What biological entity causes the prickling sensation known as sea bather’s eruption?

Tiny, translucent larvae of thimble jellyfish, often referred to as sea lice

The condition sometimes manifesting as a prickling sensation followed by a rash, known as sea bather’s eruption, is specifically attributed to sea lice, which are not true lice but rather the very small, translucent larvae produced by thimble jellyfish. These larvae can become trapped between the skin and swimwear. When pressure is applied, such as by tightening clothing or by sitting down, the trapped larvae can release toxins. This toxin release is what causes the localized irritation and subsequent rash. A key piece of aftercare recommended for this specific issue, distinguishing it from general saltwater irritation, is the immediate rinsing of the affected area with vinegar to help neutralize these biological toxins.