Chondrichthyes

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Chimaeras are a predominately deep sea branch of elasmobranchs with most found deeper than 200m (NOAA).

Chimaeras are a predominately deep sea branch of chondrichthyes with most found deeper than 200m (NOAA).

Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera.  Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles.  These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).

Placoid denticles

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While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

Placoid denticles are found on sharks, rays, and chimaeras.  Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth.  They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel.  They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.

Weberian apparatus

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The Weberian apparatus is amplifies hearing in Ostariophysian fish.

The Weberian apparatus is amplifies hearing in Ostariophysian fish.

This series of small bones links the swim bladder to the inner ear in Ostariophysian fish, including minnows, carp, catfishes, and characins.  When a sound wave hits the swim bladder, the vibration is transferred through the Weberian apparatus to the auditory region of the inner ear.  This action helps amplify sound and, as a result, these fish have very sensitive hearing and can hear sounds that many other fish are in capable of hearing.

Swim bladder

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Herring inflate their swim bladders by "gulping" air and deflate them by "burping" or "farting." (gma.org)

Herring inflate their swim bladders by “gulping” air and deflate them by “burping” or “farting.” (gma.org)

The swim bladder is an air filled organ used by some fish to maintain buoyancy at a desired depth and produce or hear sound.

  • Physostomous swim bladders are directly connected to the gastrointestinal tract so that fish with these swim bladders, such as herrings, must “gulp” air to inflate their swim bladder and “burb” or “fart” air to deflate them.
  • Physoclistous swim bladders are not connected to the digestive tract so that fish with these swim bladders must diffuse gas from the blood to fill and collapse them.

For many fish, the swim bladder has the additional role of transferring sound waves to the auditory system.  And in some fish, such as drums and croakers, the swim bladder is used to make sounds and communicate with one another and other fish.

Fish use swim bladders for buoyancy, hearing, and producing sounds.

Fish use swim bladders for buoyancy, hearing, and producing sounds.

Lateral line

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This goldfish uses its lateral line to orient upright in the water e(Pogrebnoj Alexandroff)

This goldfish uses its lateral line to orient upright in the water (Pogrebnoj Alexandroff).

The lateral line is, literally, an observable line down both sides of a fish.  It is also a sensory organ system that helps fish detect motion in the water around them.  When water along the lateral line is displaced by movement or a vibration nearby, hair cells (similar in form and function to hair cells in a human ear) translate the displacement into an electrical impulse that is transmitted to the brain.  The lateral line helps a fish orient itself upright in the water and relative to other fish (e.g., schooling fish or predators and prey).

Photophore

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Photophores from a Splitfih Flashlight fish can be seen from 100 ft away!

Photophores from a Splitfin Flashlight Fish can be seen from 100 ft away!

Photophores are organs that are used by fish (and invertebrates) to produce light either by chemical reaction or through symbiotic bacteria capable of bioluminescence.  Most fish that use photophores live in the deep sea where light from the surface is limited.  Like a firefly in the sea, some of these fish use photophores to attract mates; others use photophores as counterillumination  and camouflage; others use their photophores like search lights to find prey or avoid predators; and still others use photophores for multiple purposes.  Splitfin Flashlight Fish (Anomalops katoptron), for example, use their photophores to communicate with other flashlight fish, attract prey, and confuse predators.  They are believed to produce the brightest bioluminescence of any organism – their light can be seen from over 100 feet away!

For other examples of fish with photophores, check out the “twinkle twinkle little fish” post on the Fisheries Blog!

Thermoregulation

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Fish don't sweat to regulate their body temperature but they do have strategies for thermoregulation (www.fishlady.us).

Fish don’t sweat to regulate their body temperature but they do have strategies for thermoregulation (www.fishlady.us).

Thermoregulation is the process by which an organism controls its internal temperature.  Fish have many different mechanisms for regulating their temperature.  Most fish are ectothermic, using their environmental temperature to manage their body temperature, but some fish are endothermic, having the metabolic ability to internally manage temperature.  Poikilothermic fish are ectotherms which have no control over their body temperature; their core body temperature conforms to ambient temperature.  Eurythermic fish have evolved to survive in a wide range of environmental temperatures and stenothermic fish have evolved to survive in a narrow range of environmental temperatures.

Thermoregulation is very important for fish because temperature influences the function of many organs and the rate of many metabolic processes.  Most fish species have evolved to survive within a specific temperature ranges; outside that range, enzymes can degrade, organs can fail, and the organism can die.  Understanding thermoregulation for fish species is particularly important when considering implications for climate change.

 

Poikilotherm

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Poikilothermic fish have no control over their body temperature and their core body temperature can fluctuate broadly.  While some ectothermic stenotherms thermoregulate their body temperature by inhabiting constant temperature environments, internal temperature of poikilotherms can widely vary.

Thoughout their lives, Steelhead's internal temperature varies considerably (NPS).

Thoughout their lives, Steelhead’s internal temperature varies considerably (NPS).

 

Eurytherm

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Desert Pupfish are eurythermic, surviving in temperatures ranging from function in waters from 4º to 45ºC (Paul V. Loiselle).

Desert Pupfish are eurythermic, surviving in temperatures ranging from function in waters from 4º to 45ºC (Paul V. Loiselle).

In contrast to stenotherms, eurythermic fish can function at a wide range of water temperatures.  They are often, but not necessarily, ectotherms.  Desert Pupfish (Cyprinodon macularius), for example, can function in ambient temperatures ranging from 4 to 45 degrees Celsius.  This thermoregulatory strategy requires that organs, enzymes, and metabolic processes can operate at varying environmental temperatures.

 

Stenotherm

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Brook Trout can only survive in a narrow band of temperatures (FWS).

Brook Trout can only survive in a narrow band of temperatures (FWS).

In contrast to eurytherms, senothermic fish can only function in a narrow range of water temperatures.  Brook Trout (Salvelinus fontinalis), for example, function optimally approximately between 13 and 18 degrees Celsius.  This thermoregulatory strategy requires that organs, enzymes, and metabolic processes operate in a small temperature band and makes these fish particularly vulnerable to environmental changes.