Ecology & Distribution of Katharina tunicata
(Black Katy)

Katharina tunicata uses its muscular foot to effectively attach itself to a hard substratum. This species is primarily found at low, rocky intertidal zones where it firmly fastens itself into crevices.K. tunicata is a large chiton that persists in the shallow waters of the northeastern Pacific Ocean. It is distributed from the Aleutian Islands as far south as southern California where it resides on rocks receiving heavy wave action. The animals that I observed were primarily in the shade, although most literature claims that these animals do not object to sun exposure and are generally positively phototactic.

Several studies have looked at the ecological importance of K. tunicata in terms of how its diet relates to the animals biomass and the relative abundance of other marine invertebrates. K. tunicata feeds on a variety of different algae species and diatoms, mostly depending on its geographic distribution. When K. tunicata feeds on large algae more sunlight reaches the substrate (rocks) on which they live. This increase in sunlight promotes the growth of diatoms, which are a favored dietary component of limpets and other chitons. Studies have shown that when K. tunicata is experimentally removed from a locality the population of specific limpet species increases. However, similar studies have shown that removal of the Black Katy correlates with decreases in limpet population. These results infer that the affinity of limpets to a specific environment is dependent upon the feeding behavior of K. tunicata. Still other studies have provided insight that this feeding behavior varies regionally. Studies that have compared the role of the Black Katy as a strong "interactor" among sites in Washington state and southeast Alaska have produced significantly different results between the two regions. Despite similar ecological environments the removal of the chiton in southeastern Alaska resulted in little impact on kelp/algae abundance, while in Washington removal of the Black Katy dramatically increased algae abundance. This indicates that K. tunicata, despite similar physical ecology, interacts differently depending on geographic distribution. In Washington state the animal is a strong "indicator", and can be described as a keystone species. Conversely in the rocky intertidal of southeast Alaska the Black Katy appears to be a weak "indicator" It’s feeding behavior and existence is not completely essential to maintaining the abundance of other invertebrates.

Predators of K. tunicata also vary depending on geographic distribution, although they are usually preyed upon by Echinoderms. Green and Purple Sea Urchins as well as the Leather Star (Dermasterias imbricata) and certain sea gulls are among its predators. These animals are successfully, although not always with ease, able to pry the chitons from their substrate to which they are firmly attached.