When it comes to nudibranchs and sea slugs, two things are true: They’re quite difficult to ID at a glance, and the Indo-Pacific is absolutely crawling with them. In fact, there are about 3,000 described ­species, 40 percent of which have been found exclusively in this region.

That’s why divers and marine-biology lovers alike will rejoice at the arrival of Nudibranch & Sea Slug Identification: Indo-Pacific — 2nd Edition. This handy, hardcover identification guide is one of ­many published by New World Publications, whose easy-to-use fish-ID guides serve as the go-to companion for many divers.

The new edition features 185 new species, additional photos and an up-to-date classification. In total, 2,138 Indo-­Pacific opisthobranchs are illustrated in the full-color guide, including many species that are currently undescribed.

The book is written by three of the most respected experts in the field: ­Gosliner has studied opisthobranchs for more than 50 years; Valdés has 140 papers on the topic to his name; and Behrens has penned several field guides and books on the ­subject.

Divers can purchase a physical ­copy of the ID guide or a PDF version at ­marinelifebooks.com for $65.