Aquarium Success: Lionfishes and Other Scorpionfishes The venomous but beautiful scorpionfishes are extremely popular among home aquarists, with many species readily available in pet and aquarium stores. Written by one of the leading experts on this group of fish, this exclusive guide is the only trade book on the subject and covers topics essential to ensure a healthy and thriving tank, highlighting the top 25 species with profiles of their natural range, size, behavior, aquarium suitability, and specific care requirements.
About the Author:
Frank C. Marini III, PhD, born and raised in Hudson, Massachussets, is a long time fish nerd. Working as a stem cell biologist by day, Frank’s passion for keeping scorpionfish and their kin has lasted more than 30 years. His scientist side affords him a unique perspective into animal behavior, and his fish nerd side keeps him curious and motivated to find the keys to unlocking successful long-term husbandry of these remarkable fish. In 1995, Frank was credited with the first successful breeding of Banggai cardinalfish in captivity, and his writings have inspired thousands of fishkeepers to delve into marine fish breeding. For the past 15 years he has bred many species of marine fish, and accompanying this, he has moderated web forums on fish breeding and keeping aggressive and predatory fish in the home aquarium. Frank can be contacted at firstname.lastname@example.org.
Excerpt from Chapter 1: Scorpion Biology
Distribution and Habitat
Lionfishes and scorpionfishes are widespread in the world’s oceans but are primarily found around coral reefs. Man has aided lionfish distribution over the past 50 years, with Pterois miles entering the Mediterranean Sea from the Red Sea via the Suez Canal. Also, P. volitans and P. miles were introduced into the Atlantic Ocean along the U.S. East Coast and have become established there. Scorpions are found in water depths ranging from 2 to 1260 feet (less than a meter to 380 meters), on hard and soft bottoms, coral reefs and artificial substrates (e.g., sunken ships or oil rigs). In corals reefs and rocky outcrops, the fish hide in caves and crevices and underneath rock overhangs. Many small scorpionfishes live among the coral rubble, while some larger forms lie in exposed areas waiting for passing food. Many species are well camouflaged, as they are replete with dermal appendages, muted color tones, and skin flaps and tassels that allow them to disappear from plain sight. A sedentary lifestyle enhances their ability to blend in and ambush unsuspecting passers-by.
Scorpaeniforms have many adaptations in anatomy and behavior that serve them well in the wild, adaptations that must be taken into account when keeping these animals in captivity.
Easily outpaced by other animals, scorpionfish and lionfish are slow-moving hunters. Lionfish tend to appear conspicuous, so they rely on their unusual colorations and fins to discourage potential predators. On the reef, lionfish are top predators, as they are active hunters who ambush their prey using outstretched fins to slowly herd and corner their prey. As for scorpionfish, their strategy is different. They are extremely stealthy and sedentary, often sporting camouflaging coloration, feeding on prey that comes too close.
Lionfish are active predators of smaller fish, crabs, and shrimp. They move by slowly undulating the soft rays of the dorsal and anal fins, stalking and cornering prey with outstretched, expanded pectoral fins. Their kill shot is a lightning-quick snap of the jaws, swallowing their prey whole in an instant. In the wild and in captivity, these fish can be cannibalistic. Lionfish appear mostly nocturnal, given their tendency to retreat to hiding areas during the day. However, in captivity we clearly observe lionfish feeding throughout the day, indicating that in the wild recently fed individuals may merely be retreating to their hiding spot after eating.
Most scorpionfish remain motionless for hours, relying on concealment to aid in ambushing their prey and feeding whenever the opportunity arises. On the reef, they feed on a few organisms a day, although large scorpionfish may ingest only one larger prey every few days. Scorpionfish diets vary, with some large scorpionfish feeding exclusively on fish. Others of similar size demonstrate a wide dietary breadth, feeding on shrimp, crabs, and squid in addition to fish. Many small scorpionfish feed almost exclusively on small crustaceans and worms. Like lionfish, scorpionfish tend to hunt mainly at twilight—they’re crepuscular hunters—but they never pass up a meal opportunity, day or night.
The cavernous mouths of many scorpaeniform fishes are disproportionately large compared to their body size, so they are able to take even very large prey. Prey capture takes place in three phases: 1) orientation to the prey item, 2) seizing the prey (expansion and compression of the mouth) and 3) manipulating and swallowing the prey. When a scorpionfish sees a prey item, eye movements indicate the scorpionfish is tracking the prey. The scorpionfish raises its head slightly, visually orienting toward the prey’s eyes. They attempt to orient their bodies toward the prey’s head. This adjustment precedes the fish’s rolling and pitching its pectoral fins to ensure the prey is within range.
Just before opening its mouth, the fish depresses its mouthparts, essentially spring-loading the mouth structures with potential energy to explode wide open for the strike. Bony fishes in general have similar mouth expansion biomechanics, but in scorpaeniforms mouth movement is so rapid that it creates a large lateral expansion, resulting in additional suction. The fish literally sucks the water surrounding the prey into its mouth, rendering the prey as powerless to resist as a surfer on a bad wave. Once the prey is in its mouth, the fish holds it securely with its curved conical teeth, frequently manipulating the prey with abbreviated mouth expansion and closure—in essence, readjusting the food item to ensure it is head first. Swallowing occurs with a raking motion of the throat teeth. This entire process occurs swiftly—complete prey capture has been recorded at 45 milliseconds for a stonefish, with mouth opening and closing occurring in under 20 milliseconds (Grobecker, 1983).
Many scorpions rely on their cryptic coloration and sedentary nature to fend off predators. By being completely camouflaged, they rely on hiding in full view as a passive defense. Many have specialized body shapes and possess skin tassels that allow them to appear convincingly identical to algae tufts, rockpiles covered in sponges, or unpalatable crinoids. To supplement the camouflage, some scorpions mimic the undulating movement of local algae or debris; this is done, for example, by the leaf scorpions, whose bodies rock side-to-side, mimicking the wave action of local plants. However, if a predator stumbles across or threatens a scorpaeniform, the fish reacts. Initially it may flee, quickly settling into a new area, then quickly freezing again, remaining motionless. Alternatively, a few species possess brightly colored fin markings that are usually held in the closed, or darkened, position, such as on the bottom of the pectoral fin. These markings, called flashes or flasher fins, are rapidly exposed at a threat and may serve as a warning. Alternatively, the bright color flash may confuse or disorient predators.
Certain scorpaeniforms get their common names (stingfish, waspfish, etc.) from their ability to defend themselves with a venomous sting, although it’s used as a last-resort defensive weapon to prevent a repeated attack. Venom glands at the base of certain spines produce a venom that the fish injects via the spines. The venom immediately causes severe pain that just about guarantees a hasty retreat of the threat. Medical and scientific journals have documented numerous cases of human envenomation, and I’ve dedicated a whole section to this topic later in this chapter.
Multifocal Lensed Eyes
Lionfish eyesight consists of a multifocal optical lens in each eye, with a short focal length (Karpstam, 2007). Long focal lengths equate to high optical magnification of the image and low light-gathering ability, meaning they can see farther but require hunting during daylight hours, whereas short focal lengths correspond with higher light-capturing ability and less magnification of the target, meaning more evening time hunting of prey that swims nearby. Because lionfish possess eyes better suited for their crepuscular behaviors, they strike at food from short distances and do more low-light hunting.
Scorpaenids have many small teeth located on the upper and lower jaws in densely packed bilateral clusters, and in a small patch on the anterior roof of the mouth. These teeth appear functionally limited to grasping prey captured by the extraordinarily quick predatory strike. The pharyngeal (throat) teeth help them secure and swallow their meals.
Many scorpaeniforms lead an extremely sedentary lifestyle, and marine organisms (algae, hydroids, and bacteria that settle from the water column) can attach to them. Because of this, many have evolved a protective coating, called a cuticle. A cuticle is a thin white or opaque skin covering encasing the entire animal. The fish shed this cuticle to rid their bodies of these organisms. Some species, like Rhinopias, shed this cuticle weekly, and many others, such as lionfish, shed monthly or even less frequently. Shedding can take minutes to hours. Shedders shake their body rapidly until the cuticle tears off in small pieces. In home aquariums, cuticle shedding looks unsettling. In a lionfish, you’ll notice that the fish darts and dives, then convulses for a few seconds; a white, ghost-like tissue appears in the water. The cuticle protects the fish from illness, and excessive cuticle shedding indicates poor health, as they increase cuticle shedding during protozoan infection as a way of reducing their exposure.
Few published records of natural predators of scorpionfish and lionfish exist. One short report suggests that the piscivorous cornetfish Fistularia commersoni is a predator of Pterois miles. Judging by the presence and reverse orientation of a partially digested specimen of P. miles in the stomach of a large F. commersoni, the authors concluded that cornetfish may ambush lionfish from the rear, consuming them tail-first, allowing the venomous spines to fold harmlessly forward. In the home aquarium, venomous spines aren’t a deterrent to larger piscivores, as I’ve personally witnessed antennariids (frogfish and anglerfish) consume small lionfish. Additionally, other scorpions readily consume smaller ones. I have watched in horror as my large P. volitans engulfed two juvenile conspecifics. It is likely that natural lionfish predators include sharks, as many sharks consume noxious organisms without displaying ill effects from the poisons.
Lionfish are often seen moving about during the day, alone and in groups of two to six animals. They may live alone most of their lives, fiercely defending their home ranges from other lionfish. As juveniles, lionfish aggregate into small groups, and this cooperation probably increases their ability to gather food. During the spawning season, adult lionfish also may gather into groups, with males acting more aggressively than females. In many lionfish no clear sexual dimorphism or dichromatism exists—however, clear demarcation between sexes exists in Dendrochirus brachypterus, the dwarf fuzzy lionfish. Lionfish reproduce like many other fish, producing eggs that are fertilized outside the female’s body. There is little information about the social structure of most reef-dwelling scorpionfish species. Most observations about these fishes are from anecdotal reports from scuba divers and aquarists keeping them. We do know that most tropical species remain solitary, with occasional sightings of scorpionfish pairs, and the leaf scorpionfish have been noted in pairs and trios.
Excerpted from Lionfishes and Other Scorpionfishes: The Complete Guide to the Successful Care and Breeding of These Spectacular and Popular Marine Fish by Frank C. Marini, Ph.D. © T.F.H. Publications. Used by permission.