Requin: A Comprehensive Guide to Species and BehaviorShark (French: “requin”) species are among the most diverse and ecologically important predators in the world’s oceans. This guide covers their taxonomy, biology, behavior, habitat, conservation status, human interactions, and ways to observe them responsibly. Technical terms are defined where helpful, and examples highlight key species and behaviors.
Taxonomy and diversity
Sharks belong to the class Chondrichthyes (cartilaginous fishes), subclass Elasmobranchii, which also includes rays and skates. There are over 500 recognized shark species grouped into around 34 families. Major groups include:
- Carcharhiniformes (ground sharks) — e.g., requiem sharks, catsharks, houndsharks.
- Lamniformes (mackerel sharks) — e.g., great white, mako, thresher.
- Squaliformes (dogfish sharks) — many deepwater species.
- Orectolobiformes (carpet sharks) — includes nurse sharks and whale shark.
- Heterodontiformes (bullhead sharks) — small, reef-associated.
- Hexanchiformes (cow and frilled sharks) — primitive six- or seven-gilled forms.
Shark sizes range from the dwarf lanternshark (~20 cm) to the whale shark (Rhincodon typus), the largest fish, reaching up to 18–20 meters.
Anatomy and physiology
Sharks have distinctive features adapted to predation and survival:
- Skeleton of cartilage, which is lighter and more flexible than bone.
- Dermal denticles (placoid scales) reduce drag and protect the skin.
- Multiple rows of replaceable teeth; tooth shape varies with diet.
- Highly developed senses: acute olfaction, electroreception (ampullae of Lorenzini), good vision adapted to low light, and lateral line for detecting water movement.
- Some species maintain elevated body temperatures (regional endothermy) for faster swimming (e.g., makos, great whites).
Reproduction: sharks exhibit oviparity (egg-laying), ovoviviparity (live birth from eggs hatched internally), and viviparity (placental nourishment). Gestation periods vary widely; for example, the spiny dogfish may have a gestation of up to 18–24 months.
Feeding ecology and behavior
Diet and feeding strategies reflect form and habitat:
- Apex predators (great white, tiger shark) feed on marine mammals, fish, and carrion.
- Filter-feeders (whale shark, basking shark) consume plankton by filtering large volumes of water.
- Benthic feeders (nurse sharks, wobbegongs) eat crustaceans, mollusks, and small fishes.
- Specialist feeders: cookiecutter shark removes plugs of flesh; sevengill sharks scavenge and ambush.
Hunting strategies include ambush, ram feeding, suction, and filter feeding. Many species are solitary; others form aggregations seasonally (e.g., whale sharks at plankton blooms, hammerheads forming schools at seamounts).
Social behavior: while often solitary, some sharks display social hierarchy, mating systems where males bite females during courtship, and site fidelity (returning to the same reefs or passages).
Habitat and distribution
Sharks occupy virtually all marine habitats — coastal, pelagic, and deep sea — and some freshwater systems (bull shark, river sharks). Distribution is global, from tropical reefs to polar seas. Key habitat types:
- Coral reefs: high species diversity (reef sharks).
- Continental shelves: many commercially important species.
- Open ocean: highly migratory species (makos, great whites).
- Deep sea: slow-growing, long-lived species (grenadiers, dogfish).
Migration and navigation: many sharks undertake long-distance migrations for feeding and reproduction, using cues like temperature, currents, magnetic fields, and olfactory gradients.
Life history and population dynamics
Sharks generally have slow life histories: late maturity, low fecundity, and long lifespans — traits that make them vulnerable to overexploitation. Example life-history traits:
- Maturity age: varies — small species may mature in 2–5 years; large pelagic sharks may take 10–20+ years.
- Longevity: some species live decades (e.g., Greenland shark estimated >200 years).
- Reproductive output: from a few pups (large species) to dozens (some reef species).
These traits produce low population resilience to high mortality (e.g., fishing), causing population declines when exploitation is intense.
Conservation status and threats
Sharks face multiple human-caused threats:
- Overfishing (targeted and bycatch), driven by demand for fins, meat, liver oil, and sport.
- Habitat loss and degradation (coastal development, pollution, coral reef decline).
- Climate change altering prey availability and habitat suitability.
- Illegal, unreported, and unregulated (IUU) fishing.
Current assessments (IUCN) list many species as Vulnerable, Endangered, or Critically Endangered. Conservation measures include catch limits, shark fin bans, marine protected areas (MPAs), bycatch reduction gear, and trade regulations (CITES listings for some species).
Human–shark interactions and safety
Shark attacks on humans are rare; most species do not pose a threat. Risk factors for bites include murky water, splashing, presence of bait/fish, and entering waters where sharks feed. Safety tips:
- Avoid swimming at dawn/dusk, near fishing activity, or where schooling fish/sea birds gather.
- Swim in groups and stay near shore.
- Do not enter the water with open wounds or wearing shiny jewelry.
Shark ecotourism (diving with sharks, cage diving) can promote conservation and local economies when well-managed and regulated.
Research methods and technologies
Scientists study sharks using:
- Tagging (satellite, acoustic) to track movements and migrations.
- Genetic analysis for population structure and species ID.
- Stable isotope analysis to infer diet and trophic position.
- Underwater video and baited remote underwater video systems (BRUVS).
- Environmental DNA (eDNA) for detecting presence in water samples.
These tools inform conservation management, identify critical habitats, and improve understanding of behavior.
Notable species profiles
- Great white shark (Carcharodon carcharias): apex predator, coastal and pelagic, global temperate distribution.
- Tiger shark (Galeocerdo cuvier): opportunistic feeder, common in tropical/subtropical waters.
- Whale shark (Rhincodon typus): largest fish, filter feeder on plankton, migratory aggregations.
- Hammerhead sharks (Sphyrnidae): distinctive cephalofoil, improved sensory perception, some species form large schools.
- Greenland shark (Somniosus microcephalus): deep, cold waters, extremely long-lived, slow-moving scavenger.
How to help and get involved
- Support sustainable seafood choices and fisheries reforms.
- Back marine protected areas and responsible ecotourism.
- Reduce plastic use and pollution; participate in beach clean-ups.
- Support organizations that fund shark research and conservation.
Sharks (requins) are a vital part of marine ecosystems — diverse in form and behavior, yet fragile in the face of rapid human impacts. Protecting them preserves ocean health and the services oceans provide.
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