The coelacanth (Latimeria chalumnae Smith, 1939) is one of the famous marine fish on Earth. Coelacanths are classic examples of “living fossils” - organisms known initially as fossils, then afterward discovered still living today. The first living coelacanth was caught in 1938 offshore from southeastern Africa. Two modern species have been described. Coelacanths were once hypothesized to be the fish group transitional to the first tetrapods, but this is now known not to be the case.
Classification: Animalia, Chordata, Vertebrata, Sarcopterygii, Coelacanthiformes, Coelacanthidae
Latimeria chalumnae (above & below) - the coelacanth (models). Known distribution of the genus: Indian Ocean from South Africa to Indonesia.
Above: CMC public display (Cincinnati Museum of Natural History and Science; Cincinnati, Ohio, USA)
Below: CMNH public display (CMNH, Cleveland Museum of Natural History, Cleveland, Ohio, USA)
This is not the “angelfish” that most people have heard about. Most angelfish (or angels) are marine fish related to & in the same family as the butterfly fish (Family Chaetodontidae). The freshwater angelfish shown below is somewhat similar to the overall body form of marine angelfish.
This is Pterophyllum scalare, a cichlid fish (Family Cichlidae). Cichlids are quite variable in body form, but Pterophyllum is unusual even for a cichlid. Garden-variety fish have an elongated, fusiform body (for example, tuna). Pterophyllum has a laterally compressed and anteriorly-posteriorly truncated body. Marine angelfish also have laterally compressed bodies, but are less truncated. The body form of Pterophyllum is often seen in fish inhabiting low-energy freshwater settings having moderately thick aquatic vegetation.
Classification: Animalia, Chordata, Vertebrata, Osteichthyes, Actinopterygii, Teleostei, Perciformes, Labroidei, Cichlidae
Pterophyllum scalare (Lichtenstein, 1823) (captives) from the Rio Negro drainage basin, northern Brazil.
Some info. from Lagler et al. (1962) and Herald (1962).
The mostly brackish-water archerfish is famous for its ability to shoot narrow streams of water from its mouth (see photo). Archerfish do this to knock down potential prey (typically insects) from above the air-water interface.
Classification: Animalia, Chordata, Vertebrata, Actinopterygii, Neopterygii, Perciformes, Percoidei, Toxotidae
Toxotes jaculatrix (Pallas, 1767) (above & below) - the banded archerfish (captives). Brackish water to freshwater to marine. Natural distribution: SE Asia & Australia.
LARGESCALE FOUR-EYED FISH
Four-eyed fish (Anableps anableps) are some of the more bizarre fish on Earth. This species inhabits freshwater to brackish water settings in coastal northeastern South America. It typically hangs out just below the air-water interface. While at the surface, its eyes protrude from the water. Four-eyed fish eyes have pear-shaped lenses & each eye is divided into two parts by a thin band of horizontal tissue (each eye has two corneas and two retinas). The dorsal portion of each eye can focus on & image objects in air (using the lower retina), while at the same time the ventral portion of each eye can focus on & image objects in the water (using the upper retina).
Four-eyed fish are live-bearers - they do not lay eggs. External reproductive structures in male & female four-eyed fish are remarkable for being asymmetrically directed. So, “left-handed males” can only mate with “right-handed females” and vice-versa. Published observations indicate that dextral males, dextral females, sinistral males, and sinistral females are roughly evenly distributed in natural populations.
Classification: Animalia, Chordata, Vertebrata, Osteichthyes, Actinopterygii, Cyprinodontiformes, Cyprinodontoidei, Anablepidae
Anableps anableps (Linnaeus, 1758) (above & below) - largescale four-eyed fish (captives) from the Paria Peninsula of coastal Venezuela.
Most info. from Herald (1962) & Lagler et al. (1962) & the Newport Aquarium.
MEXICAN BLIND CAVEFISH
Many caves are known to have blind fish in subterranean bodies of water. Members of several unrelated families have become blind cavefish. Here’s the Mexican blind cavefish, Astyanax fasciatus (sensu lato), which has been recorded from throughout Central America. It completely lacks eyes (eyes are useless in a permanently dark environment) and also has no pigmentation (hence the pinkish color from blood). However, this species does still possess the genes for eyes. Why? It’s ancestors did have eyes - they’ve been lost through evolution. This is the ultimate fate for all vestigial organs. However, the genes for lost structures are often retained for long geologic intervals of time (for example, chickens still have genes that code for teeth, despite the lack of teeth in chicken beaks).
New info. - recent published research has determined that young cavefish of this species do have functioning eyes and have a light-sensitive area in their brains. The eyes degenerate with ontogeny, as does the light-sensitive area in the brain. The adults have zero light sensing ability.
Classification: Animalia, Chordata, Vertebrata, Actinopterygii, Cypriniformes, Characoidei, Characidae
Astyanax fasciatus (Cuvier, 1819) (or Astyanax mexicanus (De Filippi, 1853), depending on one's taxonomic preferences) - the Mexican blind cavefish (captives) from southern Mexico.