored (e.g., sheet pile), or
gravity structures (e.g., stone) (USACE, 1981; USACE,
1984; USACE, 2002).
Cantilever bulkheads require adequate ground embedment to retain soil and prevent overturning and are typically used where lower structures are needed. Scour at
the toe of the structure can effectively reduce the embedment length and cause failure (USACE, 1981).
110
BULKHEADS