David Shale / Siphonophore (Nanomia sp) close up of nectophores, and palpons. Gulf of Maine, Atlanic Ocean. Epipelagic species found up to 300m depth.

Alex Mustard / Nudibranchs (Doto greenamyeri) newly described species on feather hydroids. Seraya Bay, north coast of Bali, Indonesia. Java Sea.

Alex Mustard / Red eye medusa (Polyorchis penicillatus) with hitchhiking amphipods (Hyperia medusarum). Browning Pass, Vancouver Island, British Columbia, Canada. North East Pacific Ocean.

Franco Banfi / Freshwater jellyfish (Craspedacusta sowerbii) Lake Lugano, Ticino, Switzerland. August. Invasive species.

Franco Banfi / Freshwater jellyfish (Craspedacusta sowerbii) Lake Lugano, Ticino, Switzerland. August. Invasive species.

Solvin Zankl / Portuguese man-of-war (Physalia physalis) colony, underwater Sargasso Sea, Bermuda

Solvin Zankl / By-the-wind sailor (Velella velella) colony. Sargasso Sea, Bermuda.

Solvin Zankl / Portuguese Man-of-War (Physalia physalis) close up of tentacles, Sargasso Sea, Bermuda

Richard Robinson / Featherstar (Crinoidea) on red coral (Errina novazelandiae) in Doubtful Sound, Fiordland National Park, New Zealand.

Alex Mustard / Busy coral reef community feeding on plankton, with Scalefin anthias (Pseudanthias squamipinnis), Soft corals (Dendronephthya sp.) and Fire corals (Millepora dichotoma), making up classic Red Sea scen...

Alex Mustard / RF- School of Scalefin anthias (Pseudanthias squamipinnis) on reef in the current feeding on plankton, with soft corals (Dendronephthya spp.) and fire coral (Millepora dichotoma) Jackson Reef, Strait...

Alex Mustard / Busy coral reef community feeding on plankton, with Scalefin anthias (Pseudanthias squamipinnis), Soft corals (Dendronephthya sp.) and Fire corals (Millepora dichotoma) Ras Mohammed Marine Park, Sinai...

Alex Mustard / Vibrant Red Sea reef scene, with orange female Scalefin anthias (Pseudanthias squamipinnis) swarming in front of Fire coral (Millepora dichotoma) feeding on plankton brought to the reef by currents. R...

Alex Mustard / Scalefin Anthias (Pseudanthias squamipinnis) swarm over coral reef, with Fire coral (Millepora dichotoma) in the Red Sea in the morning. Yolanda Reef, Ras Mohammed Marine Park, Sinai, Egypt. Gulf of A...

Visuals Unlimited / Deep-Sea Jellyfish (Crossota)

Alex Mustard,Alex Mustard / Red eye medusa (Polyorchis penicillatus) with its feeding tentacles extended to catch planktonic prey. Note that the bell of the jelly has hitchhiking amphipods (Hyperia medusarum). Browning Pass, Van...

Doug Perrine / Flower Hat Jelly / Jellyfish (Olindias formosa), a rare hydromedusa with fluorescent tentacle tips. Endemic to Brazil, Argentina, and southern Japan.

Stephen Dalton / Green Hydra (Hydra viridis) with tentacles extended. UK, November.

Stephen Dalton / Green Hydra (Hydra viridis) with tentacles extended. UK, November.

Wild Wonders of Europe / Lundgren / Skeleton shrimp (Caprellidea sp) on a hydroid, Saltstraumen, Bodö, Norway, October 2008

David Shale / Medusa caught with 1m ring net between 0 and 150m, Mid-Atlantic Ridge, North Atlantic Ocean

Wild Wonders of Europe / Lundgren / Great spider crab (Hyas araneus) decorated with hydroids, Moere coastline, Norway, February 2009

Michael Pitts / Hydroids growing on hull of tugboat ''Blue Plunder'' wreck, Nassau, Bahamas. July 2008.

Kim Taylor / Green Hydra (Hydra viridis) walking. Digital Composite showing five movement stages. UK

David Shale / (Octophialucium funerarium) hydromedusan jellyfish, deep sea Atlantic ocean

David Shale / (Chromatonema rubrum) a hydromedusan jellyfish, deep sea Atlantic ocean

David Shale / Siphosome of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

Heather Angel / Freshwater green hydra (Hydra viridissima) moving over a petri dish. Green colour comes from the symbiotic relationship with green alga Chlorella vulgaris

Magnus Lundgren / Wild Wonders of China / Tiny hydrozoa, which is a predatory jellyfish-like animal. The hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria, Green Island, Taiwan.

Magnus Lundgren / Wild Wonders of China / Tiny hydrozoa, which is a predatory jellyfish-like animal. The hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria, Green Island, Taiwan.

Alex Mustard / Exquiste butterflyfish (Chaetodon austriacus) swims over fire coral (Millepora sp.) on a coral reef. Marsa El Shouna, Marsa Alam, Egypt. Red Sea.

Alex Mustard / Barrel sponges (Xestospongia testudinaria) grow in shallow water, surrounded by hydroids (Aglaophenia cupressina), in shallow water beneath trees and sun burst. The Passage betweeb Waigeo Island and G...

Alex Mustard / Diverse hard coral growth (Pocillopora damicornis, Pocillopora verrucosa and Millepora dichotoma), flourishes beneath the barren desert cliffs in the Red Sea. Ras Katy, Sinai, Egypt. Gulf of Aqaba, Re...

Brandon Cole / Soft corals (Dendronephthya sp.) and Fire coral (Millepora sp.) with Scalefin anthias (Pseudanthias squammipinnis), Ras Mohammad National Park. Egypt, Red Sea.

Sergio Hanquet / Colonies of hydrozoans on Neptune seagrass (Cymodocea nodosa) Tenerife, Canary Islands.

Sergio Hanquet / Colonies of hydrozoans on Neptune seagrass (Cymodocea nodosa) Tenerife, Canary Islands.

David Hall / Red-gilled aeolid nudibranch (Flabellina triophina) moving among Skeleton shrimp (Caprella sp.) The nudibranch does not prey upon amphipods; it eats stinging hydroids. Gambier Bay, Admiralty Island, A...

David Hall / Opalescent nudibranch (Hermissenda crassicornis) which preys upon stinging hydroids. It retains its prey's stinging cells (cnidocytes) which are transported to the colorful cerrata on nudibranch&#...

David Hall / Red-eye medusa (Polyorchis penicillatus), Nigei Island, Queen Charlotte Strait, British Columbia, Canada. September.

David Hall / Opalescent nudibranch (Hermissenda crassicornis) preys upon stinging hydroids. Browning Pass, Queen Charlotte Strait, British Columbia, Canada. September.

David Hall / Moon jelly (Aurelia labiata) and Cross jellies (Mitrocoma cellularia) split level view in Browning Pass, Queen Charlotte Strait, British Columbia. October.

David Hall / Cross jellyfish (Mitrocoma cellularia) swarm at night, North Wall, Browning Pass, British Columbia, Canada. September.

David Hall / Cross jellyfish (Mitrocoma cellularia) split level view at night, Hussar Bay, Nigei Island, Queen Charlotte Strait, British Columbia, Canada. Large swarms of Cross Jellies enter quiet bays at night, a...

Franco Banfi / Toothed feather hydroid (Aglaophenia pluma), South Tenerife, Canary Islands, Atlantic Ocean.

Magnus Lundgren / Amphipod riding a medusa hydroid close to the surface. Vaderoarna (Weather Islands), West coast, Sweden Magnus

Magnus Lundgren / Jellyfish (Octotiara rusellii) Balayan Bay, Luzon Island, Philippines Magnus

Magnus Lundgren / Wild Wonders of China / Tiny hydrozoa,a predatory jellyfish-like animal. Green Island, Taiwan. Lundgren

Georgette Douwma / Moluccan cardinalfish (Ostorhinchus moluccensis) next to hydroids, brittlestars and sea squirts. Tulamben, Bali, Indonesia.