No wonder they're crabby

Challenges facing stone crabs: the big picture

by: Hayley Rutger

Originally published in the Spring 2020 edition of Mote Magazine

Mote Magazine Spring 2020 article about stone crab research

From mid-October to mid-May, seafood lovers will crack open many delicious stone crab claws—but fewer than they used to. 

In Florida’s $30-million stone crab fishery, annual catch has declined by 40% since 1998.

Mote Marine Laboratory scientists are working to understand why. To support this important research, Mote Aquarium biologists are raising stone crab larvae (babies) in their Aquarium Conservation Lab.

By collecting stone crabs under government permits, hatching their eggs and raising the larvae, Mote Postdoctoral Research Fellow Dr. Phil Gravinese and colleagues are investigating which environmental challenges can affect stone crabs, to what degree, across multiple life stages.

After three years of this research, Mote’s data are coalescing into an intriguing picture of potential stone crab threats, both current and projected for the future—valuable information for fishery managers. These threats, considered together, can make the stone crab life cycle resemble an obstacle course. 

 

The challenges

Below are stone crab stressors that Mote has investigated through peer-reviewed, published research—a list that continues to grow. The next page shows how each stressor affected stone crabs at specific life stages in Mote’s laboratory-based studies, hinting at what stone crabs may experience in some habitats in the wild. 

Florida red tide icon Florida red tide (current stressor) is a higher-than-normal concentration of a toxin-producing Karenia brevis algae found in the Gulf of Mexico. Mote examined the impacts of high and medium red tide concentrations, which are each higher than the normal “background” levels.

temperature icon Elevated temperature (current stressor projected to increase): Climate change is increasing average seawater temperatures, including in regions where stone crabs live. Mote examined the impacts of increasing temperature by 2 degrees Celsius (3.6 degrees Fahrenheit), which was based on one scientific projection of global climate change for year 2100.

ocean acidification icon Ocean acidification, OA (current stressor projected to increase): OA, part of climate change, is a worldwide decrease in seawater pH driven by increased carbon dioxide in the atmosphere. Some coastal habitats in Florida are also experiencing seasonal declines in pH due to organic runoff, which can decrease pH three times faster than the rate of OA anticipated for global oceans by the end of the century. Mote investigated the impacts of OA projected for year 2100.

 Low oxygen, or “hypoxia” (current stressor), can occur because of nutrient pollution in coastal waters and in the aftermath of a severe red tide. Both of these can cause increases in decomposing organic matter, reducing oxygen in the water. Mote examined the impacts of short-term low oxygen conditions comparable to those documented in shallow, nearshore environments along Cedar Key, Florida, where stone crabs were collected for the research.

 

Precarious path to maturity

Stone crabs are vulnerable to environmental stressors at each stage of their life cycle. The more scientists learn about the impacts of these stressors, the better fishery managers can adapt their strategies to sustain the population. Here are the results of Mote’s laboratory-based studies so far.

EMBRYOS 

Stone crab embryos develop inside an egg for two weeks before hatching.

Ocean acidification (OA), exposure throughout embryonic development:

  • 28% decrease in hatching success
  • 24% slower embryonic development 

LARVAE

After hatching, stone crabs grow through five larval stages, which take 20–30 days to complete, before they molt into a post-larval stage that lasts about one week. In Mote’s research, larvae were raised in each environmental condition throughout their entire larval development to monitor survival, development and swimming behavior.

Ambient conditions (pH = 8.0, temperature = 30 °C/86 °F):

  • About 25-26 days to complete larval development
  • Normal swimming behavior (80% of larvae swam toward the surface as expected.)

Elevated temperature (+2 °C/3.6 °F):

  • 71% decrease in survival
  • 13% faster development
  • Normal swimming behavior (80% swam toward the surface as expected.)

OA, (pH = 7.6):

  • 37% decrease in survival
  • 12% longer development in later larval stages 
  • Abnormal swimming behavior (74% swam away from the surface at faster rate.

OA and elevated temperature combined:

  • 80% decrease in survival
  • Larvae developed at a similar rate to the elevated temperature condition. No changes in larval morphology (shape of body structures)
  • Abnormal swimming behavior (78% swam away from the surface at faster rate.)

Florida red tide, four-day exposure:

  • 100% mortality at high concentration (about 1 million red tide cells per liter of water)
  • 30% mortality at medium concentration (about 100,000 cells per liter)
  • Abnormal swimming behavior (60% of larvae swam downward.)

JUVENILES

Young crabs that grow and molt repeatedly and are reproductively immature.

Hypoxia, two-hour exposure:

  • 80% mortality in small juveniles
  • 12% mortality in large juveniles (but 35% of large juveniles showed immobility)

ADULTS

Adult crabs can be “sublegal,” with claws too small for harvest (roughly 1 to 1.5 years older than juvenile), or “legal,” with claws that measure at least 2.75 inches from joint to end of the claw (roughly 1.5 to 2 years older than juvenile).

Florida red tide, nine-day exposure of sublegal crabs:

  • 67% decrease in eating
  • 52% loss of reflexes
  • 42% decrease in survival

 

Lend a claw

You can help fight the challenges facing stone crabs—reduce your carbon footprint to help address climate change, use landscaping best practices to reduce nutrient-rich runoff, and support research focused on mitigating Florida red tide.

 

To view the layout of this story as it was originally published in the Sping 2020 edition of Mote Magazine click each image below, or view the full magazine online at our Publications page.

First page of this story as originally published in the Spring 2020 edition of Mote Magazine.
Infographic diagram from the original story as published in the Spring 2020 edition of Mote Magazine.