As we stand on the thin crust of this watery planet, our gaze tends to roam from horizon to heavens. We often neglect the riot of life that seethes and thrives below us, especially in the still mysterious depths of our oceans.

When we focus on sea life at all, our fancies turn to the vertebrate exhibitionists — whales, sharks, dolphins — or the delicious fish on our plates. But as the photographer and writer Susan Middleton tells us in her ravishing new book, “Spineless,” marine invertebrates make up more than 98 percent of the oceans’ known animal species. As she writes, “Beneath the ocean waves, hidden from our view, a spectacular profusion of life flourishes.”

And in the more than 250 stunning and sometimes alien images here — complemented by helpful mini-bios of each species shown — Ms. Middleton introduces the reader to the “profound assemblage of energies beyond the human.” “Spineless” represents seven years of fieldwork in three habitats: the French Frigate Shoals of the Hawaiian archipelago; the Line Islands in the Central Pacific; and San Juan Island in Washington State. And the book writhes, oozes and creeps with the salty invertebrates Ms. Middleton got acquainted with. Her intense, often color-saturated photographs pulse with spellbinding strangeness: squids, jellies and nudibranchs; whelks, bloodworms and drupes; conches, urchins and chitons.