Intense red tide can threaten stone crabs, and more research is needed
Stone crab at Mote Marine Laboratory, Sarasota, Florida. Credit: Miguel Montalvo/Mote Marine Laboratory
Florida red tide algae toxins can stress and even kill sublegal stone crabs — young adults whose claws are growing toward legal harvest size — according to preliminary research results published recently in the scientific journal Marine Environmental Research, indicating a strong need for continued studies.
Mote Marine Laboratory scientists led the project in collaboration with the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute (FWRI). The study was inspired in part by questions from stone crab fishermen.
“I was at the state’s Commercial Stone Crab Advisory Panel meeting, and some of the fishermen from the Gulf Coast were asking why their traps were relatively empty during six months of red tide,” said Mote Postdoctoral Research Fellow Dr. Phil Gravinese, first author of the study. “Also, some reported catching crabs that looked lethargic. The scientists present at the meeting didn’t have a clear answer about whether red tide was affecting the crabs, so we designed an initial study to learn more.”
The stone crab fishery, centered along west Florida, was valued at $31.3 million in the 2016–17 season, but since 2000 the average annual commercial harvest has declined by about 25 percent. Mote scientists are investigating multiple potential challenges that might contribute to this decline.
Scientists know that elevated concentrations, or blooms, of the Florida red tide alga Karenia brevis produce toxins that can kill fish and sicken or kill sea turtles and marine mammals. However, red tide impacts remain more mysterious in crabs and other crustaceans. According to FWRI, at least some crustaceans may die during red tide, but it’s unclear whether the killer is red tide toxicity, decreased oxygen related to algal bloom dynamics, or both.
So far, research suggests that red tide exposure can alter some crustaceans’ feeding behavior and movement.
Through separate published research not focused on red tide, FWRI scientists found that certain stone crab reflexes — for instance, retracting certain body parts when they are touched — offer clues to their stress levels and survival odds. These reflexes are in some ways similar to the human knee-jerk reflex checked by doctors. Through this research, FWRI successfully applied a tool called the “reflex action mortality predictor” (RAMP) to investigate how often stone crabs survive trapping and handling by fishermen. RAMP gives crabs a number score for reflex impairment, which is used to estimate the crabs’ chance of survival.
If stone crabs encounter red tide for more than a week, do their reflexes show signs of stress and does their chance of survival decrease?
Mote scientists and FWRI partners undertook the first study to answer this question. In late 2017, project partners brought 36 stone crabs from New Pass, Sarasota, to Mote’s City Island research campus. They kept 12 crabs in tanks with filtered seawater, 12 in tanks with high levels of toxic K. brevis, and 12 in tanks with a low-toxin strain of K. brevis. All K. brevis was grown in a controlled setting: Mote’s phytoplankton culture laboratory.
“For this study, we used concentrations of Karenia brevis similar to what would be found along the west coast of Florida during a moderately high bloom,” said Dr. Vince Lovko, Phytoplankton Ecology Program Manager at Mote and a co-author on the study. “It’s quite possible for stone crabs in this area to encounter concentrations of red tide ranging from ‘low,’ meaning elevated above the sparse ‘background’ concentration normally present in the Gulf of Mexico, to levels many times above what was used in this study.”
All crabs in the study were sublegal — young adults with claws too small for harvest.
“Sublegal crabs represent the next year’s harvest; what we learn from them is applicable to management of the fishery,” Gravinese said.
Over nine days, the researchers monitored their sublegal crabs for any decreases in feeding, indicating stress. They also tested the crabs’ reflexes — retracting an eye stalk or leg, closing the mouth and other movements in response to touch or pressure. They scored any reflex impairment with the RAMP method and used a mathematical model analyzing those reflex scores and other variables (such as crab size) to predict a crab’s survival odds daily.
Over the full experiment, crabs exposed to toxic red tide had the worst reflex scores and appetite, indicating greatest stress. Crabs exposed to the toxic red tide also experienced the greatest mortality.
Results, on average, for each treatment group:
- Filtered seawater only (control group): Crabs’ reflex scores dropped approximately 10 percent over the study — the least stressed group.
- Low-toxin red tide algae: Crabs’ reflex scores dropped about 32 percent. They ate 43 percent less food than the control group and experienced a 25 percent decrease in survivorship — suggesting they were more stressed.
- High-toxin red tide algae: Crabs’ reflex scores dropped by 52 percent. They ate 67 percent less food, and experienced a 42 percent decrease in survivorship compared with the control group — suggesting this group was the most stressed. The differences between this group and the control group were statistically significant — suggesting that toxic red tide algae had an impact.
Stone crabs impaired by red tide could become more vulnerable to predators, Gravinese noted. Even more concerning: Prolonged exposure to toxic red tide appears to kill them outright.
In this study, crabs exposed to high-toxin K. brevis were nearly four times likelier to die than those in seawater alone. In the high-toxin group, crabs began to perish within three days, and more than half died after nine days.
“This small-scale study is just a first step, but it suggests that these sublegal crabs may have a relatively short window of tolerance for high levels of red tide, and we need to investigate this further,” Gravinese said. “We have more question than answers right now. We do not know if more sensitive life stages, like larvae and juveniles, will respond differently. Usually larvae and juveniles have a lower tolerance to environmental stress. Additionally, stone crab larvae develop in regions where red tide blooms originate, which might serve as an additional bottleneck for the population. We are also interested in determining if larger, legal-sized crabs have a higher tolerance to red tide and if they are able to recover from red tide exposure.”
Could stone crabs avoid this danger by crawling away?
“From others’ research, we suspect that sublegal and juvenile stone crabs are not likely to travel far enough in the wild to avoid a red tide bloom, especially if it’s, say, 40 feet deep, several miles long and lasts six months,” Gravinese said. “However, adults are more mobile and do appear to travel some distance. One mark-recapture study documented that adult stone crabs were able to move about 1.5 kilometers, a little less than one mile, so their chances may be better.