Sphaeroma terebrans

Sphaeroma terebrans is a mangrove-boring isopod that was first documented in the United States as early as 1897. It is 8–10 millimetres (0.31–0.39 in) long, and is thought to have been introduced by wooden-hulled ships. The isopod is found throughout the Gulf of Mexico mainly in mangrove swamps of Louisiana and Florida. S. terebrans will also bore into boats, wooden pilings and other wooden structures.

The burrowing activities of Sphaeroma terebranshinder the growth of mangroves, and its wood boring activities limits mangroves to the upper limits of the intertidal zone.

Introduction & Identifying S. Terebrans
Sphaeroma terebrans is a wood-boring mangrove isopod that was found native to the Indo-Pacific region. It was first described when it was found in Brazil by Spence Bates in 1866. It was previously called  S. Invastator and S. Destructor by both Bates and another researcher named Richardson. The first known North American record of S. terebrans occurred in 1881 in Crescent City, Florida. It is generally described to be a reddish-brown color with a semicircular head. It is very long in shape and is compact. Owing to phenotypic plasticity, it is difficult to estimate the population of the species because of its similar characteristics to other species. Phenotypic plasticity is where the genotypes of the species alter due to external environmental conditions creating different phenotypes. However, it is possible to use the DNA barcode marker for the identification of S. terebrans, which is the mitochondrial COI gene. It is also known that S. terebrans has 13 protein-coding genes with 3 stop codons and 6 start codons. Gene flow levels are low and diversity levels are very high in environments close to each other and ones that are very similar to each other. A couple of major characteristics for identifying S. terebrans are the shape of the pleotelson and the arrangement of tubercles on the pereonite.

Location & Invasiveness
The species is known for invading areas with warm climates and subtropical and tropical waters. Regions include  Africa (Nigeria, South Africa), Florida, Venezuela, Cuba, and the Panama Canal. They are typically found in the intertidal zones of the red mangrove. The mangroves are often separated from each other by small shores, rocks, or sand giving them the appearance of small islands. These isopods are often introduced unintentionally via ships. They were primarily found in mangroves but have also made appearances in salt marshes, rotten wood, man-made structures, and free-hanging aerial roots in some regions. Even though the mangrove and the isopod have a non-herbivorous relationship,  there were several consequences and advantages for the mangroves and the surrounding environment. A positive ecological change for S. terebrans making it invasive, is that it helps to regulate the growth of the mangroves and aids in recycling dead wood, for S. terebrans own benefit. However, the isopod has also been linked to catastrophic damage to the mangrove  such as reduced root production, increased root atrophy, and nutrient deficiencies. The damage was described as ecologically catastrophic, meaning there is a natural imbalance due to intensive destruction. For example, damage for Red mangroves went from having approximately 20% damage of the roots to having 90% damage of the roots within a few years between the two statistics. However, these mangroves can combat severe levels of damage and breakage by putting energy and nutrients into growing lateral roots near the burrows to create a stable root structure. This process may even shape the mangrove islands in the intertidal zones as observed by Estevez in 1978. S. Terebrans is also found burrowed in Limnoria species. Overall, the wood properties impact the colonization patterns of S. terebrans with hardness, density, and elasticity. For example, when the wood density increases, the burrowing occurrence decreases due to difficulty chewing.

Anatomy & Physiology
Its body is described to be composed of a head, chest with seven pairs of thoracic limbs, and abdomen with five pairs of pleopods. These pairs of pleopods are used for both respiration and swimming. Each of the pleotelsons is covered with tubercles and the head of the S. terebrans is semicircular and its eyes are made of ocelli S. terebrans have 2 antennas, each with a different structure. Antenna number 1 has an 11-segment flagellum that goes to the first Peraeonite. Antenna number 2 has a 16-segment flagellum and extends to the posterior side of the second Peraeonite. These antennas are to aid with sensing its surroundings, such as temperature and sounds. The following anatomy of S. terebrans is in charge of burrowing: mandibles, cephalon, pleotelson, pleopods, and pereonites.

Characteristics
Though location-dependent,  S. terebrans have an average length of 8–12 mm and an average weight of 0.0270 g, making it a really small species and thus easily adaptable to different areas. They are usually a reddish-brown color and are sexually identifiable at around 3.5 nm in length. Overall, the entire life cycle is in the same mangrove. They rarely ever change their burrow sites. This fact allows for the S. terebrans to have a positive growth of the population. According to S. terebran's gut health analysis, they often opt for a detritivore diet. A detritivore diet includes feeding on decomposing organic waste that provides all nutrients needed. Fungi and wood particles were not found in gut health analysis, but they do filter feed as well.

Burrowing
Burrowing into the wood, usually the mangroves, creates one that is 2–3 mm long in size and is the first step for most processes in their life cycle. However, the total length of burrows is dependent on the size of the isopods creating it. S. terebrans utilize their mandibles, cephalon, pleotelson, pleopods, and first two pereonites to create the burrow and evacuate unnecessary wood chips. These burrows provide protection from abiotic - non-living chemical and physical environmental factors -  and biotic factors - living environmental factors. The female uses her telson to block the entrance to protect the juveniles immediately after they emerge from their bodies. She uses pleopods to create a flow of water and oxygen into the burrow, meaning that one of the ends of the burrow needs to be able to have flowing water. If there is no water at at least one end of the burrow, then the S. terebrans isopod will start a burrow in a new location.

Reproduction
Sphaeroma terebrans utilize an internal fertilization system, meaning that fertilization occurs in the individual isopods rather than outside of the body. However, the sperm transfer from male to female is indirect using spermatophores in water currents to transport it to the females. After the burrow has been made, as described in the burrowing section, males enter the burrow to mate with the females right before leaving and moving on to their next location. Mating occurs around the time of partial molting. Maturation of the fertilized eggs occurs when they are kept in the marsupium of the oostegites (abdominal segments). Overall, the best breeding times were when the environment has high oxygen and nutrient levels, especially after rainfall, and the temperature is around 75o Fahrenheit. Post fertilization, the embryos show as egg-shaped. Over time, the limb buds appear and the embryo takes on a comma-shaped appearance with a soft, white exoskeleton. Females are able to carry up to eighty embryos during one fertilization term but only host up to twenty juveniles in the burrow. The number of embryos the female carries is dependent on the length of her body. Two broods may be produced by one female throughout the year, one in the fall and the other in the winter-to-spring transition period. It is more likely for the females born in the summer to produce in the fall session. The juveniles are kept in the burrows for a short time, usually a minimum of forty days. Through careful observations and research, embryo mortality in the development stages was not indicated.

Research
Often, Sphaeroma terebrans are studied in the Indian and Atlantic Ocean populations as they originate from those regions and create a baseline for further research in the invasive regions. They are often tracked and investigated by their physical barriers during the time of the year and their life cycle and reproduction patterns. According to Baratti, et al., one piece of current research includes observing genetically different populations and the connections between spatial scale of this grouping and dispersal abilities. Owing to little to no physical barriers, the marine populations in this study demonstrated low genetic structure and high dispersal capabilities. When observing S. terebrans reproductive patterns in captivity, it was determined that S. terebrans needs the environment temperature to be around 24o Celsius (75.2o Fahrenheit) for the best result of reproduction occurring within 2–4 weeks. S. terebrans experiments determined that salinity levels directly impact the reproduction and burrowing rates. Salinity refers to the overall salt levels of the environment or fluid involved in the experiment. The prime salinity range discovered was 4-28 parts per thousand for the best reproduction, burrowing, and growth rates. Although S. terebrans have a preferred temperature for breeding, scientists in Veli Lake, Kerala, India, with water temperatures of 27.4o Celsius (81.32o Fahrenheit) to 33.2o Celsius (91.76o Fahrenheit) found a negative correction regarding breeding and temperature. However, there was a positive correlation between breeding and nutrients; where nutrients were abundant, more breeding occurred. Through various research, the use of the COI gene as a DNA barcode is prevalent in current and plans for future research as it can be difficult to differentiate between S. terebrans and other Sphaeroma isopod species.