The effects of moderate concentrations of Karenia brevis on stone crab reproduction.

The stone crab, Menippe mercenaria, supports a commercial fishery along Florida's Gulf coast where harmful algae blooms, known as red tides (Karenia brevis) develop. Red tides occur nearly annually and can overlap with the stone crab reproductive season. We determined the impact of moderate red tide (K. brevis) concentrations (∼105 cells L-1) on stone crab embryo development, hatching success, female stress, hatch duration, and larval survival. Crabs and larvae were exposed to a control (no K. brevis) or moderate concentrations of K. brevis. No difference in embryo development or hatching success was observed. Stress was elevated in the K. brevis treatment, resulting in prolonged hatching relative to the control. Larval survival was reduced in K. brevis relative to the control. Moderate concentrations of K. brevis results in sublethal effects on stone crabs and reduces larval survival, suggesting that mitigation that reduces bloom concentrations could provide relief to stone crab populations.

The effects of prolonged exposure to hypoxia and Florida red tide (Karenia brevis) on the survival and activity of stone crabs.

Florida red tides are harmful algae blooms caused by the dinoflagellate Karenia brevis, which occur along Florida's gulf coast almost annually. In recent years Florida red tide blooms have become more common, frequent, and intense. Florida's southwest coast, from Manatee to Collier County, has experienced repeated and prolonged K. brevis blooms since 2011 with the most recent bloom in 2017 lasting 17 months and resulting in both hypoxic and anoxic events. We therefore determined the survival and level of lethargy (e.g., lack of responsiveness or reduction in behavioral reactions) of sublegal stone crabs to K. brevis and hypoxia as both singular and simultaneous stressors. Crabs were randomly assigned to one of six treatments that included: 1) high concentration of toxic K. brevis (> 1 million cells L-1) maintained at normoxic levels (7.2 mg L-1 ± S.D. 0.47 dissolved oxygen), 2) moderate hypoxia (1.6 mg L-1 ± S.D. 0.42 dissolved oxygen) with no K. brevis, 3) moderate hypoxia (1.5 mg L-1 ± S.D. 0.43 dissolved oxygen) with a high concentration of K. brevis, 4) severe hypoxia with no K. brevis (0.69 mg L-1 ± S.D. 0.36 dissolved oxygen), 5) severe hypoxia (0.63 mg L-1 ± S.D. 0.40 dissolved oxygen) with a high concentration of K. brevis, and 6) a normoxic control (7.3 mg L-1 ± S.D. 0.61 dissolved oxygen) with no K. brevis. Survival and stone crab lethargy or responsiveness was monitored every 10-12 h for six days. Crabs simultaneously exposed to K. brevis and severe hypoxia exhibited a 43% decrease in survival and experienced increased lethargy within 24 h relative to the control (7% decrease in survival, no increase in lethargy). The increase in stress level and sluggish behavior during exposure to hypoxia was evident by a general lack of responsiveness or movement which indicates that nearshore populations of stone crabs are unlikely to emigrate away from such conditions suggesting that future harvests may be reduced following prolonged K. brevis blooms and hypoxic events.

Ocean acidification disrupts the orientation of postlarval Caribbean spiny lobsters.

Anthropogenic inputs into coastal ecosystems are causing more frequent environmental fluctuations and reducing seawater pH. One such ecosystem is Florida Bay, an important nursery for the Caribbean spiny lobster, Panulirus argus. Although adult crustaceans are often resilient to reduced seawater pH, earlier ontogenetic stages can be physiologically limited in their tolerance to ocean acidification on shorter time scales. We used a Y-maze chamber to test whether reduced-pH seawater altered the orientation of spiny lobster pueruli toward chemical cues produced by Laurencia spp. macroalgae, a known settlement cue for the species. We tested the hypothesis that pueruli conditioned in reduced-pH seawater would be less responsive to Laurencia spp. chemical cues than pueruli in ambient-pH seawater by comparing the proportion of individuals that moved to the cue side of the chamber with the proportion that moved to the side with no cue. We also recorded the amount of time (sec) before a response was observed. Pueruli conditioned in reduced-pH seawater were less responsive and failed to select the Laurencia cue. Our results suggest that episodic acidification of coastal waters might limit the ability of pueruli to locate settlement habitats, increasing postsettlement mortality.

Ocean acidification changes the vertical movement of stone crab larvae.

Anthropogenic activities are increasing ocean temperature and decreasing ocean pH. Some coastal habitats are experiencing increases in organic runoff, which when coupled with a loss of vegetated coastline can accelerate reductions in seawater pH. Marine larvae that hatch in coastal habitats may not have the ability to respond to elevated temperature and changes in seawater pH. This study examined the response of Florida stone crab (Menippe mercenaria) larvae to elevated temperature (30°C control and 32°C treatment) and CO2-induced reductions in pH (8.05 pH control and 7.80 pH treatment). We determined whether those singular and simultaneous stressors affect larval vertical movement at two developmental stages. Geotactic responses varied between larval stages. The direction and rate of the vertical displacement of larvae were dependent on pH rather than temperature. Stage III larvae swam upwards under ambient pH conditions, but swam downwards at a faster rate under reduced pH. There was no observable change in the directional movement of Stage V larvae. The reversal in orientation by Stage III larvae may limit larval transport in habitats that experience reduced pH and could pose challenges for the northward dispersal of stone crabs as coastal temperatures warm.

Karenia brevis causes high mortality and impaired swimming behavior of Florida stone crab larvae.

The dinoflagellate Karenia brevis causes harmful algal blooms commonly referred to as red tides that are prevalent along Florida's gulf coast. Severe blooms often cause fish kills, turbid water, and hypoxic events all of which can negatively impact local fisheries. The stone crab, Menippe mercenaria, is a ˜$25 million per year fishery that occurs primarily along Florida's gulf coast. On the west Florida shelf, red tides occur from fall through spring, although severe blooms can occur during the summer. During the summer, stone crabs are reproductive and release larvae that are transported offshore where K. brevis blooms originate. This study determined the effects of K. brevis exposure on the survivorship, vertical swimming behavior, and oxygen consumption of stage-1 larval stone crabs. Survivorship was determined by exposing larvae to high (> 1 × 106 cells L-1) and medium (˜1 × 105 cells L-1) K. brevis concentrations for 96-hrs and were compared to controls that had no algae present. Larval swimming behavior (i.e., geotaxis) and oxygen consumption were monitored after 6-hr exposure to K. brevis. After 96-hrs of exposure, mortality was 100% and 30% for larvae in the high and medium concentrations of K. brevis, respectively, relative to the control. Larval swimming behavior was reversed in the K. brevis treatment; however oxygen consumption rates did not differ among treatments. These results suggest that severe blooms during the summer may reduce larval supply and serve as a potential bottleneck for new individuals recruiting into the fishery in years following a K. brevis bloom.

The effects of red tide (Karenia brevis) on reflex impairment and mortality of sublegal Florida stone crabs, Menippe mercenaria.

The Florida stone crab, Menippe mercenaria, is a major commercial fishery that occurs primarily along Florida's west coast, where harmful algal blooms of Karenia brevis frequently develop. To determine sublethal and lethal effects of K. brevis on M. mercenaria, we exposed sublegal stone crabs to three seawater treatments in laboratory conditions: no K. brevis (control), a low-toxin K. brevis strain (Wilson LT), and a toxic K. brevis (New Pass strain). Total food consumed, reflex impairment and survivorship of each crab was monitored throughout the nine-day experiment. Crabs in the toxic treatment consumed 67% less food. The probability of an individual losing a reflex significantly increased with time (days), and there was a 42% decrease in survivorship in the toxic treatment. This is the first study to demonstrate negative effects of K. brevis on the stone crab, presenting the critical need of further investigation to fully understand how red tide may impact sustainability of the fishery.