Sphaeroma terebrans

Invasion History

First Non-native Panama (Caribbean) Tidal Record: 1985

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Panama Invasion History:

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Invasion history elsewhere in the world:

Outside the continental United States, Sphaeroma terebrans is found in Mexico (Montalvo-Urgel et al. 2010); Cuba (1994, USNM 280038, US National Museum of Natural History 2009); Belize (1980, USNM 205619, US National Museum of Natural History; Davidson et al. 2016); Costa Rica (Villalobos et al. 1985); Colon, Panama (Caribbean, Davidson et al. 2016); and Venezuela (1987, USNM 234047, US National Museum of Natural History 2009). We follow Carlton and Ruckelshaus (1997) in considering S. terebrans a likely very early introduction to the tropical Western Atlantic. However, paleontological studies of mangrove communities, and genetic studies are desirable to clarify the history and origin of this isopod.

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Description

Sphaeroma terebrans has a compact, convex, elliptical body, which is about twice as long as it is wide. The posterior-dorsal surface of the body, and particularly the pleoteloson, is covered with tubercles. The head is approximately semicircular, with prominent eyes, composed of many ocelli. Antenna 1 has a flagellum of 11 segments and extends to the posterior edge of Peraeonite 1. Antenna 2's flagellum has 16 segments and reaches the posterior edge of Peraeonite 2. The peraeonites are roughly equal in width. Peraeonites 2-4 each bear a broad transverse ridge, while Peraeonites 4, 6, and 7 each bear four large tubercles. The pleotelson is roughly triangular and covered with tubercles. Many of the tubercles bear clusters of hairs. Pereiopods 1-3 bear dense plumose setae on the upper (anterior) surface of segments 3-4, an adaptation for filter-feeding. The outer edges of the exopods of the uropods each bear five prominent teeth. Adults are 8-12 mm long and are brown-to-reddish brown. They frequently roll into balls when disturbed. This description is based on: Richardson 1905, Van Name 1936, Schultz 1969, Harrison and Holdich 1984, Kensley and Schotte 1989, Theil 1999, and Hossain and Bamber 2013.

Isopods identified as S. terebrans are widely distributed in subtropical and tropical waters, and show substantial morphological and genetic variation. Specimens from the Western Atlantic were described as S. destructor (Richardson 1897, cited by Richardson 1905). Sphaeroma terebrans is usually treated as synonymous with S. destructor (Van Name 1936; Miller 1968; Harrison and Holdich 1984), but sometimes with uncertainty (Kensley and Schotte 1989). Genetic studies (Baratti et al. 2005; Baratti et al. 2011) indicate a high degree of genetic diversity, suggesting that 'S. terebrans' is a complex of several species. However, some of the sharpest differences are between Clades A and B, both on the coast of East Africa, while the Western Atlantic clade (Florida-Brazil) is closely related to Clade B. The number of sites sampled was relatively small, compared to the large range of the species. Therefore, further genetic studies are needed to describe the genetic diversity and biogeographic history of the species.

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Taxonomy

Ecology

General:

Sphaeroma terebrans has separate sexes, with internal fertilization, brooded young and direct development. Females carry 10-80 embryos, with broods tending to increase with body size from 7.5 to 12 mm body length. Sphaeroma terebrans exhibits extended parental care, in which early juveniles remain in the parental burrow until they develop the ability to burrow and filter-feed. Females usually host 5-20 juveniles in their burrows (extremes 1-58). Juveniles in burrows were usually 2-3 mm in size. Males do not remain in the females' burrows after copulation. In the Indian River Lagoon, Florida, juveniles were scarce in January through March, and most abundant and June (Thiel 1999).

Sphaeroma terebrans occurs in warm-temperate to tropical climates, and tidal fresh to euhaline waters (Richardson 1905; Van Name 1936; Kensley and Schotte 1989; Wilkinson 2002). It bores into mangrove roots, Bald Cypress roots, and roots of fresh-brackish marsh plants (at least six different species). Borers were found in intertidal and shallow subtidal pilings, rotten wood, and Styrofoam floats (Richardson 1905; Rehm and Humm 1973; Estevez 1994; Wilkinson 2002). In more saline waters in Florida and the Caribbean, the most common substrates are the prop roots of Red Mangrove (Rhizophora mangle) (Rehm and Humm 1973; Brooks 2004; Brooks and Bell 2005; Davidson et al. 2016). This isopod is also occasionally found in branches and roots of White (Laguncularia racemosa) and Black Mangroves (Avicennia nitida). Burrow length is usually correlated with isopod size and the largest burrows reported (9-12 mm) were made by isopods 7-9 mm long. Animals from brackish-water areas had normal burrows at low (3 PSU) and medium salinities, but greatly reduced burrowing at high (30 PSU) salinity. However, effects of long-term acclimation and genetic variation were not examined (John 1970). There has been some debate as to whether S. terebrans obtains any nutrition from the wood or plant material that it consumes. A recent morphological study concluded that S. terebrans primary feeds by suspension-feeding on detritus and phytoplankton, and uses the wood primarily as shelter (Si et al. 2002). This isopod's burrow may limit predation.

A commensal isopod, Iais floridana has been found living in the burrows of S. terebrans in the Indian River Lagoon, Florida. This isopod has not been found elsewhere, but it closely resembles the Indo-Pacific I. singaporensis (Kensley and Schotte 1999). We consider I. floridana a likely introduction. Its effects on S. terebrans have not been studied, but a similar commensal, I. californica had no effects on the growth and survival of its host, S. quoianum, introduced to the West Coast (Rotramel 1975). Another, more unusual interaction with an isopod species, occurs in Florida, when juveniles of S. quadridentata invade the burrows of female S. terebrans, apparently benefiting from the other species parental care. The presence of the invading juveniles may decrease the duration of parental care for the females' own juveniles (Thiel 2000).

Food:

Phytoplankton, Detritus, Wood?

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Nontidal Freshwater	None
General Habitat	Tidal Fresh Marsh	None
General Habitat	Marinas & Docks	None
General Habitat	Coarse Woody Debris	None
General Habitat	Mangroves	None
General Habitat	Vessel Hull	None
Salinity Range	Limnetic	0-0.5 PSU
Salinity Range	Oligohaline	0.5-5 PSU
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Epibenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	8	Field, Bonnet-Carre Spillway, Lake Pontchartrain LA (Wilkinson 2002)
Maximum Temperature (ºC)	34	Fort Pierce, Florida (Field, Thiel 1999)
Minimum Salinity (‰)	0	Field (Richardson 1897).
Maximum Salinity (‰)	39	Field data, Florida Bay (Brooks 2004)
Minimum Length (mm)	9.5	Van Name 1936
Maximum Length (mm)	11.5	Female (Van Name 1936; Kensley and Schotte 1989)
Broad Temperature Range	None	Warm temperate-tropical
Broad Salinity Range	None	Fresh-Euhaline

General Impacts

In Florida and other tropical regions, Sphaeroma terebrans plays an important role by recycling dead wood (Becker 1971) and regulating the growth of mangroves (Rehm and Humm 1973; Ribi 1982; Simberloff et al. 1978; Perry and Brusca 1989; Davidson et al. 2014), and also as a borer in tidal marsh vegetation (Estevez 1994). This isopod is also an economically important borer on submerged wooden structures, such as pilings (Atwood 1920; Becker 1971; Wilkinson et al. 2002). It has been found burrowing in Styrofoam floats, and damaging them in Lake Pontchartrain, Louisiana; Colon, Panama; the Philippines and Taiwan (Wilkinson et al. 2002; Davidson 2012).

Herbivory- S. terebrans does not consume wood directly, but uses it for shelter, tunneling into it and filter-feeding, and also probably feeding on bacteria, fungi etc. growing on the tunnel walls (Becker 1971; Estevez 1945). However, a cellulase has been reported from some wood-boring Sphaeroma spp, so direct consumption of wood cannot be ruled out (Becker 1971). A more recent morphological study concludes that S. terebrans primary mode of feeding is suspension-feeding on detritus and phytoplankton (Si et al. 2002). Sphaeroma terebrans burrows into the aerial roots of Rhizophora mangle (Red Mangrove) (Humm and Rehm 1973), hollowing them out, and also into the rhizomes of Juncus roemerianus (Black Needle Rush) plants in tidal marshes (Estevez 1994).

Habitat Change – S. terebrans clearly has an important role in the dynamics of Rhizophora mangle (Red Mangrove) communities, but the precise nature of its effects on mangrove communities has been subject of study and debate. Initially, it was regarded as a destroyer of mangroves (Humm and Rehm 1973), but S. terebrans were also found to increase the frequency of root branching (Simberloff et al. 1978) and to differentially infest roots farther from established roots, resulting in increased density of roots near the stand (Ribi 1982). However, Perry and Brusca (1989), working with the similar Pacific S. peruvianum, found that the mangrove's responses to herbivory did not offset the loss of productivity due to Sphaeroma's burrowing. Sphaeroma terebrans effects on marsh vegetation could also encourage marsh erosion (Estevez 1994).

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