The Sioux Falls Zoo & Aquarium is excited to announce a rare hatching event at the Butterfly House & Aquarium (BHA) campus – the birth of more than 1,000 horseshoe crabs.

An aquarist found the trilobite larvae, as baby horseshoe crabs are called, during a routine water change in BHA’s new Eastern Shores exhibit. The newly-hatched trilobites are smaller than a pencil eraser, but females can grow to up 24 inches. They look like tiny adult horseshoe crabs, but with translucent shells.

Wild horseshoe crabs can lay up to 4,000 eggs at a time on sandy beaches. In a fully submerged environment, like an aquarium exhibit, horseshoe crabs typically do not lay eggs. In the wild, they have a 0.01% chance of surviving to adulthood, because the eggs and hatchlings are a food source for other crustaceans, sea birds, and fish. The wild horseshoe crab population is on the decline, due to overharvesting and habitat loss.

Once discovered, the BHA trilobites were filtered out of the sand of the exhibit and moved to a tank with slightly warmer water and lower salinity – an environment better mimicking the waters where they would naturally hatch. Eastern Shores replicates the colder, saltier areas where adult crabs thrive. It is likely many of the 1,000+ hatchlings will not survive their first molt, but at least hundreds have made it through this initial milestone and are continuing to grow. They will have to go through many more molts before reaching adulthood.

The hatchlings are currently living behind the scenes at BHA. Although they are not on public view, BHA will post updates on social media as the crabs grow. The adult horseshoe crabs can be viewed in the Eastern Shores exhibit Thursday-Sunday from 10 am-1 pm.