(DRAFT) - Taxonomy
Species rangia, common
Species Id M060007
Date 26 AUG 96
TAXONOMY
NAME - rangia, common
OTHER COMMON NAMES - common rangia, brackish water clam, Louisiana road clam and Atlantic rangia
ELEMENT CODE -
CATEGORY - Aquatic Molluscs
PHYLUM AND SUBPHYLUM - ,
CLASS AND SUBCLASS - Bivalvia,
ORDER AND SUBORDER - Veneroida,
FAMILY AND SUBFAMILY - Mactridae,
GENUS AND SUBGENUS - Rangia,
SPECIES AND SSP - , cuneata
SCIENTIFIC NAME - Rangia cuneata
AUTHORITY - Sowerby, 1831
TAXONOMY REFERENCES - 186 and 74
Taxonomy - 1 (DRAFT) - Status
Species rangia, common
Species Id M060007
Date 26 AUG 96
STATUS
Coded Status
Commercial
Sensitive
Commercial/consumption
Commercial/industrial
REFERENCES FOR STATUS - 74
COMMENTS ON STATUS -
The common rangia is an important component of estuarine ecosystems
accounting, for example, for nearly 95% of the benthic biomass in the James
River Estuary, Virginia. In low salinity estuarine areas common rangia
functions as a link between primary producers and secondary consumers. As a
non-selective filter feeder, rangia transforms large quantities of plant
detritus and phytoplankton into clam biomass. In turn, this biomass is
consumed by fishes, crustaceans, and ducks. The shells provide hard
substrate for epifaunal attachment.
The common rangia was a food item of prehistoric Indians and it is still
occasionally canned and eaten in New Jersey, Texas, North Carolina and
Mexico. Economically, common rangia is more important as a source of shells
for road building and in the manufacture of many industrial products. Much
of this shell material is dredged from buried deposits in estuaries.*74*
Common rangia are known to concentrate chemicals such as kepone.*74*
Status - 1 (DRAFT) - Distribution
Species rangia, common
Species Id M060007
Date 26 AUG 96
DISTRIBUTION
Distribution - 1 HABITAT ASSOCIATIONS
NATIONAL WETLAND INVENTORY CODES
NWI NWICLS NWIMOD NWISPEC
Estuarine, subtidal L
REFERENCES FOR NWI - 74
COMMENTS ON HABITAT ASSOCIATIONS -
-see C.Aquat.Hab
Habitat Associations - 1 (DRAFT) - Food Habits
Species rangia, common
Species Id M060007
Date 26 AUG 96
FOOD HABITS
TROPHIC LEVEL -
DETRITIVORE
FILTERER
REFERENCES FOR TROPHIC LEVEL - 74
LIFESTAGE FOOD FOOD PART
Important Detritus - Organic Not Applicable
Important Other Plant Species Not Applicable
REFERENCES FOR IMPORTANT FOOD - 74
COMMENTS ON FOOD -
Common rangia serve to link primary producers and secondary consumers in
estuarine areas. Rangia are non-selective filter feeders ingesting large
quantities of detritus and phytoplankton. In one study gut contents were
reported to contain 70% unidentifiable detritus, 10% sand, 17% algae
(possibly Anabaena or Microcystis) as well as traces of diatoms,
foraminifera, and vascular plant material. Another study reported 48
species of phytoplankton from stomach contents of common rangia, although a
large portion of the material ingested was detritus (46 to 81%, depending on
tidal conditions).*74*
Feeding of common rangia is controlled by gill palp articulations and
ciliary currents over the gills. The animal extrudes pseudofeces from the
mantle cavity, through the inhalant siphon when the valves are quickly
closed.*74*
Food Habits - 1 (DRAFT) - Environment Associations
Species rangia, common
Species Id M060007
Date 26 AUG 96
ENVIRONMENTAL ASSOCIATIONS
G = General A = Adult
LIM = Limiting RA = Resting Adult
J = Juvenile FA = Feeding Adult
RJ = Resting Juvenile BA = Breeding Adult
FJ = Feeding Juvenile P = Pupae
L = Larvae E = Egg
RL = Resting Larvae
FL = Feeding Larvae
LIFESTAGE ENVIRONMENTAL ASSOCIATIONS
LIM Water Temperature: Specified in Comments
LIM
G Dissolved Oxygen: Specified in Comments
G Turbidity: Murky water [High turbidity]
G Substrate: Mud or silt
G Substrate: Sand
G Substrate: Plants
G Relation to Substrate: Occurs in substrate [penetrating]
REFERENCES FOR ENVIRONMENTAL ASSOC_ - 74
REFERENCES FOR LIMITING ENVIRONMENTAL ASSOC_ - 74
COMMENTS ON ENVIRONMENTAL ASSOCIATIONS -
A combination of low salinity, high turbidity, and a substrate of sand,
mud, and vegetation appears to be the most favorable habitat for the common
rangia. This clam may be one of the few freshwater clams to become
established in brackish water. Conversely, others have considered
common rangia as belonging to a marine group that has become adapted to
brackish water.*74*
Temperature:
Winter kills in the shallow waters of Chesapeake Bay suggest that common
rangia have reached its limit of temperature tolerance there. Water
temperature has been reported as the most important factor stimulating
gametogenesis. The planktonic existence of larvae is apparently greatly
extended by low temperature.*74*
Salinity:
Common rangia are concentrated in areas where salinity seldom exceeds 18
ppt. One study reported a negative correlation (r=0.71) between clam
height and salinity (0 to 6 ppt). In another study, a greater number of
size classes and larger clams were found at low salinities (0 to 2 ppt)
than at higher ones in Florida and suggested that this range was optimal.
*74*
Common rangia have developed physiological responses to the frequent and
sudden salinity changes present in many estuaries. Common rangia is an
osmoconormer at salinities greater than 10 ppt, and an osmoregulator at
lower salinities. A number of amino acids (including alanine, glycine,
glutamic and aspartic) are concentrated at high salinities suggesting that
an amino acid pool is used for osmoregulation.*74*
Environment Associations - 1 (DRAFT) - Environment Associations
Species rangia, common
Species Id M060007
Date 26 AUG 96
Temperature and Salinity:
The combined effects of temperature (8 to 32 degrees C) and salinity (0 to
20 ppt) on embryos and larvae of common rangia were tested. Embryos failed
to develop at 0 ppt salinity. The optimum conditions for embryos were
temperatures of 18 to 29 degrees C and salinities of 6 to 10 ppt.
Larvae survived at all combinations of temperature and salinity tested
(except at 0 ppt). They tolerate temperatures of 8 to 32 degrees C and
salinities of 2 to 20 ppt. Growth of larvae was best at high salinity (10
to 20 ppt) and high temperature (20 to 32 degrees C). Straight-hinged
larvae were found to be more tolerant than embryos to extremes of
temperature and salinity.*74*
Oxygen:
Common rangia have been found to withstand anoxic conditions in studies of
glycolysis; however, rangia are intolerant of exposure to air.*74*
Substrate:
Common rangia are found in a wide range of soft substrates.*74* See C.
Aquat.Hab. for more information.
Relation to Substrate:
It has been reported that juveniles are capable of selecting substrate for
setting and preferred substrates high in organic content. Common rangia
move little after settling. Little movement of clams in aquaria has been
observed. It has been suggested that rangia are capable only of vertical
movement in the sediment. One study reported that clams did not move in
aquaria over a 4-month period even when given a choice of substrates.*74*
Depth:
The highest concentration of clams along the gulf coast has been associated
with shallow water areas less than 6 m deep. A general decrease in
density has been observed as depth increased from 2.5 to 4.6 m.*74*
COMMENTS ON RESTING ADULT ENVIRONMENTAL ASSOC_ -
-see C.Environ
COMMENTS ON FEEDING ADULT ENVIRONTAL ASSOC_ -
-see C.Environ
COMMENTS ON FEEDING LARVAE ENVIRONMENTAL ASSOC_ -
-see C.Environ
Environment Associations - 2 (DRAFT) - Life History
Species rangia, common
Species Id M060007
Date 26 AUG 96
LIFE HISTORY
Morphology/Identification Aids:
Adults range from 2.5 to 6.0 cm in length. The valves are obliquely ovate,
thick, and heavy. The exterior of the shell is covered with a strong,
rather smooth periostracum that ranges from light brown to grayish brown to
black. The umbones are prominent and are near the anterior end. The shell
interior is glossy white with a blue-gray tinge. The pallial sinus is small
but distinct. The posterior lateral tooth is long. Most of the variability
in form is apparently related to the differences in the height of the
umbones and the shape of the posterior margin of the shell. Rangia cuneata
var. nasutus is believed to be a rostrate form of R. cuneata and may be
confused with a closely related species, the brown rangia. The brown rangia
is 2.5 to 4.0 cm long and resembles an elongate common rangia; however,
brown rangia can be easily separated from common rangia by the short
posterior lateral tooth and the nondistinct pallial sinus. Brown rangia is
found from Louisiana to Texas and Vera Cruz, Mexico, but is much less
common than the common rangia.*74*
Spawning:
The reproductive cycle and environmental conditions necessary for spawning
are well known for common rangia. Most rangia spawned from March to May and
from late summer to November in Louisiana and from February to June and
September to November in Mexico. In both areas, spawning may be continuous.
In Virginia, gametogenesis began in early April and continued throughout the
summer; gametes were ripe from May through November. Gametogenesis was
initiated when water temperature rose to 15 degrees C, and spawning was
initiated by a rapid increase or decrease in salinity. In upstream areas of
the James River, Virginia, clams required a salinity increase of about 5 ppt
associated with reduced freshwater output, but in downstream areas they
required a salinity decrease of about 10 to 15 ppt associated with increased
freshwater output. Spawning peaked at 5 ppt in fall. In Florida, ripe
gametes and spawning were reported from July through November; spawning
peaked in September. Temperature and salinity increases were suspected of
triggering spawning.
In spawning, common rangia release gametes directly into the water. Sex
ratios were reported to be near 1:1 in Louisiana and Mexico, but females
outnumbered males in Virginia. The minimum length of mature adults in Lake
Pontchartrain, Louisiana, was 24 mm. From data on annual growth increments,
one report inferred that a clam could reach minimum length in 2 to 3 years.
In the James River, Virginia, it was reported that gonads were mature in
clams as small as 14 mm, which were probably clams in their second year of
life.*74*
Larvae and Postlarvae:
In a Louisiana study, the average diameter of eggs was about 69
micrometers. Ciliated blastula developed 8.5 hours after fertilization, a
pelagic trochophore at 26.3 h, and a veliger at 34.3 h (93 micrometers in
mean diameter). In a study conducted in Virginia, shelled larvae appeared
within 24 h after fertilization. The length of different life stages were
as follows: Straight-hinged larvae 0.75 to 130 micrometers; unbowed larvae
120 to 175 micrometers; and pediveligers (metamorphosed) 160 to 175
micrometers. Pediveligers began to settle, lose the velum, and attain gills
Life History - 1 (DRAFT) - Life History
Species rangia, common
Species Id M060007
Date 26 AUG 96
at 175 to 180 micrometers. Metamorphosis began after 7 days.
Most settling of larvae in the James River, Virginia, took place between
September and March when the animals were 230 to 500 micrometers long and
averaged 300 micrometers. A second settling period occurred in midsummer.
In Louisiana juveniles were collected as small as 375 micrometers while
another study observed several small clams attached to a hydroid colony.
How the juvenile rangia disperse is uncertain. They may be transported to
upstream areas in the more saline bottom water in an incoming tide, or by
swimming during low flow or both. One study reported that larvae were
capable of selecting substrate for setting and preferred substrates high in
organic content.*74*
Adult Activity:
Common rangia move little after settling. A 1963 study observed little
movement of clams in aquaria. It has been suggested that rangia are capable
only of vertical movement in the sediment. One study reported that clams
did not move in aquaria over a 4-month period even when given a choice of
substrates.*74*
Life Span:
The life span of the common rangia has not been confirmed. If one relates
the mean length (about 40 mm) of rangia collected in Louisiana, to estimates
of growth rate, the average life span is about 4 to 5 years. A clam of the
maximum expected length of 75 mm, would be 10 years old. A maximum life
span of 15 years has been estimated.*74*
Growth Rate and Size:
Annual growth increments of common rangia in the Gulf of Mexico are reported
to vary from 0 to 20 mm. The largest predicted length of 75.6 mm would
represent 10 years growth.
Maximum length reported was 94 mm for a common rangia from Grand Gosier
Island, Louisiana. In one study the largest clams were found in the lower
salinity areas of estuaries, whereas another study found that clam size
increases with salinity. In Virginia, Clams living in sand were found to be
typicially larger than those living in mud.*74*
Population Trends:
By far the largest concentrations of living clams are along the Louisiana
coast. The minimum standing crop of clams estimated to be between the
Atchafalaya River and Sabine Lake, Louisiana, was between 24 billion and 48
billion clams. Because of the relatively slow growth rate of rangia, it has
been suggested that no more than 5% of the living clam population should be
harvested annually if current production of fossil shells is to be
maintained; however, at an annual recruitment of 5% the estimated shell
deposits in Lake Pontchartrain would be nearly exhausted in 35 years; at 3%
another study estimated depletion in 18 years.*74*
Predators and Parasites:
Common rangia are preyed upon by fish, crustaceans, mollusks and ducks. In
addition, moon shell snails (Polinices spp.) may be predators as suggested
by drill holes in rangia shells. Common rangia are abundant in the diets of
blue catfish, freshwater drum, spot, black drum, river shrimp, and blue crab
Life History - 2 (DRAFT) - Life History
Species rangia, common
Species Id M060007
Date 26 AUG 96
in Lake Pontchartrain, Louisiana. The smaller rangia are subjected to the
greatest predation pressure. Clams as large as 40 mm (length or height),
however, are eaten by fishes such as sheepshead and black drum. A potential
group of predators are the ctenophores (i.e., Mnemioposis) which sometime
appear in tremendous numbers at certain times of the year. Ctenophores can
cause mass mortality of larvae if coincidental with rangia spawning.
The common rangia is parasitized by larvae of fellodistomatid trematodes.
Cercariae and sporocysts of this parasite are found in the gonadal tissue,
giving it an orange coloration and effecting castration. Only large clams
are infected.*74*
Competitors:
Potential competitors of common rangia may be reduced by the wide range of
salinities tolerated by this clam. Polymesoda caroliniana has feeding
habits identical to those of rangia, but is spatially separated from rangia;
it is found primarily in intertidal areas or in small numbers in the
shallow nearshore subtidal areas. In contrast, rangia live largely in the
subtidal zone. Other potential competitors are apparently not adapted to
fluctuating salinities.*74*
Density:
The density of clams varies greatly. The highest density of adult clams was
818/m2 in Lake Maurepas, Louisiana, and 238/m2 in Vermilion Bay,
Louisiana.*74*
Effects of Pollution:
Common rangia are known to concentrate chemicals such as kepone. Peak
kepone levels in common rangia during summer, in the James River Estuary,
were related to increased metabolism and feeding rate. The concentration of
kepone was 2 to 4 times greater in rangia than in the water column. The key
factors affecting kepone uptake were water temperature, dissolved oxygen
concentration, lipid index of clam tissue, turbidity, kepone concentration
in the water, and the duration of exposure. Kepone is adsorbed by
particulate matter, which enhances its uptake by filter feeders such as
common rangia. Uptake of oil related products such as benzopyrene,
naphthalenes, and various aromatic hydrocarbons has also been reported.
All of these compounds were accumulated primarily in the viscera and fat
bodies of clams under direct exposure and most were readily released when
clams were returned to clean water. Low levels of these contaminants,
however, were retained by the clams in each case. The effects of low
concentrations of contaminants on common rangia are not known.*74*
Feeding:
Feeding of common rangia is controlled by gill palp articulations and
ciliary currents over the gills. The animal extrudes pseudofeces from the
mantle cavity, through the inhalant siphon when the valves are quickly
closed.*74*
REFERENCES FOR LIFE HISTORY- 74
Life History - 3 (DRAFT) - Management Practices
Species rangia, common
Species Id M060007
Date 26 AUG 96
MANAGEMENT PRACTICES
RESULT MANAGEMENT PRACTICE
Beneficial Regulating harvest of species being described
Beneficial Regulating commercial harvest levels
Beneficial Maintaining unique or special habitat features [wetlands, caves,
REFERENCES FOR BENEFICIAL MANAGEMENT PRACTICES - 74
COMMENTS ON MANAGEMENT PRACTICES -
Clam shells are harvested by large commercial hydraulic dredges. Because of
the relatively slow growth rate of rangia, it has been suggested that no
more than 5% of the living clam population should be harvested annually if
current production of fossil shells is to be maintained; however, at an
annual recruitment of 5% the estimated shell deposits in Lake Pontchartrain
would be nearly exhausted in 35 years; other researchers have estimated
depletion in 18 years at 3%.*74*
Mortality of rangia can result from shell erosion, which can be accelerated
in highly aerated sediments in which carbonic acids are released.*74*
The substrate of some coastal waters is mainly shells which are often
dredged commercially. For example, the common rangia makes up much of the
hard substrate of Lake Pontchartrain in Louisiana. The effects of shell
dredging on the substrate and benthos are too complex and controversial to
discuss in this profile.*74*
Common rangia are known to concentrate chemicals such as kepone.*74*
Management Practices - 1 (DRAFT) - References
Species rangia, common
Species Id M060007
Date 26 AUG 96
References
74* LaSalle, M.W., A. A. de la Cruz. 1985. Species Profiles: Life
Histories and Environmental Requirements of Coastal Fishes and
Invertebrates (Gulf of Mexico) -- Common Rangia. U.S. Fish and
Wildlife Service Biol. Rep. 82(11.31) pp 16.
186 * Turgeon, D.D., A.E. Bogan, E.V. Coan, W.K. Emerson, W.G.
Lyons, W.L. Pratt, C.F.E. Roper, A. Scheltema, F.G. Thompson,
J.D. Williams. 1988. Common and scientific names of aquatic
invertebrates from the United States and Canada: mollusks.
American Fisheries Society Special Publication 16. (ed.):277.
References - 1