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Potentially Misidentified Species:
At least two additional Sphaeroma species also occur in Florida, S.
walkeri (also a Florida non-native) and S. quadridentatum. Distinguishing
these species based only on appearance is beyond the abilities of non-experts,
although habitat preference information may provide a partial remedy. Nelson
and Dematriades (1992) indicate that sabellariid Phragmatopoma lapidosa
wormrock reefs are a preferred habitat for S. walkeri in the IRL region
of Florida. S. quadridentatum reportedly does not burrow, instead
opportunistically inhabiting crevices it finds (Thiel 2000).
Florida is also home to related wood-boring isopods known as mangrove gribbles
belonging to genus Limnoria. Limnoria species tend to be slightly smaller than
S. terebrans.
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
The individuals from which Sphaeroma terebrans was first described were
collected in India, although the species now occurs in several mangrove forest
systems worldwide including Australia, Sri Lanka, east Africa, South Africa,
Costa Rica, Brazil, the eastern (north to South Carolina) and Gulf regions of
the United States, and elsewhere (Animal Encyclopedia: Sphaeroma terebrans). In addition
to burrowing into living and dead wood, S. terebrans can burrow into other
hard substrata such as hard packed sand (Ray 2005).
Carlton and Ruckelshaus (1997) indicate that the species has occurred in Florida at least as far back as 1897. Collection information for
elsewhere in the U.S. is incomplete, but reports indicate S. terebrans
also occurs in Chesapeake Bay (SERC) and in Lake Pontchartrain, Louisiana, where it is found in littoral
cypress trees (Poirrier et al. 1998, Wilkinson 2004).
IRL Distribution:
In the IRL, Sphaeroma terebrans is found primarily in burrows excavated from
the aerial roots of the red mangrove (Rhizophora mangle) although it can
also make burrows in fallen trees and in the roots of other species [Animal
Encyclopedia]. The species likely occurs wherever red mangroves occurr within the
IRL.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
The average size of female S. terebrans in the Indian River Lagoon is
reported to be 8-10 mm for females and 6.5-8.5 mm for males and the lifespan is
approximately 10 months (Thiel 1999).
Abundance:
Mangrove boring isopods can be extremely abundant within their wood burrow
habitats. Poirrier et al. (1998) recorded densities of greater than 500
individuals per cubic decimeter of infested cypress wood in Lake Pontchartrain.
Where they occur in Florida, S. terebrans can also be widespread. A
survey conducted in Tampa Bay by Brooks and Bell (2005) indicated that 25%-86%
of the R. mangle aerial roots examined were occupied by the isopods.
Reproduction:
Reproduction is sexual and occurs within the aerial root burrows excavated by
the animals (Thiel 1999).
Thiel (1999) found reproductive individuals year-round in the Indian River
Lagoon but noted twin reproductive peaks occurring in the fall and again in the
late spring/early summer. Reproduction occurs in a manner that is unlike that
known from other isopods. Mating in most isopods involves internal
fertilization by means of a specialized male reproductive structure known as
the appendix masculina. Male S. terebrans lack this organ, however,
and instead release sperm external to the female during mating and rely on water
currents set up by the beating of the female pleopods to carry sperm into the
genital opening (Messana 2004).
Males typically abandon females after copulation and do not participate in extended care
(see below) of the offspring (Thiel 1999).
Embryology:
Embryonic development occurs within the mother and early juveniles emerge fully
formed. There is a degree of parental care in the species (short compared to
other peracarid crustaceans), with female S. terebrans commonly hosting
their offspring for a period of time in family burrows within mangrove aerial
roots (Thiel 1999, 2000). Reproductive females in the IRL typically hosted
5-20 juveniles in their burrows during this stage (Thiel 1999).
IV. PHYSICAL TOLERANCES
Temperature:
As an inhabitant of intertidal mangrove aerial roots, Sphaeroma
terebrans appears capable of enduring reasonably wide daily and seasonal
variations in temperature. Individuals may occasionally experience lethal
winter low temperatures, as reported in 1996 in Lake Pontchartrain LA, for
example (Poirrier et al. 1998).
Since the distributional ranges of the tropical-subtropical mangrove tree
species that serve as the primary hosts of this isopod are themselves
temperature-limited, S. terebrans are probably only exposed to lethal
low temperatures at their latitudinal distribution limits.
Salinity:
Poirrier et al. (1998) relate work of authors from India indicating
Sphaeroma terebrans is extremely euryhaline. Lethal salinities occurred
below 0.5 ppt and above 50 ppt, although a somewhat more narrow range between 4
ppt and 28 ppt was reported as optimum for growth and reproduction. Boring
activity was shown to decrease with sudden salinity increase. Poirrier et al.
(1998) indicated that S. terebrans was abundant in littoral
cypress trees and other wooden structures in low salinity (0.5- 5 ppt) Lake
Pontchartrain waters.
V. COMMUNITY ECOLOGY
Trophic Mode:
Despite their wood boring habits Sphaeroma terebrans has long been assumed to
be a filter feeder or a grazer of the epiphytic material that grows on burrow
walls (Poirrier et al. 1998). Recent morphological studies of the mouthparts
and gut support the contention that S. terebrans is primarily a filter
feeder (Si et al. 2002). This is in contrast to wood boring isopods of genus
Limnoria who consume the wood they excavate as their principle food
source.
A laboratory feeding study by Benson et al. (1999) complicates the established
view somewhat by indicating that juvenile S. terebrans survive on a diet
of pure cellulose significantly longer than individuals given no food. The
authors also present enzyme assay analyses and electron microscopy findings
further indicating that S. terebrans can use wood as a food source. The
relative importance of wood in the natural diet of the species remains unknown.
Associated Species:
In addition to the close association of Sphaeroma terebrans and its preferred
host habitat, the red mangrove Rhizophora mangle in the Indian River Lagoon,
Thiel (2000) reports that juveniles of the Sphaeroma congener S.
quadridentatum may be found living within family burrows of reproductive
female S. terebrans.
VI. INVASION INFORMATION
Invasion History:
Sphaeroma terebrans was introduced to the United States more than a
century ago. Carlton and Ruckelshaus (1997) cite an 1897 description by H.
Richardson as the first evidence of this species occurring in Florida coastal
waters. The early date of introduction and the wood-boring habit of the
species suggest the species arrived on or in the hulls of wooden sailing ships
(ERDC 2005).
Carlton and Ruckelshaus (1997) report that in the western Atlantic, S.
terebrans now occurs from Brazil north into South Carolina and from Liberia
to the Congo in the eastern Atlantic.
Potential to Compete With Natives:
There is considerable debate as to how ecologically damaging Sphaeroma
terebrans boring is to host mangrove trees in Florida. Early studies
(e.g., Rehm and Humm 1973) suggested that boring damage caused by S.
terebrans to R. mangle prop root tips was sufficient to destroy the prop
root and was the underlying reason for shrinking fringing mangrove habitats
observed at the time. Some evidence for the ability of S. terebrans to
damage the mangrove Rhizophora mucronata in east Africa is also
presented by Svavarsson et al. (2000).
Simberloff et al. (1978)
presented an opposing viewpoint that the S. terebrans wood boring may
actually be beneficial to red mangroves by promoting increased branching of
aerial prop roots that allow the trees to better withstand wave action. These
authors conclude that wood borers may benefit or harm host plants and the true
impacts may be difficult to assess. Brooks and Bell (2002) reported that while
some lateral root production occurred in response to S. terebrans
boring, the most common response was repair of damaged root tissue rather than
the production of new lateral roots.
Possible Economic Consequences of Invasion:
Negative ecological impacts of Sphaeroma terebrans boring on mangrove health
would likely carry economic consequences as well, owing to the importance of
mangroves both as nursery and refuge habitat as well as their role in preventing shoreline erosion.
In addition to natural wood habitats, S. terebrans burrows in wooden
boats, piers, pilings, and bridges which can result in negative economic
consequences (Poirrier et al. 1998).
VII.
REFERENCES
Benson L.K., Rice S.A., and B.R. Johnson. 1999. Evidence of cellulose digestion
in the wood boring isopod Sphaeroma terebrans. Florida Scientist
62:128-144.
Brooks R.A. and S.S. Bell. 2002. Mangrove response to attack by a root boring
isopod: root repair versus architectural modification. Marine Ecology Progress
Series 231:85-90.
Brooks R.A. and S.S. Bell. 2005. The distribution and abundance of Sphaeroma
terebrans, a wood-boring isopod of red mangrove (Rhizophora mangle)
habitat within Tampa Bay. Bulletin of Marine Science 76:27-46.
Brusca R.C., Coelho V., and S. Taiti. 2001. A guide to the coastal isopods of
California. Part of the
Tree of Life Web Project.
Carlton J.T., and M.H. Ruckelshaus. 1997. Nonindigenous marine invertebrates
and algae. Pp 187-201 in: Simberloff, D., D.C. Schmitz, T.C. Brown (eds),
Strangers in Paradise. Island Press, Washington, D.C.
Carlton J.T. and E.W. Iverson. 1981. Biogeography and natural history of
Sphaeroma walkeri Stebbing (Crustacea: Isopoda) and its introduction to
San Diego Bay, California. Journal of Natural History 15:31-48
Fuller P. 2007. Sphaeroma walkeri. USGS Nonindigenous Aquatic Species Database,
Gainesville, FL. Available online.
Messana G. 2004. How Can I Mate Without An Appendix Masculina? The Case of
Sphaeroma terebrans Bate, 1866 (isopoda, Sphaeromatidae). Crustaceana
77:499-505.
Nelson W.G. and L. Demetriades. 1992. Peracarids associated with sabellariid
sorm rock (Phragmatopoma lapidosa Kinberg) at Sebastian Inlet, Florida,
U.S.A. Journal of Crustacean Biology 12:647-654.
Poirrier, M.A., C.D. Franze and S.M. Arthur. 1998. The Occurrence of the Wood
Boring Isopod, sphaeroma terebrans, in Littoral Cypress of Lake Pontchartrain
and Lake Maurepas. Abstract of paper presented at the 1998 Basics of the Basin
research symposium. Available online.
Ray G. 2005. Invasive animal species in marine and estuarine environments:
Biology and ecology. US Army Corp of Engineers Engineer Research and
Development Center document ERDC/EL TR-05-02. 64 p.
Richardson H. 1897 Description of a new species of Sphaeroma.
Proceedings of the Biological Society of Washington 11:105-107.
Si A., Bellwood O., and C.G. Alexander. 2002. Evidence for filter-feeding by
the wood-boring isopod, Sphaeroma terebrans (Crustacea: Peracarida).
Journal of Zoology 256:463-471.
Svavarsson J., Osore M.K., and Olafsson E. 2000. Does the wood-borer
Sphaeroma terebrans (Crustacea) shape the distribution of the mangrove
Rhizophora mucronata? Ambio 31:574-579.
Thiel M. 1999. Reporductive biology of a wood-boring isopod, Sphaeroma
terebrans, with extended parental care. Marine Biology 135:321-333.
Thiel M. 2000. Juvenile Sphaeroma quadridentatum invading
female-offspring groups of Sphaeroma terebrans. Journal of Natural
History 34:737-745.
Laura Lee Wilkinson. 2204. The biology of Sphaeroma terebrans in Lake
Pontchartrain, Louisiana with emphasis on burrowing. Unpublished thesis,
University of New Orleans.
Report by:
J. Masterson, Smithsonian Marine Station
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