Spiny Dogfish vs Small Sharks: Key Differences

Spiny dogfish differ from other small sharks mainly in their two venomous spines, extreme schooling behavior, and a slower growth rate that makes them unusually long-lived for their size. While most small sharks are solitary or loosely social, spiny dogfish travel in massive packs of hundreds or thousands, and they possess a defensive mechanism no other small shark has. This article compares the biology and behavior of the spiny dogfish with other common small sharks like the smoothhound, catshark, and bonnethead, covering anatomy, diet, reproduction, and habitat.

What Is a Spiny Dogfish?

The spiny dogfish (Squalus acanthias) is a small shark in the order Squaliformes, family Squalidae. It rarely exceeds 1.2 meters (4 feet) and typically weighs 3–8 kg (7–18 lb). Its common name comes from its habit of hunting in dog-like packs and from the two sharp, venomous spines located in front of each dorsal fin. These spines are unique among the small sharks commonly encountered in temperate waters.

Other small sharks often compared with the spiny dogfish include:

• Smoothhound sharks (genus Mustelus) – similar size, but no spines and no venom.
• Catsharks (family Scyliorhinidae) – smaller, bottom-dwelling, often patterned.
• Bonnethead sharks (Sphyrna tiburo) – a small hammerhead, tropical and social.
• Spurdogs – a regional name for the same spiny dogfish species.

These species overlap in size, but their biology and behavior diverge in key ways.

Anatomy and Defensive Adaptations

The most obvious difference is the spiny dogfish’s two dorsal spines. Each spine sits just in front of the dorsal fin and secretes a mild venom from glandular tissue at its base. When threatened, the shark arches its back and can inflict a painful, though rarely dangerous, sting. No other small shark has this defense. Smoothhounds rely on speed and camouflage; catsharks hide in crevices; bonnetheads use their flattened heads to pin prey.

Spiny dogfish also lack an anal fin, a trait shared with all dogfish sharks but not with smoothhounds, catsharks, or bonnetheads, which all have anal fins. Their skin is covered in sharp dermal denticles that feel like sandpaper, common to all sharks but especially rough in dogfish. The spiny dogfish has a more cylindrical body and a pointed snout, whereas smoothhounds have a flatter head and catsharks often have elongated, cat-like eyes.

Another key anatomical difference is the teeth. Spiny dogfish have small, sharp, cusped teeth in both jaws, suited for grasping and slicing. Smoothhounds have flat, pavement-like teeth for crushing shellfish. Catsharks have small, multi-cusped teeth for gripping small fish and invertebrates. Bonnetheads have flattened teeth at the back of their jaws for crushing crabs.

Behavior and Social Structure

Spiny dogfish are among the most social sharks in the ocean. They form schools that can number in the thousands, migrating seasonally along continental shelves. This schooling behavior is rare among small sharks. Smoothhounds are often solitary or found in small groups, especially during mating. Catsharks are mostly solitary, though some species aggregate in rocky reefs. Bonnetheads form schools of 10–100 individuals, but these are smaller and more loosely organized than dogfish schools.

Spiny dogfish also show a distinct diel pattern: they feed more actively at night and rest during the day, often in dense clusters near the bottom. Their pack-hunting strategy helps them corral prey like herring, squid, and small fish. In contrast, bonnetheads are daytime foragers, and catsharks are typically nocturnal hunters that rely on smell and electroreception.

Diet and Feeding Habits

Spiny dogfish are opportunistic predators with a broad diet. They eat small fish (herring, capelin, sand lance), squid, octopus, and sometimes jellyfish. They also consume crustaceans and even marine mammal carrion. Their sharp teeth and fast strike let them take prey up to a third of their own body length.

Smoothhounds specialize in hard-shelled prey: crabs, lobsters, shrimp, and small clams. Their flat teeth are adapted to crush, not cut. Catsharks feed on small fish, polychaete worms, and mollusks, often scavenging. Bonnetheads have a notable preference for blue crabs, which can make up over 80% of their diet in some areas, though they also eat shrimp and small fish.

This dietary difference affects their behavior. Smoothhounds and bonnetheads spend more time foraging on the seafloor, while spiny dogfish are more pelagic, chasing schools of fish in open water.

Reproduction and Life History

Spiny dogfish have one of the longest gestation periods of any vertebrate: 18–24 months. They are viviparous, giving birth to live young (typically 2–11 pups per litter). Pups are born fully formed, about 20–30 cm long, and are immediately independent. Females mature only after 12–15 years, and the species can live 40–70 years, making them extremely slow-growing and late-maturing.

Other small sharks have faster life histories:

• Smoothhounds: gestation 10–12 months, litters of 4–20 pups, mature at 4–7 years.
• Catsharks: many are oviparous (lay egg cases, “mermaid’s purses”), with incubation of 5–11 months, and mature at 2–5 years.
• Bonnetheads: gestation 4–5 months, litters of 4–15 pups, mature at 2–3 years.

This contrast matters for conservation: spiny dogfish are far more vulnerable to overfishing because their population cannot rebound quickly. Many fisheries have collapsed in the past due to this slow reproduction.

Habitat and Distribution

Spiny dogfish are found in temperate and subarctic waters worldwide, from the coasts of North America, Europe, and Asia to New Zealand and Chile. They prefer depths of 10–200 m but have been recorded down to 900 m. They migrate long distances, sometimes over 1,000 km, following water temperatures and prey.

Smoothhounds also inhabit temperate waters but tend to stay closer to the bottom on continental shelves. Catsharks are mostly deep-water or reef-associated, often in tropical and temperate zones. Bonnetheads are strictly warm-water sharks, found in the western Atlantic and eastern Pacific, in bays and estuaries.

These habitat preferences affect human encounters. Spiny dogfish are commonly caught by anglers and as bycatch in trawl fisheries. Their spines make them a nuisance to handle, leading many fishers to consider them pests. In contrast, smoothhounds are valued for their meat and are often targeted by commercial fisheries.

Economic and Ecological Importance

Spiny dogfish are harvested for their meat (often sold as “rock salmon” or “flake” in some regions) and their fins. However, their slow growth makes them prone to overfishing. In the 1990s, the Northeast Atlantic stock collapsed, leading to strict quotas. Today, some populations are recovering, but the species remains classified as Vulnerable by the IUCN.

Smoothhounds are also commercially important, especially in Europe and the Americas, where they are sold fresh or frozen. Catsharks are less targeted but are common bycatch. Bonnetheads are not a major commercial species but are caught in small-scale fisheries.

Ecologically, spiny dogfish are mesopredators that help control prey fish populations. Their schools also serve as prey for larger sharks, seals, and killer whales. Smoothhounds and bonnetheads play similar roles in their respective ecosystems, but their more specialized diets make them more sensitive to changes in prey abundance.

Frequently Asked Questions

Are spiny dogfish dangerous to humans?

Spiny dogfish are not aggressive toward humans, but their venomous spines can cause painful wounds if the shark is handled carelessly. The venom is mild and not life-threatening, but it can cause swelling and infection. Anglers should avoid grabbing a dogfish by the back.

Why are they called “dogfish”?

The name comes from their habit of hunting in packs, like dogs, and their dog-like, persistent chasing of prey. It is not a reflection of any relation to dogs.

Can you eat spiny dogfish?

Yes, spiny dogfish are edible and are sold in many countries as “rock salmon” or “flake.” However, due to their slow reproduction, consumers should check that the fish comes from a sustainably managed stock.

What is the lifespan of a spiny dogfish compared to other small sharks?

Spiny dogfish can live 40–70 years, which is exceptionally long for a small shark. Smoothhounds live about 10–20 years, catsharks 10–15 years, and bonnetheads about 10–12 years.

Do all small sharks have spines?

No, only dogfish sharks (family Squalidae) have dorsal spines. Other small sharks, like smoothhounds, catsharks, and bonnetheads, have no spines at all.

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