There are five chapters, looking in turn at anatomy, life history, taxonomy and evolution, ecology, and relationships with humans. Each chapter has ten double-page spreads on different aspects of the biology, followed by ten double-page spreads on individual species of jellyfish which illustrate the chapter theme. This five-fold/ten-fold structure has a formal elegance reminiscent of the more symmetrical jellyfish themselves.
The "jellyfish" Gershwin covers are a non-monophyletic group that includes members of three different phyla — Cnidaria (Hydrozoa, Scyphozoa, a variety of groups with uncertain relationship), Ctenophora (comb jellies), and even Chordata (salps) which are more closely related to humans than to the other jellyfish. They span a huge diversity in size (from the giant heart jellyfish at 50 metres down to species a few millimetres across) and life cycle (with thirteen different methods of cloning, and some species that are immortal). And jellyfish have a role in most of the ocean problems humans have created: overfishing, climate change, ocean acidification, garbage and chemical pollution, eutrophication, and even coastal construction ("jellyfish polyps prefer artificial substrates over natural ones").
Ecologically, jellyfish operate what is almost a parallel food chain; among their primary competitors are "small pelagics" or forage fish.
"Jellyfish interact with numerous other types of organisms. In shallower coastal waters, many of these interactions are mutually beneficial for both the jelly and the associate. One example is the symbiotic algae found in blubber jellyfish. However, in the open ocean the relationships between jellies and other species generally come down to two types: predator-prey and host-parasite. In these waters, jellies are frequently parasitized by small insect-like crustaceans or other jellies."
The fifty jellyfish covered are individually amazing.
PURPLE PEOPLE EATER
"The name Pelagia noctiluca, Latin for "drifting night light", comes from the brilliant bioluminescence the species displays after dark. Masses of Pelagia glowing cool blue-green must have been quite a wonderous sight for sailors in the 1600s and 1700s when the species was discovered.
Pelagia noctiluca, known as the Purple People Eater or Mauve Stinger, is a common pest along tropical and subtropical coastlines of the world, especially in and around the Mediterranean. It is a handsome species, with a mauve hemispherical body bearing eight long, fine tentacles and four ruffled oral arms. But it travels en masse, and its sting is fierce. In recent years its blooms have shut down swimming along entire coastlines of Spain and France and have put salmon farms out of business in Ireland.
Pelagia has a broad diet and a voracious appetite. Its prey includes hydromedusae, ctenophores, small crustaceans, fish eggs, and other plankton. Its dense swarms can quickly consume just about every living thing in the water.
In addition to being so familiar and fearsome, Pelagia noctiluca is also one of the most aberrant and intriguing jellyfish when it comes to life history. It has no polyp form. Whereas in most scyphozoans the planula larva settles to the seafloor and turns into a polyp, which then produces medusae through a process of strobilation, the Pelagia noctiluca planula flattens out and transforms directly into an ephyra, or larval medusa. The whole developmental process takes about 92 hours..."
The Portuguese Man-of-War comes in varieties with left- and right-handed floats, making it less likely that a wind will drive an entire population on-shore.
Tesserantha connectens is a possibly apocryphal deep-sea stauromedusan, known only from one detailed 19th century report.
Leaf-Crawler Jellies lack the ability to swim, and spend their entire lives crawling among algae and sea grasses.
Nomura's Giant Jellyfish has a bell up to two metres in diameter and weighs up to 200kg; in blooms, huge numbers drift from eutrophied areas off the Chinese coast into Japanese fishing grounds.
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