This is the index to every glossary entry on Shark Sider: the anatomical, reproductive, ecological and fisheries terms used throughout the species pages, each defined in a line here and explained in full on its own page. They are grouped by what they describe rather than alphabetically, because the terms that confuse people are usually the ones sitting next to each other.
Three groups account for most of the confusion. The three reproductive modes — oviparous, ovoviviparous and viviparous — are routinely mixed up. The sensory structures on a shark's head look alike and do entirely different jobs. And the conservation vocabulary is routinely assumed to carry legal weight that it does not have.
On this page
Anatomy and the senses
Most of the structures on a shark's head are small, look broadly similar and are constantly confused with one another. These six entries separate them.
- Ampullae of Lorenzini — the electroreceptors: pores across the snout opening into jelly-filled canals that read the faint electric fields of living animals at close range.
- Barbels — the paired fleshy feelers beside the nostrils of nurse, bamboo and carpet sharks, loaded with touch and taste receptors for working over the sea floor.
- Cirri — short flexible outgrowths of skin, most obvious as the branched fringe along the head of a wobbegong, where they break up the animal's outline against the reef.
- Gills — the breathing apparatus: filaments on cartilage arches that pull dissolved oxygen out of the water, opening to the outside through five to seven slits on each side.
- Placoid scales — also called dermal denticles, the tooth-like scales that cover shark skin, giving it the texture of coarse sandpaper and reducing drag as the animal swims.
- Spiracles — the small inlets behind the eyes of bottom-dwelling species, which let a shark lying still on sand draw water over its gills without opening its mouth.
The full picture is assembled in the guide to shark anatomy.
Reproduction, light and activity
The reproductive terms are the ones readers most often get the wrong way round, so they are worth reading together rather than one at a time.
- Oviparous — the female lays an egg case and the embryo develops outside her, living entirely on the yolk sealed inside. Catsharks, horn sharks and bamboo sharks work this way.
- Ovoviviparous — the eggs are retained and hatch inside the female, but the young are still fed by their own yolk rather than by the mother, and are born alive.
- Viviparous — the mother feeds the embryos directly through a yolk-sac placenta once the yolk runs out. Hammerheads, requiem sharks and the blue shark all do this.
- Bioluminescent — able to produce light chemically, through skin organs called photophores. The lanternsharks and the kitefin shark are the best-documented luminous sharks.
- Nocturnal — feeding and moving mainly at night and resting by day, the pattern followed by nurse sharks, whitetip reef sharks and a long list of other reef species.
These behaviours sit alongside the wider material on the shark facts hub.
Distribution, genetics and naming
Four terms deal with where a shark is found, what it is called and how the two are established.
- Endemic — native to one defined area and occurring naturally nowhere else, whether that area is an island group or an entire continental shelf.
- Sympatric — sharing overlapping geographic ranges with another species, so that the two can meet in the wild. It says nothing about whether they interbreed.
- Genetic — relating to genes and the DNA they are written in. Genetic work is what separates populations that look identical and identifies fins in trade.
- Scientific nomenclature — the two-part Latin naming system that gives every species one unambiguous name, in place of common names that change from country to country.
Where those names sit in the wider classification is covered in what is a shark and across the full list of shark types.
Conservation and fisheries
Two terms carry most of the weight in anything written about shark declines, and both are regularly misunderstood.
- Bycatch — the animals a fishery catches without targeting them. For most shark species it is the main source of mortality, and much of it is discarded dead.
- IUCN Red List — the global inventory of extinction risk, placing each assessed species in one of nine categories. It measures risk and carries no legal force of its own.
Both feed directly into the question of whether sharks are endangered and what can be done about it.
Frequently asked questions
What is the difference between oviparous, ovoviviparous and viviparous?
Oviparous sharks lay egg cases and the embryo develops outside the mother on its own yolk. Ovoviviparous sharks retain the eggs until they hatch inside her, but the young are still fed by yolk. Viviparous sharks nourish the embryos directly through a yolk-sac placenta and give birth to live young.
What are the sensory organs on a shark's head called?
The main ones are the ampullae of Lorenzini, jelly-filled pores that detect electric fields, and the lateral line, which senses water movement. Barbels beside the nostrils of some species carry touch and taste receptors, and cirri are skin outgrowths used mainly for camouflage rather than sensing.
What is the difference between endemic and sympatric?
Endemic is about exclusivity: a species found naturally in one defined area and nowhere else. Sympatric is about company: two or more species whose ranges overlap, so members of each can encounter the other. A species can be both, being endemic to one region and sympatric with another species inside it.
What are dermal denticles?
They are the tooth-like scales covering shark skin, also called placoid scales. Each has a base set in the skin, a core of dentine and a hard enamel-like outer layer, built on the same plan as a tooth. They give shark skin its sandpaper feel and reduce drag as the animal swims.
Does the IUCN Red List protect sharks?
No. The Red List assesses how close a species is to extinction and publishes the evidence, but it has no legal force. Protection comes from CITES trade controls, regional fisheries agreements and national law, which often cite Red List assessments as the basis for acting but are separate instruments.
References
A small team focused on researching sharks and making marine biology easy to understand. Every page is reviewed against current species records and the SharkSider attack database, and updated when new incidents or research appear.
Published December 1, 2014 · Last updated September 23, 2026 · Part of the Shark Facts section.
