Morphological and functional characterization of the ptychocyte, a stingless stinging cell

Cnidocytes (stinging cells), unique to cnidarians (corals, anemones, jellyfish), have diversified into distinct types with variable forms and functions. Nematocytes, cnidocytes found in all cnidarians, are used for prey capture and defense. When triggered, a pressurized capsule inside the nematocyte releases a harpoon-like structure attached to a hollow tubule that pierces prey and delivers venom. Ptychocytes, a cnidocyte unique to tube anemones (sister to corals and sea anemones) discharge a long spineless tubule used exclusively to build the tube in which the animal lives. Given that nematocytes and ptychocytes are specialized for different functions, we hypothesized that they might respond to firing cues in different ways. To test this, we examined the morphology, function, and distribution of nematocytes and ptychocytes in the North American Tube Anemone, Ceriantheopsis americana. We determined that ptychocytes have apical sensory structures like the cones previously described on nematocytes. Surprisingly, the body wall has a dense population of multiciliated cells that appear to function in tube formation. To determine how divergent selection pressures may have affected firing dynamics, we compared the discharge kinematics of cnidocytes from C. americana and the model sea anemone, Nematostella vectensis. Both nematocytes and ptychocytes from C. americana fired slower than nematocytes from N. vectensis, suggesting the rapid discharge speed of sea anemone nematocytes resulted from modification to these cells after sea anemones and tube anemones diverged from their common ancestor. By comparing the morphology and function of different cnidocytes, we can reconstruct the steps that gave rise to cnidocyte diversity.