How Do Marine Iguanas Feed? Diving for Algae in Cold Seas

How do marine iguanas find food?

The marine iguana is the only lizard in the world that forages in the sea, and its whole day revolves around feeding and warming up. Each iguana usually has a favorite feeding area that it returns to day after day.

The population splits into two feeding groups. Only about 5 percent of marine iguanas, all of them large males, swim offshore and dive. Most females, young iguanas and smaller males feed in the intertidal zone, the strip of shore that is covered at high tide and exposed at low tide. A few medium-sized animals use both methods. This division means the big divers face less competition from the many smaller iguanas on the shore.

Diving for algae: the big males

Diving is reserved for the largest iguanas, because big bodies lose heat more slowly in cold water. The minimum size of divers depends on the island, from about 0.6 kg (1.3 lb) on Genovesa, where iguanas are small, to about 3 kg (6.6 lb) on Fernandina.

Divers usually head out around midday, when they are fully warmed by the sun. They swim slowly with side-to-side sweeps of their flattened tails, averaging only about 0.45 m (1.5 ft) per second, and can manage about twice that only in short bursts. On the bottom they grip the rocks with long, curved claws against the surge and tear off mouthfuls of algae.

Marine iguanas are naturally buoyant and float to the surface unless they swim or hold onto rocks. Their limb bones are unusually dense compared with those of land iguanas, which acts as ballast and helps them stay down. Large divers often feed only every second or third day.

Grazing at low tide: females and young

Smaller iguanas feed on algae left exposed on rocks when the tide goes out. They time their trips to low tide, run down to a patch of algae, take a few quick bites and dash back to higher ground before the next wave breaks over them. Watching a colony at low tide can look like a nervous game of tag with the sea.

This strategy keeps small iguanas out of deep, cold water, where they would chill quickly and could be swept away. It also means they can feed most days. Even hatchlings can swim, but they avoid the water, and young iguanas usually only begin entering shallow tide pools at one to two years old.

What algae do marine iguanas eat?

Marine iguanas are herbivores that eat almost nothing but marine algae, mainly fine red algae and green algae such as sea lettuce (Ulva). At least ten kinds of algae are eaten regularly. They avoid species with chemical defenses, and brown algae are of little use to them because they cannot digest them well.

A 1 kg (2.2 lb) iguana eats about 37 g (1.3 oz) of fresh algae a day. In a big colony that adds up: the roughly 1,900 iguanas at Punta Espinoza on Fernandina are estimated to eat 27–29 tonnes of algae a year, which the fast-growing algae can replace. On North Seymour Island some iguanas also eat coastal land plants, which helps them survive when algae are scarce.

Digesting seaweed needs help from microbes. Young iguanas acquire the necessary gut bacteria by eating the droppings of adults during their first months.

Built for grazing on rocks

Several features make the marine iguana an efficient algae grazer:

• Blunt snout: a short, flat face lets it press its mouth close against rock surfaces.
• Sharp three-pointed teeth: the side points are almost as large as the central one, ideal for scraping algae, and the teeth are replaced often.
• Flattened tail: the main source of power when swimming.
• Long claws: for climbing slippery rocks and holding on underwater.
• Salt glands: feeding in the sea brings in a lot of salt, which the iguana sneezes out, as explained in Why do marine iguanas sneeze?

The partly webbed feet mentioned in some books are not really an adaptation for swimming; the webbing is slight, no greater than in the green iguana.

Why marine iguanas must warm up to feed

Water around the Galápagos is often 11–23 °C (52–73 °F), while a marine iguana works best at a body temperature of about 35–39 °C (95–102 °F). A feeding trip can cool it by as much as 10 °C (18 °F). Like other reptiles, it cannot make its own body heat, as explained in Are reptiles cold-blooded?

So feeding and basking go together. Iguanas warm up on dark lava rocks before a trip and return to bask afterward. In the water their heart rate drops to about 30 beats a minute, reducing heat loss, and on land it rises to around 100 beats a minute, spreading warmth through the body. The colder the water, the shorter the feeding trip. A warm body also helps digestion, since the gut bacteria work best at high temperatures.

When feeding stops

Marine iguanas can go without food for long periods. They stop feeding in stormy weather with big waves, sometimes for more than a week. Territorial males may barely eat for weeks during the breeding season and can lose up to a quarter of their weight.

The real crisis comes in El Niño years, when warm water kills off the red and green algae. Many iguanas starve, and survivors can even shrink in length to cope, as described in Do marine iguanas shrink?