By Aimee van der Reis
Lionfish are mesmerising. They have majestic manes, bold colours and an incredible ability to ‘hover-swim’, all of which makes them great to photograph.
These cryptic fish are found belly-side against rocks/wrecks/reefs with their fins flared in all directions. They are definitely not afraid of the limelight, and you often see them upside down, or confidently parading. I knew they were venomous but never seemed threatening, and so after a recent trip to Port Vila, Vanuatu, I decided to investigate a bit more…
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The lionfish taxonomic family is Scorpaenidae which contains other venomous species such as scorpionfish (Scorpaena cardinalis) and in fact they can also be grouped on the structure of their venom organs. Their venoms are similar but potency differs with the lionfish envenomation symptoms being the least severe and the notorious stonefish the most severe with scorpionfish somewhere in between (1, 2)!
What’s in the venom?
Lionfish venom has so far been found to contain a toxin; acetylcholine and hyaluronidase. The toxin affects neuromuscular transmission (think motor neurons and muscle contractions) and is a heat labile antigenic substance (which evokes an immune response, eg swelling) (1, 3, 4). Research suggests that the toxin produces nitric oxide which causes muscle relaxation and thus the inhibition of neuromuscular functions (2, 4).
The non-proteinaceous substance, acetylcholine, is essential for muscle contraction to occur and is a vital substance occurring naturally in the human body (3).
Hyaluronidase is an enzyme that functions as toxin-spreading factor which can also possibly act as an allergen (5) which may explain why some people react more adversely to lionfish venom than others.
No precise treatment or antidote exists for lionfish venom and only in severe cases will the antivenin for stonefish be considered for it. (4, 6, 7).
Lionfish hunt use their natural camouflage while ambushing or stalking their prey. At the right moment they lunge with lightning speed, then generally swallow their prey head first (8). As you may have noticed, they do not tend to move much when hovering while staring at you through their beady eyes. This is most likely because they are a confident solitary territorial fish, probably sizing you up trying to see whether they could get their mouth around your head or not… Their spines coupled with their venom are purely a form of defence playing no role as a weapon as they are already very successful hunters.
Invasion of a non-native species is usually a man-made accident with dire consequences. High densities of invasive Indo-Pacific lionfish (P. volitans and P. miles) have established themselves on the east coast of Florida (USA), the Caribbean and the Gulf of Mexico. And they are claiming more territory for example near Rio de Janeiro, Brazil and in the Mediterranean Sea (14, 15).
The origin of the lionfish invasion in the western Atlantic remains unknown though thought to be due to the release of ornamental pet lionfish when aquarium owners tired of them. There is evidence lionfish were present in the Florida region as early as 1985 when, in 1992, six lionfish were seen alive and well in a Florida bay after a private aquarium became a victim of Hurricane Andrew (16-18). Genetic evidence showed as few as three P. volitans and one P. miles females were needed for the founding of the Atlantic populations, and that their introduction was more likely to have occurred through a single event with ocean currents the main means of dispersal (14, 18-20).
If you are wondering why these strong, independent female lionfish didn’t need a man…well, this particular genetic study was based on a gene region in the mitochondrial DNA passed on from females to their offspring, thus researchers could only determine how many females initially played a role in this lionfish population explosion. Maybe the Hurricane Andrew lionfish are partly to blame though the accuracy of this report is questionable to say the least…
Unprecedented invasion speed
Nevertheless the speed and scale of the lionfish invasion was unprecedented, spurred on by their (I) venomous defence mechanism, (II) lack of predatory pressures, (III) ability to reproduce all year round (though temperature and food dependent), (IV) thrive in a variety of habitats, (V) immunity to certain fish pathogens, (VI) unique hunting techniques and (VII) high rates of survival despite long periods of food scarcity (8, 26-33).
The invasion led to competition with native fish (eg groupers) seeking similar prey on the reef (34). Researchers across certain study sites found a 65% average decline in the biomass of affected native fish (26) The lionfish diet is generalist by nature and known to include many different species including (35, 36) trumpetfish, chromis, grouper, parrotfish, snapper, pufferfish and squirrelfish.
Shrimp identified have included mantis shrimp and cleaner shrimp (37, 38). The diet includes somewhat larger (adult) species than the lionfish itself and thus it is likely they are targeting the juveniles which in turn may alter the functioning of the food web and thus the structure of coral reef ecosystems (38).
Underwater visual censuses suggest the densities of lionfish in invaded areas are far greater than in their natural habitats, up to 400 fish per hectare (29)! That’s up to 15× the density of their own natural habitats!
So what options are there to decrease these invasions and restore the natural populations? Larger native fish may learn to eat the lionfish while preyed-on smaller native fish may come to identify lionfish as a threat (39). A more proactive approach has been lionfish removal events such as the Reef Environmental Education Foundation’s (REEF’s) lionfish derbies, and promoting lionfish as a desirable fish to eat, such as National Oceanic and Atmospheric Administration’s (NOAA) “Eat lionfish” campaign. These actions also provide education about handling and preparing a potentially harmful fish which is in fact completely safe to eat (34).
Developing a market for them would be cost-effective for controlling their populations, alleviate over-exploited native fish, relieve stress occurring on the reefs, and provide an opportunity for small scale commercial fishing (27). Where a market for lionfish has been explored it looks promising, and may be a means to control this invasive species.
What are the possible side effects if injury occurs? (1, 2, 9-13)
Mild to medium side effects:
Redness around site of injury, severe pain (immediate throbbing), swelling and possible discolouration of the skin where the injury occurred, pins and needles, dizziness or feeling faint (coupled with looking pale), nausea, sweating, and possibly bullae (type of blister) formation at site of stings.
Severe side effects:
Delirium, seizures, anaphylactic shock (if stung more than once), limb paralysis, vomiting (watch out for dehydration) and shortness of breath. After several weeks you may feel a loss of sensation (anesthesia), abnormal perception of sensation (paresthesia), or an increase in sensation (hypesthesia). Local necrosis (dead tissue) at site of injury could occur.
Actions to take:
Firstly, it is important to get the diver out of the water as soon as possible after injury.
Pain will be most severe an hour to an hour and a half after the venom has been injected and persist for 6-12 hours or longer. The persistent pain is likely due to the fact that the toxin cannot be inactivated immediately after injury has occurred. Immersing the affected area in hot water, no more than 45°C for 30-60 minutes will help neutralize the toxin and provide relief. If the spine and/ sheath is embedded it should be removed. But when cleaning the wound use warm saline solution as alcohol based solutions may cause further tissue damage. The wound will need to drain so should be left open. It is always advisable to consult a doctor and wise to make sure that tetanus protection is up to date (Clostridium tetani, the bacterium responsible for tetanus, can be found in marine sediments. Better safe than sorry!).
- Lionfish are suction feeders, feeding on crustaceans and fish mostly at dusk and dawn and use their cryptic nature to their advantage by ambushing their prey (32, 40-42). Research suggests cannibalism occurs, but this is linked to fish size and density within an area (43).
- They can hover due to their specialized bilateral swim bladder muscles allowing them to alter their centre of gravity (the reason they are often seen upside down/head down). So they can orientate themselves strategically before striking their prey (44).
- Lionfish can produce jets of water in the direction of their prey when stalking which are thought to confuse or distract the prey, and make them orientate head-first for an easy swallow (8).
- Lionfish fins give the illusion the fish is larger than it actually is. Specifically their pectoral fins allow it to ‘herd’ potential prey into areas of no escape. They also have been seen to team up to hunt and alternate the fish that gets to strike and eat the prey (45). Their pectoral fins also are handy for flushing benthic invertebrates out of substrates by palpation (applying pressure to determine if invertebrates are present in the substrates) (46).
- Research has described distinct vocalization of lionfish. They make different calls when alone (repetitive pulsecalls), or together (multiple fish vocalize concurrently with less rapid repetitive pulse-calls at a lower frequency), or when agitated (hum call) (47). It is thought this calling may have be to do with seeking a friend to hunt…
- They occupy a wide range of thermal environments, from 13 to 32°C. But about 23°C suits best. They have been found at depths greater than 75 m (29, 32) too which means they populate a range of habitats including reefs, mangroves, soft bottoms, nearshore seagrass beds and near estuaries (28, 29).
- The bacterial community found on lionfish skin is diverse and is capable of producing antibacterial metabolites and thus help defend the lionfish from fish pathogens (the bacteria promotes disease resistance to its host) (30).
- Tagged lionfish movement records show they tend to move relatively little depending on the lionfish density in the area, body size and seascape structure (41).
- They can change their physiology to meet their energy demands, for eg lowering their metabolism when food is scarce (31, 32).