Marine Botany

Pugetia fragilissima & Pugetia firma

Habitat

The habitats in which algae are capable of growing depend on a variety of abiotic and biotic factors. Adequate locations need to appropriate levels of light, temperature, salinity, water motion, and nutrient availability. Space constraints, due to competition with other algae or animals may force an alga species to adapt to a sub-optimum habitat, thus promoting specialization. Considering the nutrient rich waters where Pugetia firma and Pugetia fragilissima live, for these two species it would appear that space and light are the two central factors for habitat. In their book Seaweed Ecology and Physiology, Lobban and Harrison claim that, as a general rule, benthic marine algae “live attached to the sea bed between the top of the intertidal zone and the maximum depth to which adequate light for growth can penetrate.” In the case of Pugetia firma that range extends from the low intertidal to a depth of 100 ft. Pugetia fragilissima is subtidally limited, growing between 10-100ft.

Other red algae may be able to out-compete Pugetia on the well-lit surface of rocks, but Pugetia’s cup-like morphology allows it to utilize rock faces. Pugetia firma is found primarily growing on the side of rocks with its holdfast connecting the center of the thallus to the face. Presumably to gain more light, the bottom portion of the thallus will extend perpendicularly from the face. When present subtidally, Pugetia firma tends to dominate the entire face of the substrate, however rocky faces covered with Pugetia firma are relatively spread apart. In the intertidal, Pugetia firma is scarcer, with only one or two specimens attached in any given area. The thicker thallus of intertidal Pugetia firma allows it to withstand tidal energy, while also decreasing its surface area to volume ratio and thus desiccation. Although rocky walls are Pugetia firma’s primary habitat, they have been documented growing on tunicates, stalks of Laminariales and other hard surfaces such as the metal pipeline in breakwater cove.

Pugetia fragilissima can also grow in similar fashion to Pugetia firma on rocky faces, however it also grows on a variety of other substrates. Most commonly, Pugetia fragilissima is found growing on tubeworms, but it is also found epiphytically on many other red algae including Chondracanthus and Calliarthron. In general, Pugetia fragilissima is found along sandy substrates growing on the tubes of two closely related polychaete worms, Phyllocheatopterus prolifica and Spichaetopterus castarum. Thus, Pugetia fragilissima has found a way to colonize sandy flats, which are generally uninhabited by most algae. Often small tubeworm casings with large bundles of Pugetia fragilissima will dominate these flats, appearing to be the only living organisms.

Interestingly, the density of these tubeworm-Pugetia colonies seems to increase with proximity to rocky substrate or artificial reef structures, such as pipelines. A thorough density survey has yet to be done, but from observations made for this study, it appears that these polychaete worms don’t live or at least don’t host Pugetia fragilissima further than 20m from some hard substrate. Freya Sommer, from Hopkins Marine Station, hypothesized that these worms may actually burrow deep enough to anchor to hard substrate, thus explaining both their proximity to rocky outcroppings and difficulty associated with digging them out. Increased nutrient output from rocky systems is another possible explanation for the distribution pattern.

© 2005 Laure Sierra Katz.
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Last updated: Oct. 21, 2005