Diving Deep: the Fangtooth Fish

Anoplogaster cornuta, commonly known as the fangtooth fish, is a pelagic fish that grows to a maximum of 16 centimeters in length. Pelagic refers to aquatic organisms that can be found anywhere between 500 and 2,000 meters below sea level (see Figure 1).

With a name including ‘fangtooth,’ these organisms can be visualized as vicious predators within the deep ocean. Their elongated teeth bring about a fearsome visage. Certainly, to an unaware deep sea visitor, this fish may seem key to avoid. Despite its formidable name, the common fangtooth fish is hardly an eager, frightening hunter–in part to its small size, but also through its feeding behavior.

Habitat and General Description

Figure 1. Illustration of the habitable range (in meters) of the common fangtooth fish.

A. cornuta can be identified through its large caudal (or head) region and two milky eyes. These organisms have incredibly poor vision, which would be expected to pose a disadvantage, considering they dwell at incredible, poorly-lit depths. To combat this visual disadvantage, fangtooth fish possesses a highly developed lateral line system (see Figure 2). A lateral line is a sensory organ that runs laterally (horizontally) along the body and is highly sensitive to changes in the surrounding environment. This organ senses vibrations in the water driven by nearby movement, as well as changes in pressure.

Figure 2. Illustration of the lateral line location in the common fangtooth fish.

Highly developed lateral line systems, or those with high levels of efficient detection are useful for avoiding predation. In tandem with this form of defense, the common fangtooth has another useful mechanism to protect their lineage. These organisms have ultra-black skin, meaning their skin contains high concentrations of pigment that allow them to absorb nearly 100% of all available light! This allows the fish to hide in plain sight by blending in with its dark, deep surroundings (see Figure 3). 

Figure 3. A vintage photograph (film) of a collected, and dried, group of common fangtooth fish.
Figure 4. Illustration of gill components
and their functions.

Feeding Behavior and Morphology (Anatomy)

The most notable feature of the fangtooth fish’s morphology by far is its set of long, sharp teeth and cavernous jaw. In fact, the fangtooth has the largest teeth-to-body-size ratio of any known fish in the ocean. With record-setting lengths, how does this little fish close its mouth without puncturing its brain? The answer is with specialized pouches! These pouches are in the roof of the fish’s mouth and extend into deep sockets, allowing the teeth of its lower jaw to safely slide inside without doing harm to its noggin. Interestingly, juvenile fangtooth fish have much smaller teeth, and only a single row. As a result, juveniles filter feed zooplankton with gill rakes (bony or cartilaginous structures extending from the gill arch, see Figure 4) until they develop their name-worthy fangs.

As adults, fangtooth fish are opportunistic feeders, meaning a large portion of their hunting pattern involves consuming what happens to get close enough to capture. Indivudals of A. cornuta engage in diel migration, where they remain in the ocean depths during the day and migrate to shallower waters at night for feeding. Here, these organisms can enjoy a diverse diet of small prey that happen to get within range. Examples of prey include juvenile squid and juvenile or small adult fishes. Whereas these fish often consume prey at or beneath body size, these organisms have been known to consume prey much larger. Regardless of size, the fangtooth fish does indeed have poor visual acuity, and as a result relies on specialized chemoreceptors (or chemical sensing organs) to smell the organism underwater. Once a prey is obtained, it is unlikely to escape, making this fish an effective hunter should the opportunity arise.

Additional Resources

If you would like to learn more about this incredible species, you can view the BBC Studies Blue Planet video below titled: “Fangtooth in the Abyss.”

References


Monterey Bay Aquarium | Animals A to Z | Meet the Common Fangtooth

Monterey Bay Aquarium | Ocean Twilight Zone | Creature Feature: Fangtooth

Smithsonian | Ocean, Find Your Blue | Fangtooth Fish


Diving Deep: the Sea Angel

 © Monterey Bay Aquarium

Using a pair of winglike structures, the sea angel propels itself gracefully through the deep waters of the ocean. Sea angels look quite ethereal, with translucent bodies and internal organs of pink and orange. However, despite its celestially inspired name, the sea angel is not so angelic in disposition as they in fact are fierce predators of the deep. 

Development and Habitat

Sea angels, Clione limacina, are invertebrates within the phylum Mollusca. Despite their shell-less appearance, these organisms are classified with other snails! Though bare in adulthood, these organisms were not always in this state. Representatives of C. limacina are born with shells that are shed upon adulthood, leaving them with soft gelatinous bodies for later in life. The visible ‘wing’ structures, also known as parapodia, are homologous (or similar due to common ancestry) with the muscular foot used by land snails for locomotion.

Sea angels have a wide distribution in the earth’s oceans, ranging from temperate to arctic zones. Regardless of temperature, these organisms are known to dwell within the mesopelagic zone of the ocean, 200-1,000 meters, occupying only as deep as 600 meters.  

Diet and Reproduction

 © Monterey Bay Aquarium

Individuals of C. limacina are sequentially hermaphroditic, meaning they have both male and female reproductive organs with the ability to swap sexes if needed. When the mating pool becomes limited, this is an incredibly useful adaptation! These creatures are also very small, only growing to be about 5 cm at the most, but are fierce predators nonetheless.  

Clione limacina has a preferred diet of one its own close relatives, its sister species: the sea butterfly! Sea butterflies, like sea angels, are born with shells. Differing from the sea angels, however, sea butterflies retain their shells throughout their lifetime. Unfortunately, the presence of a shell doesn’t offer much protection against their ravenous cousins. To deal with the pesky shells of sea butterflies, C. limacina has an adaptation of tentacle-like structures, called buccal cones, that originate from their heads and latch onto prey. These buccal cones have a radula, a mouth with teeth-like structures, and hooks to scoop the sea butterfly out of its shell like a kiwi from its skin! The sea angel then devours its prey whole and flaps away to hunt down another sea butterfly delicacy. Altogether, the process of locating prey and feeding can take anywhere from two to 45 minutes. 

A favorite for now, unfortunately C. limacina might need to find a more sustainable favorite as sea butterflies are becoming increasingly endangered by ocean acidification. As the acidity of the ocean increases (or pH decreases), the calcium carbonate making up the shell of the sea butterfly disintegrates, leaving the organism vulnerable to predation and environmental variables.

Fear not, however, as the sea angel has other nearby food sources. Some of these are phytoplankton, or floating photosynthetic organisms. In fact, consuming these is the mechanism behind the sea angels’ vibrant colorations!

 © Monterey Bay Aquarium

Sea angels are an incredible example of the diverse life dwelling in the depths of our oceans, and a great reminder that size is no indication for how well-adapted an ocean predator can be.

Check back soon for more of the Diving Deep series!

References


Monterey Bay Aquarium | Animals A to Z | Meet the Sea Angel

Smithsonian | Ocean, Find Your Blue | Angels of the Sea

Diving Deep: the Anglerfish

The Anglerfish, to some, is a true deep-sea nightmare–and not just to Marlin and Dory on their search for Nemo! This species was discovered in 1833 by an English naturalist named James Yate Johnson. At the time of discovery, not much was known about the ecology and lifestyle of this ghoulish fish, as the only details came from deceased specimens. Whereas in recent years deep-sea divers have added much to our compendium of anglerfish, much of their lifestyle is still shrouded in mystery. There are over 200 species of anglerfish extant today, varying in lifestyle and size, but they all have one thing in common: an elongated cranial spine tipped with a bioluminescent organ. 

Most anglerfish live in the bathypelagic region of the open ocean—in other words, they live in the deep, dark, and cold regions, about 2000 m (6600 ft) below the surface. In complete absence of light with scarce food in the ocean depths, these ambush predators evolved their own ways to hunt and survive—by underwater fishing! The anglerfish uses its modified cranial spine to imitate other organisms in the darkness, with the photophore at the tip using a process of bioluminescence to create a blue-green light to lure in other small marine animals. Once a prey fish is in adequate range, the anglerfish will use its powerful jaws to suck in its meal whole. Though some anglerfish can reach up to four feet long and 110 pounds, most are fairly small. Despite this stature, members of this species are known to be able to swallow prey twice their size.

Museum specimen of an Anglerfish, genus Acentrophryn; image taken by Hongseok Kim in Seoul, South Korea.
Museum specimen of an Anglerfish, genus Ceratias; image taken by Hongseok Kim in Seoul, South Korea.

The bizarre appearance and predation style of anglerfish aren’t the only factors that make these fish so interesting. In fact, one of the most fascinating and unusual practices of some anglerfish species is their disturbing mating habit! For around 90 years after the first anglerfish was discovered, scientists and researchers were baffled as the only anglerfish they were finding were all females. That’s right, every anglerfish with a lure is female! These females were sometimes found with small growth like fish attached to their bellies and were believed to be their offspring. The behavior and ecology of male anglerfish were a total mystery until 1924 when Charles Tate Regan dissected a smaller, attached fish on a female and discovered they were neither growths nor offspring, but the female’s mate! 

Male anglerfish are substantially smaller than females, only about one inch long, and lack lures, large maws, and the frightening teeth of their female counterparts. Since males are not equipped for survival on their own, they spend their lives seeking out a female mate— and mate they do. Male anglerfish have small hook teeth that they use to latch onto a female’s body. Once attached, the male anglerfish releases an enzyme that dissolves the membranes of his mouth and her skin so that their bodies can fuse together, blood vessels and all! The male will lose the body parts no longer necessary to him, including eyes, fins, and sometimes even his own internal organs. He is then entirely dependent on the female for nutrition and survival and essentially becomes a fleshy lump, ready to release sperm into the water when his mate chooses to release eggs for fertilization. Even more fascinating, a female can carry up to six males at one time!

Not every species of male anglerfish is destined to such a dark fate, but this seemingly parasitic form of reproduction sure does sound like the stuff of science fiction. So, if you’re ever feeling down in the dumps about your love life, just remember: at least you’re not an anglerfish!