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Global oyster populations have declined an estimated 85% since the 1900s (Coleman, 2014). A significant driver of oyster mortality at this time are boring sponges, a set of sponge species that use oysters shells and other calcareous materials as a habitat. Boring sponges secrete acids that wear down oyster shells and use drill-like movement to create holes that the sponges inhabit. While the boring sponges gain no nutrients from their hosts, it is possible that boring into oysters protects the sponges from predation. However, the presence of boring sponges places oysters at a higher risk of predation and can result in stunted growth and early mortality. The boring sponge Cliona celata, sometimes called the red boring sponge, poses a particular threat to Crassostrea virginica, the dominant oyster species on the North Carolina coast.
The main threats boring sponges pose to oysters are structural and developmental damage. As sponges bore into oysters, they make oyster shells brittle by dissolving calcium carbonate and forming holes. The weakened shell can make oysters more vulnerable to predation (Coleman,2014). For example, Watts et al (2018) found that pea crabs, another species that uses oysters as a host and decreases oyster health, are more commonly found in shells infested by boring sponges. They hypothesized that this is because the weakened shells of boring sponge infested oysters are easier for pea crabs to get into. The presence of both pea crabs and boring sponge resulted in swifter oyster death compared to the presence of just one of the two. To compensate for the weakening of their shells, oysters produce additional layers of shell to protect the inner cavity that holds their organs. This deforms the shape of the inner cavity and reduces the amount of space the oyster’s organs can occupy, which is developmentally costly. The energetic cost of allocating resources to protecting the inner cavity can stunt overall growth. These developmental, energetic, and structural costs caused by boring sponges collectively contribute to premature oyster death.
In addition to threatening oyster structure and development, boring sponges can negatively impact the market for oysters. Scientific accounts of the negative impact of boring sponge on the oyster market exist back to 1951. 25-30%of farmed oysters have severe boring sponge infestation. The holes left by sponges make oysters less visually appealing and thus more difficult to sell. Decomposing sponges release a sulfurous odor that deters buyers. Damaged oyster shells are often brittle and harder to open. The shells may crumble upon opening, making them dangerous to eat. As little as a 1-2 cm hole near where a knife would be inserted to open an oyster can render it unmarketable (Carver et al, 2010). The defense mechanism of the oyster, creating new layers of shell and reducing the inner cavity size, also reduces the amount of meat available.This is also unappealing to buyers (Carroll et al, 2015). Though oyster harvest is implicated in the decline of oyster populations, healthier oysters means fewer must be discarded, and fewer need to be harvested. For this reason, reduction of boring sponge infestation should be a primary concern for oyster sellers and consumers.
Future climate predictions indicate an increase in temperature and ocean acidification, which may exacerbate boring sponge’s impact on oysters. A more acidic environment might weaken oyster shells by partially dissolving their calcium carbonate shell. This would make it easier for boring sponges to make holes in oysters’ shells. Duckworth and Peterson (2012) tested how boring sponges behave under the predicted ocean temperature and acidity for the year 2100. They found that the sponges bored into oysters twice as fast than they do under current oceanic conditions. While the 2100 conditions killed approximately 20% of the boring sponges, the remaining 80% destroyed oyster shells at an above average rate. Without rapid action, climate change can and will worsen the severity of the boring sponge threat.
Oyster reef restoration is an increasingly common method of oyster conservation, and boring sponges must be considered in restoration efforts. Reducing boring sponge presence in restored reefs is especially important because oyster larvae are significantly less likely to settle in spaces already inhabited by boring sponge (Barnes et al, 2010). Typical oyster reef restoration practices use a base of recycled oyster shells, which oyster larvae settle on and grow into anew reef. However, Dunn et al (2014) theorized that using a calcareous base may increase the rate of boring sponge predation. They conducted an experiment in which they compared restored reefs with calcareous bases (oyster shell and limestonemarl) to those with non-calcareous bases (concrete and gravel). At high and low salinities, the reef base didn’t impact rates of boring sponge damage. In low salinity environments, boring sponges were not naturally present at high enough levels to cause significant damage. At high salinities, other predators such as the Atlantic oyster drill preyed on oysters quickly enough to cause low overall restoration success. However, at mid salinities boring sponge preferentially colonized the calcareous bases, which allowed oyster larvae on the non-calcareous bases to grow more before showing signs of sponge damage. In future oyster reef restoration projects, the type of base used and the salinity level of the selected area may be worth considering to boost restoration success.
Though boring sponges have long been a natural adversary to oyster survival, they are of specific concern now because of changing climate conditions that may worsen their impact on oysters. Given that the causes of rapid climate change are largely anthropogenic, it is our responsibility to minimize the damage. As of now, there are no methods to completely prevent boring sponge colonization of natural reefs, but reducing sponge colonization of restored reefs may be possible. Choosing a non-calcareous base and mid-level salinity appears likely to slow sponge colonization. More research into methods of controlling boring sponge populations is necessarily in the short term. A long term solution to boring sponge damage should include regulations on the anthropogenic activities that cause increased ocean temperatures and acidity. Without action to combat the effects of boring sponge colonization, oyster reefs are at risk of significant population decline.
Post by Cady Bailey, Biodiversity team
References
Barnes, Brian B, et al. “Oyster reef community interactions: The effect of resident fauna on oyster (Crassostrea spp.) larval recruitment.” Journal of Experimental Marine Biology and Ecology, vol. 291, August 2010, pp.169-177. https://doi.org/10.1016/j. jembe.2010.06.026
Carroll, John M, et al. “Are Oysters Being Bored to Death? Influence of Cliona Celata on Crassostrea virginica Condition, Growth and Survival.” Diseases of Aquatic Organisms, vol. 117, no. 1, 17 Nov. 2015, pp. 31–44.,doi:10.3354/dao02928.
Carver, Claire E, et al. “Infection of Cultured Eastern Oysters Crassostrea virginica by the
Boring Sponge Cliona celata, with Emphasis on Sponge Life History and Mitigation Strategies.” Journal of Shellfish Research, vol. 29,pp. 905-915. 1 December 2010. https://doi.org/10.2983/035.029.0423.
Coleman, Sara E. “The Effects of Boring Sponge on Oyster Soft Tissue, Shell Integrity, and Predator-Related Mortality .” The Effects of Boring Sponge on Oyster Soft Tissue, Shell Integrity, and Predator-Related Mortality, 2014.
Duckworth, Alan R., and Bradley J. Peterson. “Effects of Seawater Temperature and pH on the Boring Rates of the Sponge Cliona Celata in Scallop Shells.” Marine Biology, vol. 160, no. 1, 14 Sept. 2012, pp. 27–35.,doi:10.1007/s00227-012-2053-z.
Dunn, Robert P, et al. “Effects of Substrate Type on Demographic Rates of Eastern Oyster (Crassostrea virginica).”Journal of Shellfish Research, vol.33, no. 1, 1 April, 2014, pp 177-185. https://doi.org/10.2983/035.033.0117.
Watts, Jessica C, et al.“Examination of the Potential Relationship between Boring Sponges and Pea Crabs and Their Effects on Eastern Oyster Condition.” Diseases of Aquatic Organisms, vol. 130, no. 1, 28 Aug. 2018, pp.25–36., doi:10.3354/dao03257.
