Species sensitivity assessment of five Atlantic scleractinian coral species to 1-methylnaphthalene

Coral reefs are keystone coastal ecosystems that are at risk of exposure to petroleum from a range of sources, and are one of the highest valued natural resources for protection in Net Environmental Benefit Analysis (NEBA) in oil spill response. Previous research evaluating dissolved hydrocarbon impacts to corals reflected no clear characterization of sensitivity, representing an important knowledge gap in oil spill preparedness related to the potential impact of oil spills to the coral animal and its photosymbiont zooxanthellae. This research addresses this gap, using a standardized toxicity protocol to evaluate effects of a dissolved reference hydrocarbon on scleractinian corals. The relative sensitivity of five Atlantic scleractinian coral species to hydrocarbon exposure was assessed with 48-h assays using the reference polycyclic aromatic hydrocarbon 1-methylnaphthalene, based on physical coral condition, mortality, and photosynthetic efficiency. The threatened staghorn coral Acropora cervicornis was found to be the most sensitive to 1-methylnaphthalene exposure. Overall, the acute and subacute endpoints indicated that the tested coral species were comparatively more resilient to hydrocarbon exposure than other marine species. These results provide a framework for the prediction of oil spill impacts and impact thresholds on the coral animal and related habitats, essential for informing oil spill response in coastal tropical environments.

Species sensitivity assessment of five Atlantic scleractinian coral species to 1methylnaphthalene *D. Abigail Renegar 1 , Nicholas R. Turner 1 1 Nova Southeastern University, Halmos College of Natural Science and Oceanography Dania, FL, USA *drenegar@nova.edu Semi-quantitative scoring matrix for the assessment of sublethal stress in corals To estimate sub-acute effects, specific physical characteristics of each coral are semiquantitatively scored. The scoring system assesses relative deviations from the normal state of each species; initial range-finding experiments are very useful to establish the full spectrum of possible or expected effects in each category to the test species of interest.
The physical characteristics scored include color, polyp extension/retraction, tissue swelling/distension, tissue attenuation, and mucus production. These observable, physical characteristics are not independent, as changes to one characteristic may or may not reflect alterations in another. Each of the five characteristics are scored on a scale of 0 (normal limits) to 3 (severely affected), with a precision level of 0.5 (thus half-scores are permitted) (Supplemental Table 1). The maximum score for an individual coral is 12; tissue swelling and tissue attenuation are scored separately but are considered as one category, as the two criteria are opposite responses to stress (a coral which receives an attenuation score of 3 cannot have a swelling score greater than 0, as there is no tissue to swell). To determine the EC50, the scores for each criterion were summed and divided by the total maximum score possible (12) to obtain a single percent effect for each coral fragment at each time point.
Color: Scores for changes (loss) in coral color were used to assess relative lightening (or bleaching) of coral fragments. Due to inherent variation in coral fragment color at the beginning of each exposure, this score was based on the lightening of color of each fragment relative to the fragment's initial color. A score of 0 was given to a coral which has maintained its coloration, and a coral with slight lightening on color received a score of 1. A score of 2 represents a coral that is moderately bleached, and a score of 3 indicated significant bleaching/loss of color. To score color correctly, the corals must be evaluated and photographed under the same lighting conditions at each time point, and photos taken before and after the exposure for verification must be consistently white balanced.
Polyps: Polyps which were normally extended or slightly retracted received a score of 0. Polyps which were retracted and partly closed received a score of 1; this score would be representative of a "startled" polyp. Great care must be taken during scoring to avoid disturbing the corals and therefore causing a polyp score not related to exposure conditions. Fully closed polyps received a score of 2, and those with very tightly retracted polyps received a score of 3. It is important to note that the degree of polyp retraction associated with scores of 2-3 was not readily reversible; the polyps were completely contracted/recessed and no tentacular tissue was visible. This extreme retraction represented a significant disruption in normal coral behavior, and frequently included exposure of skeletal elements around the polyp; corals with high polyp scores often had very little tissue outside of the corallites. Thus, severe polyp retraction could be accompanied by tissue attenuation. • 0 (normal): normal mucus production; no mesenterial filaments apparent • 1 (mild): slightly elevated mucus production, no mesenterial filaments apparent • 2 (moderate): moderately elevated mucus production; mesenterial filament extrusion possible • 3 (severe): mucus sheets evident; possible mesenterial filament extrusion Tissue swelling: A tissue swelling score of 1 indicated a slight localized or diffuse swelling, usually of the coenenchyme. More extensive coenenchyme swelling and/or a greater degree of polyp distension received a score of 2, and extreme swelling of coral tissues and/or extreme polyp distension received a score of 3. Scores of 2-3 often indicated variable severity of diffuse swelling throughout the coenenchyme of the entire coral. In some cases, severe swelling manifested as large bubbles under localized areas of tissue, engulfing the coenenchyme and polyp. These "bubbles" frequently resulted in eventual tissue lysis of the affected area.

Supplemental
Tissue attenuation: A minor thinning of the coenenchyme, and/or slight recession of tissue from corallite ridges resulted in a score of 1. A greater degree of thinning, typically accompanied by clear visualization (but not exposure) of skeletal elements received a score of 2. Severe tissue thinning was characterized by exposure of skeletal elements in the coenenchyme and/or the polyp (sclerosepta and columella exposed within polyps) and was given a score of 3.
Mucus production: All corals continually produce some amount of mucus; an amount produced somewhat greater than this level received a score of 1. Low mucus scores were indicative of a localized mucus release, or a thin halo that was observable with backlighting. A score of 2 was given to corals producing more significant amounts of mucus, with more visible mucus strings or thin sheets extending upward from the coral. Partial mesenterial filament extrusion was possible but not frequently observed. Corals producing copious amounts of mucus, in thick sheets or with pools of mucus around the base of the coral received a score of 3. Again, mesenterial filament extrusion was possible but not typically seen in these exposures.
Mucus production was observed to have a high temporal variability, depending on the toxicant concentration. Low concentrations usually resulted in slower, consistently elevated mucus secretion that occurred over the duration of the exposure. High concentrations resulted in copious mucus production early in the exposure which then decreased later in the exposure, likely related to a depletion of energetic resources used to support mucus production. Mucus secretion was therefore not a reversible/irreversible stress, but rather a time/concentration dependent depletion of organism resources. For this reason, mucus scores that reached a high-to severely-impacted level (2.5-3) were fixed at subsequent time points, even if mucus production subsequently decreased as a result of tissue degradation or necrosis.