Species Description: Unlike many marine isopods, the mangrove boring isopod Sphaeroma terebrans and other members of family Sphaeromatidae have compact, convex bodies, giving them an appearance similar to terrestrial isopods (pillbugs). Also similar to terrestrial species, S. terebrans and most members of the family can roll into a ball (conglobulation). This represents a convergent evolution of form (species exhibit similarities in form and function but are not closely related).
Sphaeromatids to not represent a secondary marine invasion by a terrestrial isopod line (Brusca et al. 2001). Sphaeroma terebrans is reddish-brown in color and the pleotelson (the terminal body segment) is rough-surfaced and slightly pointed.
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 inhabits 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.
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 occur within the IRL.
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 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).
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.
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, which 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.
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).
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