I hiked in the Mokuleʻia Forest Reserve to see kāhuli or Oʻahu tree snails crawling on the native vegetation.
During the night the temperature dropped to the low 60’s and rain fell intermittently on our campsite at about 2,200 feet elevation. When I got up at first light, the landscape was drenched — perfect for snailing. Pahole Natural Area Reserve ranges in elevation from 1,200 to 2,590 feet, and receives average annual rainfall of about 60 inches. The reserve protects some of the best remaining native mesic forests on the island which is home to rare and endangered kāhuli, also known as pūpū kuahiwi or pūpū kani oe.
After a quick breakfast, I wasted little time making my way down the trail to search the native trees for snails.
When the Hawaiian Islands were explored after Western contact in the 1800’s, 41 different species of Oʻahu tree snails were described and classified in the genus Achatinella. Sadly, most species are extinct today due to shell collectors, habitat destruction, and predation by rats and carnivorous snails. Only a dozen or so species remain today and all are on the federal list of endangered species. The most common species today — Achatinella mustelina — is found only in the Waiʻanae Mountains. The snails are mostly active at night and sleep during the day. I scoured the trees in the area for quite some time searching each leaf and branch where I saw a number of adult tree snails sleeping.
Adult kāhuli 3/4 of an inch long shares leaf with tornatelides snail 2 millimeters in length.
The shells of adult kāhuli are smooth, glossy and conical in shape with five to seven whorls and are up to 3/4 of an inch long. Kāhuli are moisture loving creatures and require regular rain or fog drip to live. The snails usually retract into their shells and hide in the vegetation during the day to conserve water. But if the vegetation is wet they can sometimes be seen crawling around during the day.
When I examined the trunk of an olopua tree, I was thrilled to see a trio of kāhuli — possibly a family, huddled together in a crook between branches! A full-grown adult three-quarters of an inch long, a juvenile half-an-inch long, and a cute little baby a quarter-inch long! It takes up to 3-5 years for kāhuli to become reproductively mature and the snails can live for 10 years.
The baby kāhuli wandered off from the others and crawled up the tree branch. Oʻahu tree snails are unique in the snail world in that they give live birth to their offspring. Adults can give birth to up t0 7 times a year.
When baby kāhuli are first born they are only about 4.6 mm in diameter. This specimen is just under a quarter inch long or about 6.35 mm.
The carnivorous snail — Euglandina rosea — that was brought to Hawaiʻi to control the African snail population, has a much faster reproductive rate. Each carnivorous snail can lay hundreds of eggs per a year! Sadly, the carnivorous snail prefers to eat kāhuli over African snails and has decimated their population causing 80% of the species to go extinct.
Time passed quickly as I patiently watched the snails return to their spot, retract into their shells and go to sleep. I hope these snails escape predation by euglandina and rats, grow up to be big and strong, and have many offspring so the kāhuli population can rebound.
Hawaiian Tree Snail Conservation Lab, University of Hawaii
Oahu Tree Snails, Biomes of the World
Oahu Tree Snail, Earth’s Endangered Species
Oahu Tree Snails, Endangered Species in the Pacific Islands, US Fish & Wildlife