Abstract:
This dataset includes results from laboratory experiments with oil, sediment and fiddler crabs of the genus Uca, to assess whether their burrowing (and potential for bioturbation) is affected by exposure to oil. Experiments are focusing on survival and burrowing activity for two different species of fiddler crab.
Suggested Citation:
Franco, M., Klerks, P.. 2017. Effects of oil on two species of fiddler crabs (Uca longisignalis and Uca panacea). Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N76Q1VNR
Data Parameters and Units:
Burrowing activity: Day, Tank, Species, Oil concentration (mg/cm^2), Number of burrows, Sex, Individual, Carapace width (mm), Burrow diameter (mm), Extract volume (mL), Absorbance for TBARS, TBARS amount (nmoles), TBARS concentration (nmoles/mL), Absorbance in protein assay, Protein concentration (mg/mL), TBARS concentration (nmoles/mg protein), burrow depth (cm), amount of sediment excavated (g dry wt.), estimated sediment volume removed (cm3)
Methods:
Fiddler crabs were collected at the coastal marshes and sand banks of Elmer’s Island and Grand Isle State Park in Grand Isle, Louisiana. Two indoor large holding tanks, one for each of the species, were filled with sediment and water from the collection sites. Each tank was filled using one 10-gallon bucket of sediment and approximately 5 liters of water. Crabs were kept in these tanks at the University of Louisiana at Lafayette Ecology Center. The species collected were identified using the guidelines reported by Crane (1975). Experiments were conducted with Uca longisignalis and U. panacea. To evaluate burrowing behavior and oxidative stress, three experiments were conducted using 10-gallon aquaria (20”L x 10”W x 12”H) placed next to each other in two rows. The first two experiments, one for each of the species, used 15 mesocosms. The third experiment included both species at the same time and it required 18 mesocosms. The rationale for this third experiment was to avoid differences from environmental factors and to provide a direct comparison between species since the other experiments (one for each species) were carried out at different times. The aquaria used in all of the experiments were filled with approximately five gallons of sediment from the collection sites in Grand Isle, LA. Sediment was added to each tank making a shallow slope of about 22° from one side (20 cm high) to the other (base of the tank). For Uca longisignalis, sediment consisted of a mixture of approximately 1.5 gallons clayey sediment and 3.5 gallons sandy sediment with a total organic content of X%. For Uca panacea, 5 gallons of sandy sediment were used with a total organic content of X%. Water with an average salinity of 3.5 ppt was added to the side with low sediment to a maximum height of 6.5 cm. Fiddler crabs are semi-terrestrial organisms; thus, this design allowed the addition of water without completely covering the sediment surface as wet sediment impedes burrowing construction. Five different concentrations of Louisiana Sweet crude oil (GoM Marlin Platform Dorado well) were used in experiments where species were individually evaluated. The concentrations were: 0, 8, 20, 40, 55 mg of oil per cm2 of sediment surface for Uca longisignalis, and 0, 5, 15, 25 and 35 mg of oil per cm2 of sediment surface for Uca panacea. For the third experiment, only three concentrations of crude oil were used and they were selected based on the results from the previous two experiments. These concentrations were: 0, 10 and 35 mg of oil per cm2 of surface sediment. Crude oil was mixed with 100 grams (wet weight) of sediment before being added to each mesocosm. The oil-sediment mixture was evenly spread over the sediment surface avoiding direct contact with water in the aquarium. Three replicates for each concentration were established in each of the experiments. Three male and three female fiddler crabs were placed in each mesocosm for individual species and three males and two females were used per mesocosm in the third experiment. The carapace size (mm) of each fiddler crab was measured before adding them to the mesocosms. Fiddler crabs in each mesocosm were marked with a different paint color on their carapaces to keep track of them during an experiment. Each experiment lasted for 10 days, and daily monitoring was done to record sediment temperatures in all mesocosms and to add water when it was necessary. Crabs were removed from mesocosms at the end of the exposure period, placed individually in plastic bags and stored at -80°C. The experiments were conducted in a greenhouse at the Ecology Center of the University of Louisiana at Lafayette. Burrowing Frequency and Burrow Diameter Daily observations were made during the exposure period. The number of burrows in each tank was recorded on days 1, 5 and 10. Burrow diameters were measured at the end of the exposure period. Diameters were measured on the sediment surface using calipers. Two measurements were taken for each burrow, and the average of the two measurements was used for statistical analysis. Bioturbation Fiddler crab bioturbation activity was assessed by measuring the depth of burrows constructed and the volume of sediment removed by the individuals when constructing their burrows. For this purpose, 40 PVC cores of 7.5 cm diameter and 20 cm height were used. They were placed in eight rows of five cores each, and filled with moist sediment, from the fiddler crab collection sites, to a height of 15 cm. For Uca longisignalis, sediment consisted of a mixture of clayey and sandy sediment. For Uca panacea, only sandy sediment was used. Twenty individuals from each of the fiddler crab species, two concentration of crude oil (0 and 35 mg/cm2), and 10 replicates for each treatment were used. Crude oil was mixed with 50 grams (wet weight) of sediment. This mixture was evenly spread over the sediment surface in each of the treatment cores. One fiddler crab, previously measured for its carapace width, was added to each core and exposed for 5 days. Carapace width of U. longisignalis ranged between 18-21 mm, and U. panacea between 15-17 mm. Cores were covered with a mesh screen to avoid individuals climbing out. Water was daily sprayed to keep sediment moist and reduce crab mortality from desiccation. At the end of the exposure period, crabs were carefully removed from the cores. A flexible plastic strap was used to measure the depth of burrows in each core. The strap was carefully inserted into each of the burrows until the bottom was reached. A ruler was used to measure how deep the strap went. Fiddler crabs remove sediment when burrowing and pile it up next to the burrow openings. Thus, the mass of sediment dug by fiddler crabs when burrowing was used as a proxy to determine the volume of the burrows. Sediment found above the 15 cm mark was taken from the cores, placed in plastic bags and dried in an oven (Isotemp® (655G) by Fisher Scientific) at 50°C for 48 hours. The weight of dry sediment was recorded, and the volume of the burrow was calculated using the sediment density (1.72 g/cm3 for U. longisignalis and 1.57 g/cm3 for U. panacea). Oxidative Stress Oxidative stress was quantified by determining lipid peroxidation levels in the hepatopancreas. Lipid peroxidation was assessed by employing a modified TBARS assay reported by Dorts et al. (2009). Fiddler crabs were removed from the -80° freezer and dissected to obtain the hepatopancreas. A standard weight of 100 mg of tissue was placed in a clean microcentrifuge tube and homogenized with 500 µL of 0.05M potassium phosphate buffer. A 350 µL aliquot was transferred to another clean microcentrifuge tube and the same volume of 5% trichloroacetic acid (TCA) was added. The mixture was incubated in ice for 15 minutes, followed by the addition of 350 µL of 0.67% thiobarbituric acid (TBA). The solution was centrifuged at 2,200 rpm for 10 mins at 4°C. The supernatant was transferred to another clean centrifuge tube and boiled at 85-95°C. This solution was used to measure absorbance at 535 nm using a spectrophotometer (GenesysTM 20 (4001/4) by Thermo Fisher Scientific). Protein concentrations of the crude homogenates were determined using the Bio-Rad Protein Assay based on the Bradford method. Lipid peroxidation was quantified as nM TBARS per mg protein of crude homogenate, using a molar extinction coefficient of 156,000 M-1 cm-1.
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