What Eats Seaweed?

Various organisms eat seaweed, including small crustaceans like bristle worms (Polychaeta) and snails (Gastropoda), as well as large mammals like manatees (Trichechus spp.). Seaweed is also consumed by crustaceans such as crabs, shrimp, and lobsters, along with starfish, sea urchins, betta fish, turtles, eels, pufferfish, sea lions, penguins, and certain bird species like the albatross.

Seaweed plays a significant role in marine ecosystems, providing food and shelter for a wide range of organisms, and contributes to the overall health of our oceans.

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Not intentionally. Dolphins are carnivores and eat a variety of small fish, squid and shrimp. The large mammals sometimes hunt in groups but also feed alone.

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Dolphins are highly intelligent and social marine mammals found worldwide. As carnivores, dolphins have evolved to hunt and consume a diet comprised primarily of fish, squid, and other oceanic prey. But when swimming through seas teeming with kelp forests, seaweed beds, or marine algae, you may wonder—do dolphins ever eat seaweed? While these aquatic acrobats may ingest bits of seaweed incidentally, they do not intentionally feed on marine vegetation.

Table of Contents

  • What is Seaweed and Where Does it Grow?
  • Dolphin Physiology and Metabolism
  • What Marine Animals Eat Seaweed?
  • Do Dolphins Eat Seaweed?
  • Can Seaweed be Part of a Healthy Dolphin Diet?
  • Nutrients in Fish vs. Seaweed
  • Seaweed as Food and Medicine for Humans
  • Locations with Abundant Seaweed Growth
  • Impact of Seaweed on Dolphins

What is Seaweed and Where Does it Grow?

The term seaweed refers to several species of multicellular, marine macroalgae. Rather than having true roots, stems, and leaves, seaweed attaches to rocky substrates, coral reefs, shells, and other ocean surfaces. Seaweed is classified into three broad groups based on pigmentation: brown, red, and green algae.

There are over 10,000 known species of seaweed found along rocky coastlines, continental shelves, seagrass meadows, and coral reefs worldwide. Seaweed grows in tidal areas as well as in deeper, offshore locations. Major habitats include:

  • Intertidal Zone – Seaweed attaches to hard substrates and is exposed during low tides. It grows in tidepools and on rocky shores prone to wave action.
  • Kelp Forests – Dense aggregations of brown algae like giant kelp form underwater towers up to 60 m tall, creating a canopy and habitat for other organisms.
  • Coral Reefs – Seaweed is abundant on coral reefs, providing food for herbivorous fish and crustaceans. It competes for space with corals.
  • Sargassum Mats – Floating golden-brown seaweed forms massive offshore rafts providing key habitat and refuge for marine life.
  • Seagrass Meadows – Underwater seagrass beds interspersed with diverse seaweed species create critical nursery habitat for many animals.

Some key facts about seaweed:

  • It covers about 3% of the total ocean surface, primarily growing along coastal regions.[1]
  • Annual global seaweed production is estimated at over 30 million metric tons, primarily for use in food, medicines, cosmetics, and fertilizers.[2]
  • Seaweed plays a vital ecological role as a primary producer, carbon sink, food source, and marine habitat.

Dolphin Physiology and Metabolism

Dolphins belong to the taxonomic order Cetacea which includes whales, dolphins, and porpoises. As marine mammals, they are fundamentally adapted to obtain their energy and nutrient requirements from animal-based prey rather than plant materials. Here’s why dolphins are obligate carnivores:

  • Dolphins have a streamlined digestive system with a simple stomach and short intestine compared to land-based herbivores. This allows for rapid digestion of animal protein, fats, and bones.[3]
  • They lack cellulase and other enzymes required to break down the complex carbohydrates found in plant cell walls.[4]
  • Up to 50% of their daily caloric intake comes from blubber reserves or fat contained in prey species.[5]
  • Dolphins have a higher baseline metabolic rate than land mammals given the heat conduction properties of water. This increases their energy requirements which animal prey efficiently provides.[6]
  • To support their high activity level and metabolism, around 4-6% of their body weight must be consumed in prey daily.[7]

Given these evolutionary adaptations, dolphins require a diet high in protein, fat, and calories which seaweed lacks. Next we’ll compare seaweed-eating marine animals.

What Marine Animals Eat Seaweed?

While seaweed forms an indispensable part of many marine food chains, dolphins have evolved as apex predators not directly dependent on plant matter for energy. But many key ocean species do directly consume seaweed. Herbivorous marine animals include:

Fish – Many fish nibble or graze directly on live seaweed and algae. This includes coral reef species like parrotfish, damselfish, surgeonfish, and butterflyfish that consume significant volumes of seaweed daily.[8]

Urchins – Spiny sea urchins like the red sea urchin have specialized mouthparts called Aristotle’s lantern which they use to scrape algae off surfaces.[9] Their grazing helps keep coral reefs free of overgrown seaweed.

Turtles – Green sea turtles are unique among marine turtles for being herbivorous, feeding on seagrasses and multiple types of algae and seaweed.[10] Their serrated jaws help slice and consume large volumes of vegetation.

Manatees – Slow-moving sea cows spend 6-8 hours daily grazing on seagrasses and algae, consuming 10-15% of their body weight.[11]

Crabs – Numerous crab species shred and eat seaweed using their claws and chelipeds, especially in the intertidal zone. Examples are spider crabs, porcelain crabs, and California rock crabs.[12]

Snails – Marine snails like limpets, sea hares, and periwinkles directly graze on seaweed. Their tongue-like radula helps scrape and shred the vegetation.[13]

Do Dolphins Eat Seaweed?

Given their anatomy and physiology as obligate carnivores, dolphins are not capable of directly consuming seaweed as a major food source. There are several reasons dolphins do not actively feed on marine vegetation:

  • Lack of digestive adaptations – Dolphins lack the specialized teeth, enzyme secretions, gut flora, and lengthy intestinal tracts to break down and absorb nutrients from fibrous seaweed.[14]
  • Insufficient protein and fat – Dolphins require high levels of protein and fat, which seaweed lacks. Fish and squid provide complete, highly digestible protein.
  • Potential nutritional deficiencies – Seaweed may not provide adequate levels of certain vitamins and minerals dolphins require. Iodine in seaweed could potentially reach toxic levels if large amounts were consumed.[15]
  • Higher energy content of animal prey – Because of their high metabolism, dolphins need calorie-dense foods. Squid, fish, and crustaceans contain 2-10 times more calories per ounce than seaweed.[16]

While dolphins do not intentionally eat seaweed, they may occasionally ingest small amounts while chasing prey in areas with abundant seaweed growth. Such ingestion tends to be incidental and minimal.

Can Seaweed be Part of a Healthy Dolphin Diet?

Given dolphins’ evolutionary adaptations, there is no evidence that seaweed could provide a beneficial dietary component for dolphins. Here are some considerations:

  • Lower protein content – Fish and squid consist of 15-20% protein, while seaweed contains 5-10% protein at maximum, and in less digestible forms.[17]
  • Lack of lipids – The blubber and fatty prey dolphins consume provides high energy density. Seaweed contains almost no digestible lipids.
  • Indigestible complex carbs – The tough cell walls and carbohydrate complexes in seaweed cannot be adequately broken down by a dolphin digestive system.
  • Potential nutrient deficiencies – Seaweed lacks key nutrients like vitamin B12, vitamin D, and calcium compared to whole fish and squid prey. [18]
  • Risk of excess iodine – While dolphins need some dietary iodine, the high levels in seaweed could cause thyroid dysfunction if frequently consumed.[19]
  • Higher risk of gut issues – Too much seaweed could potentially cause indigestion, gas, bloating, and intestinal blockages due its fibrous nature.[20]

Overall, seaweed would not offer sufficient protein, energy, or digestible nutrients to constitute a healthy part of a dolphin’s diet. Their physiology is specifically adapted to derive nutrition from animal rather than plant matter.

Nutrients in Fish vs. Seaweed

Dolphins have evolved to take advantage of the dense nutrition provided by fish, squid, and other prey species. Here’s a comparison of the nutritional composition per 100 grams of seaweed versus common dolphin prey:

NutrientSquid[21]Mackerel[22]Seaweed (Kombu)[23]
Protein15 g18 g3 g
Fat0.6 g15 g0.6 g
Vitamin B126 μg7 μg0 μg
Calcium37 mg19 mg168 mg
Iron0.5 mg1.5 mg2.0 mg
Zinc1.3 mg0.9 mg1.2 mg

As the table illustrates, seaweed is considerably lower in protein, fat, and calories compared to animal prey. And while it does contain some vitamins and minerals, they are not always in the most bioavailable or appropriate forms for dolphin physiological needs.

Seaweed as Food and Medicine for Humans

Although seaweed may not provide adequate nutrition for dolphins, it has been an important dietary component for coastal human populations worldwide dating back thousands of years. Edible seaweeds are a staple ingredient in Asian cuisines. Global demand for seaweeds has risen rapidly, with annual production exceeding 30 million tons.[2] Key facts about seaweed as human food include:

  • Provides an abundant source of dietary iodine which is often lacking in inland diets
  • Contains unique bioactive polysaccharides like fucoidan that may have antiviral, anticancer effects[24]
  • Offers a nutritious salt substitute that is naturally low in sodium
  • Imparts beneficial prebiotics, dietary fiber, vitamins, and minerals
  • Yields beneficial nutritional components like algal oil rich in omega-3s
  • Used as a thickening and stabilizing agent in many foods

In addition to nutritional usage, seaweed has been part of traditional medicine systems for treatment of goiters, injuries, and digestive issues.[25] Modern medical applications are also being developed based on seaweed’s anti-inflammatory, antimicrobial properties.

Locations With Abundant Seaweed Growth

While dolphins roam oceans worldwide, some areas where they live have especially abundant seaweed growth that may lead to higher incidental ingestion:

Sargasso Sea – This region of the North Atlantic has massive rafts of floating sargassum seaweed that serve as critical habitat for many species. The dense brown macroalgae provides food, refuge, and nursery grounds.[26]

Western Australia – Extensive seagrass meadows with diverse seaweed assemblages are found along the coast. Seaweed forests support the largest population of dolphins in Australia.[27]

Gulf of California – Home to many dolphins, this area has huge kelp forests and one of the most diverse seaweed assemblages in the world with over 500 species.[28]

Irish Sea – Kelp species like oarweed and cuvie are interspersed in seagrass beds used by local dolphins for feeding and nursing calves.[29]

Patagonia – Large kelp beds along the Argentinean and Chilean coast provide habitat for marine mammals and shorebirds that dolphins feed on.[30]

In all these regions, while dolphins may occasionally ingest bits of seaweed incidentally, it does not provide a meaningful nutritional or energy source.

Impact of Seaweed on Dolphins

In areas where dolphins live alongside substantial seaweed growth, here are some potential impacts:

  • Trace nutrition – Ingesting mouthfuls of seaweed while hunting for fish in kelp forests or sargassum rafts provides small amounts of minerals and pigments such as iodine, chlorophyll, and antioxidants.[31][32]
  • Risk of toxicity – Toxins like ciguatoxin accumulate up the food chain from microalgae to fish. Eating contaminated prey can cause poisoning in dolphins, especially from toxic algal blooms.[33]
  • Habitat benefits – Living adjacently with seaweed beds provides refuge to hide calves from predators. Abundant fish that associate with seaweed offer prime hunting grounds.[34]
  • Entanglement risk – Floating rafts of Sargassum occasionally trap dolphins and other marine animals. Disentanglement programs help reduce risk.[35]
  • Gut impaction – Consuming large amounts of seaweed could potentially cause intestinal gas, bloating, obstructions, or dehydration though cases are unconfirmed.[36]

Overall, while dolphins may derive trace secondary nutrition from incidental seaweed ingestion, it is not a ecologically vital food source given their fundamental carnivorous physiology and metabolism.


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