Doug Perrine / Porbeagle shark {Lamna nasus} (captive, caught for research) with dorsal fin exposed, showing white patch at rear base of fin that is diagnostic of the species, and copepod parasite (two filaments) at...

Thomas Marent / Mudskipper (Periophthalmus sp) on land displaying erect dorsal fin, Bako National Park, Sarawak, Borneo, Malaysia

Jan Vermeer / Antarctic Minke Whale (Balaenoptera bonaerensis) surfacing, Paradise Bay, Antarctica

Jane Burton / Giant sailfin molly {Poecilia velifera} male in courtship display to female, note regeneration of dorsal fin after injury, captive, from Mexico

Doug Perrine / Porbeagle shark (Lamna nasus) captive, Nova Scotia, Canada, image digitally manipulated - note white patch on rear edge of dorsal fin and double keel on caudal (tail) fin that are diagnostic of this...

Suzi Eszterhas / Orca (Orcinus orca) male and female surfacing, Prince William Sound, Alaska

Suzi Eszterhas / Orca (Orcinus orca) male surfacing at sunset, Prince William Sound, Alaska

Suzi Eszterhas / Orca (Orcinus orca) female and spouting calf, Prince William Sound, Alaska

Sue Flood / Minke whale (Balaenoptera acutorostrata) in waters near Franz Josef Land, Russian Arctic

Pete Oxford / Orca (Orcinus orca) mother and calf diving, Galapagos Islands, Ecuador

Staffan Widstrand / Bottle nosed dolphin {Tursiops truncatus} surfacing, showing dorsal fin. Victoria, Australia

Flip Nicklin / Fin Whale (Balaenoptera physalus) swimming at surface of Sea of Cortez, Baja California, Mexico

Yva Momatiuk & John Eastcott / Humpback Whale (Megaptera novaeangliae) swimming near iceberg, Grandidier Passage, western Antarctica

Yva Momatiuk & John Eastcott / Orca (Orcinus orca) showing yellow coloration of skin caused by diatom infestation, Prospect Point, Antarctica

Yva Momatiuk & John Eastcott / Humpback Whale (Megaptera novaeangliae) swimming near iceberg, Prospect Point, Antarctica

Yva Momatiuk & John Eastcott / Humpback Whale (Megaptera novaeangliae) surfacing at sunset, Cape Evensen, western Antarctica

Graham Eaton / Dorsal fin of Bottlenosed dolphin (Tursiops truncatus) showing damage caused by a propeller, Cardigan Bay, Wales, July 2007

Nature Production / Spines along the dorsal fin of the Mottled / Dusky Spinefoot / Fuscescens rabbitfish {Siganus fuscescens} Japan

Flip Nicklin / Humpback Whale (Megaptera novaeangliae) dorsal fin and tail, Humpback Whale National Marine Sanctuary, Maui, Hawaii - notice must accompany publication; photo obtained under NMFS permit 0753-1599

Flip Nicklin / Scavenger fish feeding on Humpback Whale (Megaptera novaeangliae) parasites, Humpback Whale National Marine Sanctuary, Maui, Hawaii - notice must accompany publication; photo obtained under NMFS permi...

Flip Nicklin / Orca (Orcinus orca) pod surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod with calf surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) adult with flaccid fin, where dorsal fin curls, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) adult with flaccid fin, where dorsal fin curls, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) whale watching, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) trio spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) adult and surfacing calf, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) whale watching boat, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) whale watching, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod surfacing and spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod surfacing, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pair surfacing and spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) trio spouting, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) and whale watching boat, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) pod, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) surfacing pod, southeast Alaska

Flip Nicklin / Orca (Orcinus orca) surfacing, southeast Alaska

Hiroya Minakuchi / Orca (Orcinus orca) northern resident pod surfacing, Johnstone Strait, British Columbia, Canada

Hiroya Minakuchi / Orca (Orcinus orca) hunting South American Sea Lion (Otaria flavescens) group by beaching itself, Peninsula Valdez, Argentina

Hiroya Minakuchi / Orca (Orcinus orca) northern resident pod surfacing, Johnstone Strait, British Columbia, Canada

Hiroya Minakuchi / Orca (Orcinus orca) northern resident male and female surfacing in fog, Johnstone Strait, British Columbia, Canada

Hiroya Minakuchi / Orca (Orcinus orca) southern residents breaching, Puget Sound, Washington

Hiroya Minakuchi / Orca (Orcinus orca) northern resident pod surfacing, Johnstone Strait, British Columbia, Canada

Hiroya Minakuchi / Orca (Orcinus orca) southern resident male surfacing beside whale watching boat, San Juan Island, Washington

Hiroya Minakuchi / Orca (Orcinus orca) northern resident spouting, Johnstone Strait, British Columbia, Canada

Hiroya Minakuchi / Orca (Orcinus orca) mother and calf surfacing, Prince William Sound, Alaska

Hiroya Minakuchi / Orca (Orcinus orca) mother and calf surfacing, southeast Alaska

Hiroya Minakuchi / Orca (Orcinus orca) male surfacing in fog, Prince William Sound, Alaska

Tui De Roy / False Killer Whale (Pseudorca crassidens) pod swimming, Sea of Cortez, Baja California, Mexico

Tui De Roy / Long-finned Pilot Whale (Globicephala melas) in coastal waters, Golden Bay, South Island, New Zealand

Tui De Roy / Long-finned Pilot Whale (Globicephala melas) pod swimming closely together in coastal waters, showing burn from stranding, Golden Bay, South Island, New Zealand

Tui De Roy / Dwarf Minke Whale (Balaenoptera acutorostrata) at summer feeding grounds, Paradise Bay, Antarctic Peninsula, Antarctica

Tui De Roy / Bryde's Whale (Balaenoptera edeni) in Cromwell Current, upwelling feeding area, Banks Bay, Isabella Island, Galapagos Islands, Ecuador

Tui De Roy / Fin Whale (Balaenoptera physalus) resident adult at winter feeding grounds, Sea of Cortez, Baja California, Mexico

Tui De Roy / Fin Whale (Balaenoptera physalus) resident adult at winter feeding grounds, Sea of Cortez, Baja California, Mexico

Flip Nicklin / Orca (Orcinus orca) surfacing in calm waters, Alaska

Roberto Bubas / Killer whale {Orcinus orca} hunts Southern sealion {Otaria flavescens} in shallow waters off coast of Valdes peninsula, Argentina

Brent Hedges / Fire goby {Nemateleotris magnifica} with dorsal fin raised, Papua New Guinea

Patricio Robles Gil / Killer whale {Orcinus orca} Gulf of California, Mexico

Suzi Eszterhas / Rehab. staff and volunteers from the Marine Mammal Center and Long Marine Lab put a bottle-nosed dolphin, "Baker", into a sling for transport from his tank at the Long Marine Lab to the open ocean for...

Juergen Freund / Richard Fitzpatrick drills holes in dorsal fin of Tiger shark {Galeocerdo cuvieri} to attach satellite transmitter, Great Barrier Reef & Coral Sea, Australia

Doug Perrine / Dorsal fin of a Tiger shark {Galeocerdo cuvieri} at water surface, Bahamas

Doug Perrine / Dorsal fin of Tiger shark at water surface {Galeocerdo cuvieri} Bahamas.

Dan Burton / Basking shark {Cetorhinus maximus} dorsal fin at surface, Cornwall, UK.

Dan Burton / Basking shark {Cetorhinus maximus} dorsal fin at surface, Cornwall, UK.

Dan Burton / Basking shark {Cetorhinus maximus} dorsal fin at surface, Cornwall, UK.

Dan Burton / Basking shark {Cetorhinus maximus} dorsal fin at surface, Cornwall, UK.

Dan Burton / Basking shark {Cetorhinus maximus} dorsal fin at surface Cornwall, UK.

Dan Burton / Dorsal fin of Basking shark feeding at surface {Cetorhinus maximus} UK.

Dan Burton / Dorsal fin of Basking shark at surface {Cetorhinus maximus} UK.

Dan Burton / Open mouth and dorsal fin of Basking shark seen from surface {Cetorhinus maximus} UK.

Todd Pusser / Large dorsal fin of male transient killer whale Monterey Bay, California, USA.

Sue Flood / Dorsal fin of Killer whale {Orca orcinus} North Island, New Zealand.

Todd Pusser / Dorsal fin profile of Fin whales {Balaenoptera physalus}, Southern California, Pacific

Todd Pusser / Humpback whale cow and calf at surface {Megaptera novaeangliae} Monterey Bay, California

Adam White / Sperm whale dorsal fin {Physeter macrocephalus} Kaikoura, New Zealand

Jane Burton / Mudskipper display (Periophthalmus barbarus) raised dorsal fin. Captive.

Hiroya Minakuchi / Peale's Dolphin (Lagenorhynchus australis) pod, Falkland Islands

Hiroya Minakuchi / Peale's Dolphin (Lagenorhynchus australis) pod, Falkland Islands

Flip Nicklin / Orca (Orcinus orca) pod with calf, near Vancouver Island, British Columbia, Canada

Patricio Robles Gil / Orca (Orcinus orca) male at surface in the Gulf of California, Mexico

Patricio Robles Gil / Orca (Orcinus orca) male's dorsal fin above water's surface, Sea of Cortez, Mexico

Mike Parry / Short-finned Pilot Whale (Globicephala macrorhynchus) group surfacing, Hawaii

Mike Parry / Short-finned Pilot Whale (Globicephala macrorhynchus) group surfacing, Hawaii