2.5 million specimens don't collect themselves!
The Field Museum's Division of Fishes houses approximately 2.5 million specimens of fish, including whole specimens in alcohol, skeletal specimens, tissue samples, and cleared and stained material. That is a lot of fishes! But the fishes did not just arrive overnight; scientists and researchers have been adding to the collection at The Field Museum since 1894.
Museum collections serve as records of the natural world, as well as critical resources for scientists wanting to study the biodiversity of the planet. The fishes collection has grown through continued fieldwork by ichthyologists that use a vast array of tools and strategies for sampling living and extinct fishes from around the world. Tune into our podcast this week with a special paleoichthyological guest, Sarah Gibson of the University of Kansas. Listen to us as we discuss the many different ways we collect fishes in the field.
The allure of sharks
Sharks, and cartilaginous fishes in general, have long mesmerized scientists and the public alike. They are fascinating creatures! Fossil remains indicate that sharks have been evolving on this planet for well over 400 million years. They have been described in popular culture as "perfect killing machines," and it is hard to argue with their abilities to hunt and secure prey of all shapes and sizes. Be it the incredible filter-feeding of the immense Whale and Basking sharks or the pure power and incredible bite of the Great White Shark, few organisms in nature portray an aura of such efficiency. Sharks are among the earliest known jawed-vertebrate lineages, and since their initial evolution, they have wasted no time in putting those jaws to work. After 400 million years they still remain some of the oceans most incredible predators with no signs of slowing down.
Sharks are venomous and bioluminescent?
However, there are more to sharks then just teeth! A number of lineages have independently evolved venom, including the horn sharks and angel sharks. The horn sharks release venom out of spines that stick out from each of their dorsal fins, making them an unattractive meal for even larger predatory fishes. The spines of the dorsal fins in some squaliform (or dogfish sharks) are also known to be venomous, including species within the lanternsharks. Sharks have also independently evolved bioluminescence, the ability to generate and emit light, as they have invaded the deep sea. These include predominantly deep-sea taxa, such as the lanternsharks and the bizarre Cookie Cutter Shark that sucks on to prey and rips off hunks of flesh before speeding away. These deep-sea sharks all use bioluminescence to hide from potential prey items, to avoid being eaten themselves, and for communication.
For most fishes, reproduction involves eggs and milting, which is like crop-dusting with sex cells (aka gametes). The vast majority of fishes are oviparous, which means they lay eggs that are fertilized and develop outside the mother's body. In these situations, males typically milt, which is the release and spreading of their gametes, onto the eggs that have been deposited in the environment. In ovoviviparous fishes, the eggs develop inside the body of the mother, and male gametes have to be passed into the females’ body through specialized structures, such as claspers (modified pelvic fins) in sharks or gonopodiums (modified anal fins) in guppies. Live birth (viviparity) has also evolved in a number of lineages of fishes, including sharks, guppies, and rockfishes. In viviparous fishes, the young develop within the mothers’ body.
Male, female, or both?
While most fishes have separate sexes, a number of lineages are simultaneous or sex-switching hermaphrodites. Some fishes, such as clownfishes, can change their sex once during their lifetimes either from female to male, or male to female depending on environmental and/or behavior scenarios. A small number of fish species are simultaneous hermaphrodites capable of producing both male and female gametes at the same time (e.g., lancetfish, some species of moray eels as seen above). Scientific studies have identified that at least some of these species (e.g., the mangrove killifish Kryptolebias marmoratus) are capable of self-fertilization!
How does a fish make light?
Bioluminescence, the production and emission of light from a living creature, is widespread among different groups of marine fishes (e.g., anglerfishes, flashlight fishes, dragonfishes). Most organisms produce light through a chemical reaction between luciferin (a small molecule) and oxygen. The enzyme luciferase speeds up this reaction, resulting in the production of light. But unlike the incandescent lightbulbs in your home, this light gives off almost no heat. Some fish species have the ability to produce the chemical compounds necessary for bioluminescence themselves (such as lanternfishes), while others rely on symbiotic bacteria to create and generate light (including the beloved anglerfish in our logo).
Why would a fish want to make light?
The majority of bioluminescent fishes are found in the deep sea. Below 1,000 meters there is no visible sunlight in the ocean. As a result, many organisms that live below this depth have evolved bioluminescent structures, and fishes use this light in a variety of ways. Some fishes use light for camouflage, specifically counterillumination. This is where the fish emits light around its belly to match any light coming from overhead, making it invisible to predators looking upwards for shadows in the water column. Others use light to attract and catch prey, such as the beckoning luminescent lure of the anglerfish. Fishes will even use light for communication in order to recognize each other in the darkness of the deep or to communicate with potential breeding partners.
