Features in common between the two include the anatomy of the mouth, the shape of the radular teeth and the paths taken by the dorsal veins.

In Florida however it feeds on seagrasses including Halophila engelmannii, Halodule wrightii and Thalassia at depths of up to a few metres.

[4][5] Elysia catulus on the other hand, feeds on the seagrass Zostera and is found on the North American coast between South Carolina and Nova Scotia.

This is likely to be because the life span is less than a year and recruitment, and the establishment of new populations, comes as a result of planktonic larvae arriving from elsewhere and settling.

[5] This may be because the plant has large epidermal cells which are easy to exploit or because of the absence of tannins, a chemical defence against herbivores used by some seagrasses.