Seahorses are renowned for mating for life ... It showcases the ocean’s little-known creatures with more than 300 images taken during Smith’s research dives in over 23 countries.

Dwindling natural habitats are causing a significant decline in certain seahorse species. To bolster populations, researchers in Australia are building seahorse hotels and leaving the lights on.

Remembering that pygmy seahorses are a popular photographic target of tourists diving among corals, the researchers began searching for pictures of them on the iNaturalist website—a place where ...