Neurologic System
The teleost brain has similar components to the brain of other vertebrates: the telencephalon (forebrain); the diencephalon (epithalamus/pineal body, thalamus, and hypothalamus); the mesencephalon; and the metencephalon (cerebellum and myelencephalon or medulla oblongata). Olfactory bulbs are connected from the nares to the telencephalon. In most fish, the spinal cord extends to the tail, but it ends earlier at the urophysis in some “higher” teleosts. An exception is the ocean sunfish (Mola mola) whose spinal cord is shorter than its brain (Helfman et al. 2009). There are 10 cranial nerves, as with other vertebrates (Roberts and Ellis 2012).
Table A1.3 Examples of sexual dimorphism in bony fish.
Source: Lodé (2012). © John Wiley & Sons.
| Feature | Changes | Examples |
|---|---|---|
| Size/shape dimorphism | Larger body in males | Discus (Symphysodon spp.), rainbowfish (Melanotaeniidae) |
| Wider head in males | Channel catfish (Ictalurus punctatus) | |
| More “humped” head in males | Freshwater angelfish (Pterophyllum scalare) | |
| Large, rounded coelom in gravid females | Most species | |
| Skin and fin changes | Brighter coloration of males during the breeding season | Zebrafish (Danio rerio), dwarf gourami (Trichogaster lalius), squarespot anthias (Pseudanthias pleurotaenia) |
| Pearl organs or nuptial tubercles on males during the breeding season | Goldfish (Carassius auratus) | |
| Different coloration of adult males and females | Kenyi cichlids (Maylandia lombardoi), California sheephead (Semicossyphus pulcher), striped killifish (Fundulus majalis) | |
| Longer fins in males | Bettas (Betta splendens) | |
| Intromittent organs | Anal fin modified into intromittent organ in males (gonopodium) | Four‐eyed fish (Anableps spp.), guppies, mollies, mosquitofish (Poeciliidae) |
| Ribs and pelvic bones modified into intromittent organ in males (priapium) | Priapium fish (Phallostethus spp.) | |
| Urogenital changes | Urogenital pouch or patch in males | Seahorses, sea dragons, and pipefish (Syngnathidae) |
| Rounder, larger, more concave genital papilla in females | Tilapia (Tilapia, Oreochromis spp.), carp and koi (Cyprinus carpio) |
There is a blood–brain barrier, but it is not well‐characterized. The blood–brain barrier shows lower permeability in most bony fish than in elasmobranchs (Jeong et al. 2008). An exception is sturgeon (Acipenseridae), which are similar to the elasmobranchs (Bundgaard and Abott 2008).
Electric Organs
Some species are able to generate electric charges using electrocytes within electric organs. These are disc‐like modified muscle cells. When stimulated, ions rush across cell membranes and create a small electric current. Stacks of cells essentially create batteries in series and produce an additive effect. These organs are innervated by the spinal cord and the potential voltage and frequency depend on fish species, activity level, and size (Kramer 1996; Helfman et al. 2009). Interestingly, these animals usually suffer no ill effects themselves from the electricity. It is unknown why, but reasons may include their body size, the directionality of the current, adipose under the skin, and structural proteins which provide electroprotection.
Anatomy of Cartilaginous Fish
This section highlights the differences between cartilaginous fish and bony fish (Box A1.1).
Table A1.4 Ovarian types in fish.
Source: Stoskopf (1993). © 1993, Elsevier.
| Definition | Examples | |
|---|---|---|
| Cystovarian | Ova released into oviduct | Most bony fish |
| Gymnovarian | Ova released into coelom, then ostium, then oviduct | Lungfish (Dipnoi), sturgeon (Acipenseridae), bowfin (Amia calva), cartilaginous fish |
| Semicystovarian (secondary gymnovarian) | Ova released into coelom, then through urogenital pore | Salmonids (Salmonidae) |
Table A1.5 Example of reproductive modalities in bony fish.
Source: Lodé (2012). © John Wiley & Sons.
| Definition | Examples | |
|---|---|---|
| Ovuliparity | Ova expelled externally, then fertilized | Salmonids (Salmonidae), sticklebacks (Gasterosteidae) |
| Oviparous | Internal fertilization, then ova expelled externally | Most teleost species |
| Ovoviviparous | Internal fertilization, retention of ova in body for embryo development, live births | Some rockfish (Sebastidae) |
| Viviparous (histotrophic or lecithotrophic) | Embryo development in body, nutrients provided by body: glandular, oophagy, adelphophagy | Guppies, mollies (Poecilidae) |
| Viviparous (hemotrophic or matrotrophic) | Embryo development in body, nutrients provided by body: pseudoplacentation or placentation | Four‐eyed fish (Anablepidae), cusk‐eels (Ophidiidae), some blennies (Clinidae), some rockfish (Sebastidae), splitfins (Goodeidae), halfbeaks (Hemiramphidae) |
Body Plan
Pelagic sharks share a similar, hydrodynamic body shape, e.g. requiem sharks (Carcharhinidae) and ground sharks (Triakidae). Epibenthic, benthic, and demersal sharks typically have large heads, more