Marine Vertebrates & Mammals

Secondarily Marine Vertebrates

  • Marine organisms with terrestrial ancestors.

  • Last week covered reptiles and seabirds.

  • This week focuses on marine mammals.

Objectives

  • Critically discuss, what is a marine mammal?

  • Discuss origins, diversity, main types, features of marine mammals.

  • Describe how marine mammals are associated with the sea and with the land.

General Information: What is a Marine Mammal?

  • A mammal that is primarily ocean dwelling and depends on the ocean for its food.

  • Feeding in the sea is the common link.

  • Different groups represent different transitions from terrestrial ancestors.

  • Parallel transitions are seen with reptiles and seabirds.

  • Varying degrees of marine adaptation exist among marine mammals.

Examples of Marine Adaptation

  • Polar Bear:

    • Amphibious, spends time on land and sea ice.

    • Good swimmer, swims for kilometers.

    • Depends on seals and whales for food.

  • Seal:

    • Spends a lot of time at sea.

    • Comes on land to breed.

  • Whales and Dolphins:

    • Fully aquatic.

    • Lives and gives birth in the sea.

Marine Mammal Numbers

  • Approximately 130 out of 6,500 living mammal species are marine (about 2%).

  • Marine mammals are spread over three different orders: carnivores, sirenians, and cetaceans.

Carnivores

  • Includes cats and dogs.

  • Marine carnivores include polar bears, sea otters, and pinnipeds (seals, sea lions).

  • Multiple independent transitions to marine lifestyle within carnivores.

Family Tree of Carnivores

  • Sea otters and marine otters belong to the mustelids (badgers, weevils, minks).

  • Pinnipeds evolved from a bear-like ancestor.

  • Polar bears evolved from a terrestrial brown bear.

Polar Bear

  • Lives in the Arctic.

  • Largest of all bears.

  • Evolved from a brown bear about 15,000 years ago.

  • Opportunistic feeders, but primarily eat seals and whales.

  • Hunts from sea ice.

Sea Otter

  • Wholly marine; gives birth in the water.

  • Lives in the North Pacific.

  • Historically widely distributed, now restricted to a few locations (fragmented population due to hunting for pelts).

  • Relies solely on fur for insulation (lacks blubber).

  • Spends a lot of time grooming due to fur dependence.

  • High metabolic rates lead to a ravenous appetite.

  • Feeds on large amounts of invertebrates and fish.

  • Considerably large for otters, reaching up to 45 kg and 1.5 meters.

Pinnipeds

  • Evolved from a bear-like creature about 20-25 million years ago.

  • Found mainly in cold waters of both hemispheres.

  • Give birth on land (varying degrees of terrestrial time).

  • Retained four limbs modified as flippers for propulsion.

  • Fusiform body shape (cigar-shaped) for streamlining.

  • Reduced tail.

  • Three main types:

    • Ataridae: Sea lions and fur seals (external ear flaps/pinnae, can walk on land by rotating pelvis, forelimb propulsion).

    • Focidae: True seals (no ear flaps, poor terrestrial locomotion, hind limb propulsion).

    • Walruses: (Only one living species, tusks are elongated canine teeth, squirt suction feeding).

Ataridae (Sea Lions and Fur Seals)

  • Thick fur in adults (especially fur seals).

  • External ear flaps (pinnae).

  • Can walk on land by rotating the pelvis and using all four limbs.

  • Underwater propulsion mainly through forelimbs.

  • Example: Antarctic fur seal (thick fur), Australian sea lion (rotates pelvis to walk).

Focidae (True Seals)

  • No ear flaps (earless seals).

  • Cannot rotate their pelvis, resulting in poor terrestrial locomotion (rolling/undulating).

  • Underwater propulsion mainly through hind limbs.

  • Examples:

    • Weddell seal: Thick pup fur, visible digits in the forearm.

    • Elephant seals: Southern species around Antarctica, named for long proboscis on males, sexually dimorphic (males larger), polygamous, deep diving (up to 1,500 meters), feed on cephalopods.

    • Leopard seals: Restricted to Antarctica, large predators, feed on penguins (shake and beat against water), have attacked humans.

    • Crabeater seals: Multi-lobed teeth for filter-feeding on krill.

Walrus

  • The modern walrus is the only living representative of a once more diverse group.

  • Recognizable by tusks (elongated canine teeth, more developed in males).

  • Three populations: Pacific, Atlantic, and Laptev Sea.

  • No external ears.

  • Can theoretically rotate pelvis but too fat to walk properly on land.

  • Mainly use forelimbs in water, some hind limb use.

  • Tusks used as tools (digging, hauling onto ice), dominance displays.

  • Feed on benthic invertebrates in soft sediments (bivalves).

    • Use "squirt suction feeding": expel water to displace sediment and suck in food.

    • Favor right forelimb when feeding (89% of the time).

Sirenians

  • Thought to have evolved from elephant-like ancestors.

  • Two main types: dugongs and manatees.

Dugongs

  • Square jaw deflected downwards.

  • Triangular tail (like a dolphin).

  • Distribution: Historically broadly distributed in coastal waters of the Indo-West Pacific.

  • Threatened by overfishing and bycatch.

Manatees

  • Rounded jaws.

  • Rounded tail.

  • Three species:

    • Amazon species (freshwater).

    • West African species (freshwater).

    • West Indian species (Caribbean and Florida, largest, up to 1,500 kg).

Feeding Habits of Sirenians

  • Dugongs mainly feed on seagrasses.

  • Herbivorous marine mammals.

    • Large size relates to staying warm; low surface area to volume ratio reduces heat loss.

Revision Questions

  • Approximately what percentage of the total number of living species of mammal is marine? 2%

  • Which of the following lists the animal groups in decreasing order of the number of living marine species?
    *Invertebrates, fishes, birds, mammals, reptiles (There are ~350 species of birds, 130 mammals, and <100 reptiles).

  • True or false? Members of the Ataridae have external ear flaps. True

  • Which of the following uses suction squirt feeding? Walrus

  • Dugongs spend most of the time most of their lives in the water but give birth on land. False (they give birth in the water)

Cetaceans

  • Whales, dolphins, etc.

  • Two main types:

    • Baleen whales (Mysticetes).

    • Toothed whales (Odontocetes).

General Characteristics of Cetaceans

  • All are fully aquatic, never leave the water.

  • Most are marine, but some freshwater forms exist.

  • Distant relatives of cows, hippopotamuses, and camels.

  • Earliest whales (now extinct, known from fossils) are called archaeocytes.

  • Modern forms diverged from archaeocytes about 40-35 million years ago (long marine history).

  • Often described as mammals that look like fish (parallels with dugongs).

Physical Adaptations

  • Forelimbs are rigid paddles.

  • Hind limbs are vestigial (rudimentary, dysfunctional).

  • Propel themselves by a tail fluke.

Forelimbs

  • Humpback whales: largest forelimbs, up to 131\over3 of the body length, used for stopping and steering.

  • Cetacean skeletons: same bones as other mammals (scapula, humerus, radius, ulna, phalanges), but humerus, radius, and ulna are fused (no elbow).

  • Phalanges are lengthened for a large surface area paddle.

Hind Limbs

  • Rudimentary hind limb in embryos, but not visible externally in adults.

  • May find tiny rudimentary bones within the body wall.

Tail Fluke

  • The tail fluke of a humpback whale may reach up to 4 meters across.

  • Made of fibrous connective tissue (no bone or cartilage).

  • Orientation is horizontal, not vertical (unlike fish), relates to evolutionary history from land tetrapods, using spinal muscles to move the tail up and down.

  • Easier to move tail up and down than side to side due to land tetrapod ancestry.

Fusiform Body

  • Streamlined body to overcome viscous forces of the water.

  • Minimal external protuberances (largely hairless, no external ears) to reduce turbulence.

  • Thick blubber for insulation and food storage, highly vascularized (can control blood flow for temperature regulation).

Breathing

  • Breathe entirely through nostrils (blowholes on top of the head for easy breathing).

  • Mysticetes (baleen whales): two blowholes (equivalent to two nostrils), flaps of muscle close off blowhole when diving.

  • Odontocetes (toothed whales): one blowhole (not directly equivalent to nasal opening), use modified nasal canals to produce sonar.

    • Mysticetes: produce low-frequency sounds.

    • Odontocetes: produce high-frequency sonar for echolocation.

Baleen Whales (Mysticetes)

  • Filter feeders (krill, fish, squid) using baleen plates.

  • Four types:

    • Pygmy right whale (smallest, ~6 meters).

    • Right whales and bowhead whales (Northern Hemisphere only; asymmetrical upper jaw, ~18 meters, slow-moving).

    • Rorqual whales (throat pleats, humpback, blue, minke whales; throat pleats allow expansion of the gape for feeding).

    • Gray whale (North Pacific, feeds on benthic invertebrates).

Baleen Plates

  • Evolved from ancestors with teeth, now rudimentary.

  • Plates attached to the upper jaw, made of keratin (like hair and fingernails).

  • Base attaches to the upper jaw, edge consists of fibrous strands for straining food.

  • Large numbers of plates in the mouth of a whale.

Feeding Modes

  • Skimming (right whales and bowhead whales): move through the water with mouth open and strain what they come across (slow-moving whales).

  • Gulping (rorqual whales): depress tongue in mouth and consume large volume of water, often hurt prey.

Toothed Whales (Odontocetes)

  • Diverse group with around 72 species.

  • Some are large (sperm whale), some are deep diving (beak whales).

  • Include whales, dolphins, and porpoises.

Differentiating Whales, Dolphins, and Porpoises

  • No scientific basis in names; just common names.

  • Whale = large; dolphin/porpoise = smaller (cutoff ~10 meters, but variable usage).

  • Small group of odontocetes in a particular family are scientifically called porpoises.

Odontocete Characteristics

  • Range from small porpoises to the large sperm whale.

  • Have teeth in their jaws.

  • Homodont teeth (all teeth are the same, no specialization, used to grip prey for swallowing).

  • Some species have greatly reduced teeth (especially cephalopod feeders):

    • Use suction feeding (adjust mouth/tongue position to create negative pressure).

    • Varied diet.

Sperm Whale

  • Characterized by melon on the top of its head

  • Well known for being deep diving, though, not the deepest diver in marine mammals.

  • Suction Feeder.

  • No teeth in the top jaw.

  • Teeth only present in the bottom jaw, Single-tooth can weigh a kilo.

Killer Whale

  • Broadest diet.

  • Distinct population.

  • Some populations are fish specialists, some are generalist.

  • Classic homodont structure (conical teeth all the same).

Narwhals

  • Have tusk, elongated tooth.

  • Most Pronounced in males.

  • Females do not have this.

  • Part of a deep diving group that's restricted to the Artic.

  • Social group, often see them swimming in large numbers.

Revision Questions

  • Which of the following taxa includes the crabeater seal? A true seal; Focidae

  • Baleen plates occur in members of the ____? Mysticetes

  • The hind limbs of dolphins have evolved into tail flukes. False (Hind limbs are vestigial. Tail fluke is independent cartilage and structure)

  • The sperm whale is a type of odontocete. True

  • Deepest diving marine mammal is ____? Cuviers Beaked whale