Georgette Douwma / Stinging hydroid (Aglaophenia cupressina). North Sulawesi, Indonesia.

Georgette Douwma / Stinging hydroid (Aglaophenia cupressina). North Sulawesi, Indonesia.

SCOTLAND: The Big Picture / Hydroids (Nemertesia ramosa) on reef with fish swimming in plankton bloom in background, South Arran Marine Protected Area, Isle of Arran, Scotland, UK, August.

SCOTLAND: The Big Picture / Plumose anemone (Metridium senile) and hydroids (Nemertesia ramosa) with fish swimming in plankton bloom in background, South Arran Marine Protected Area, Isle of Arran, Scotland, UK, August.

SCOTLAND: The Big Picture / Edible crab (Cancer pagurus) sitting between hydroids (Nemertesia ramosa) and Plumose anemone (Metridium senile) with fish swimming in plankton bloom in background, South Arran Marine Protected Area,...

SCOTLAND: The Big Picture / Common sunstar (Crossaster papposus) amongst hydroids (Nemertesia ramosa) and Jewel anemones (Corynactis viridis) with fish swimming in plankton bloom in background, South Arran Marine Protected Area,...

SCOTLAND: The Big Picture / Common sunstar (Crossaster papposus) amongst hydroids (Nemertesia ramosa) and red algae with plankton bloom in background, South Arran Marine Protected Area, Isle of Arran, Scotland, UK, August.

Graham Eaton / Sea beard hydroid (Nemertesia antennina) Cardigan Bay, Wales, UK. April.

Nick Upton / Hydroid colonies (Obelia dichotoma) attached to rocks in a rockpool, Rhossili, The Gower peninsula, Wales, UK, June.

Alex Mustard / Cross jellyfish (Earleria cellularia) swims below the surface in the evening. Browning Pass, Vancouver Island, British Columbia, Canada. North East Pacific Ocean.

Alex Mustard / Mixed assemblage of jellyfish, mainly cross jellyfish (Earleria cellularia), at the surface. Browning Pass, Port Hardy, Vancouver Island, British Columbia, Canada. Pacific Ocean.

Lewis Jefferies / Crystal jellyfish (Aequorea victoria) floating above Golden Kelp forest (Laminaria ochroleuca) Lundy Island, Bristol Channel, Devon, UK.

Alex Mustard / Coral reef scene with multiple species of Princess anthias (Pseudanthias smithvanizi), Threadfin anthias (Pseudanthias huchti), Redfin anthias (Pseudanthias dispar) and feathery hydroids (Aglaophenia...

Joel Sartore / Photo Ark / Crystal jelly (Aequorea victoria) portrait, Aquarium of the Pacific, California, USA. Captive.

Henley Spiers / Crystal jellyfish (Aequorea victoria) drifting past mussel farming lines, Shetland, Scotland, UK, North Sea. July.

Alex Mustard / Crystal jellyfish (Aequorea sp.) floating above a forest of Kelp (Laminaria hyperborea), Island of Coll, Inner Hebrides, Scotland, UK, Atlantic Ocean.

Alex Mustard / Crystal jellyfish (Aequorea sp.) floating above a forest of Kelp (Laminaria hyperborea), Island of Coll, Inner Hebrides, Scotland, UK, Atlantic Ocean.

David Fleetham / Stinging hydroid / Fire hydroids (Aglaophenia cupressina)Yap, Micronesia, Pacific Ocean. Hydroids are small predators that look like plants and are related to jellyfish.

Henley Spiers / White-cross jellyfish (Staurophora mertensii) bloom, Berwickshire Marine Reserve, Scotland, North Sea, UK. June.

Franco Banfi / Crystal jellyfish (Aequorea victoria) in deep water, Trondheimsfjord, Norway, Atlantic Ocean.

Shane Gross / Umbrella jelly / Aggregating jellyfish (Eutonina indicans) eating a small fish, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Tony Wu / Aggregating jelly (Eutonina indicans) hydrozoan, measuring about 4cm in diameter, Hokkaido, Japan, Pacific Ocean.

Alex Mustard / Crystal jellyfish (Aequorea sp.) in shallow, sunlit, water, Talland Bay, Cornwall, UK, English Channel. July.

Alex Mustard / Crystal jellyfish (Aequorea sp.) at the surface, with Snell's window, Cornwall, UK, English Channel.

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...

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.