dactylozooid

A dactylozooid is a specialized zooid found in certain colonial marine invertebrates, most notably hydrozoans and siphonophores. These zooids are primarily defensive in function and are characterized by their stinging tentacles, known as nematocysts. Dactylozooids are typically elongated and possess a single, prominent tentacle that can be extended to capture prey or ward off predators. They are part of a polymorphic colony, meaning the colony is composed of different types of zooids, each with a specific role.

In hydrozoan colonies, such as those of the order Milleporina (stinging hydroids), dactylozooids are often intermingled with gastrozooids (feeding zooids) and gonozooids (reproductive zooids). They emerge from pores on the surface of the colony's calcareous skeleton. Their nematocysts are potent and can deliver a painful sting to humans, even after the colony has been removed from the water.

In more complex colonial organisms like siphonophores, which include famously predatory species such as the Portuguese man o' war, dactylozooids form specialized hunting structures called tentilla. These are highly modified tentacles, often covered in numerous nematocysts, that trail below the main body of the colony to ensnare small organisms. The structure and arrangement of dactylozooids can vary significantly between different species, reflecting their diverse ecological roles and predatory strategies.