Ascophyllum nodosum Linnaeus 1753
Ascophyllum nodosum is a large brown seaweed found on intertidal and subtidal rocks, usually in sheltered waters. Morphological variants occur in salt marshes, mudflats, and occasionally floating in plankton. The plant has a leathery, narrow, branched, linear thallus, lacking a midrib, marked by numerous inflated vesicles, also known as floats (generally ~20-30 mm long,~ 20 mm wide). The plant is often attached by a discoid holdfast, but can be free-floating or entangled with other plants. The pattern of primary branching is dichotomous, while secondary branching is bilateral, with short, forked or club-like branchlets, 1-2 cm long. The plants are dioecious, developing receptacles, which begin first as linear structures and expand to a globular shape when mature. Plants grow to 600 mm and sometimes over 1000 mm. The color is usually olive-green, but can range from yellow to brown. Description based on: Baardseth 1970, Gosner 1978, and Schneider and Searles 1991.
An unattached form of A. nodosum has been distinguished from the attached form as separate species ('scorpioides' or 'mackaii') (e.g. Taylor 1957) or as a variety or ecotype ('forma'; 'ecad', etc.) (South and Hill 1970; Chock and Mathieson 1979). These forms tend to have flattened and very dichotomous (branched) thalli, and tend to lack air bladders and receptacles (sexual organs) (Gibb 1957; South and Hill 1970). The 'ecad' or ecotype interpretation seems to be more common in recent literature (e.g. Chock and Mathieson 1979). Gradients among morphotypes occur in marshes (Brinkhuis 1976; Chock and Mathieson 1979) and frond fragments from the 'scorpiodes ' ecad will take on the morphology of the drifting 'mackai' ecad when transplanted to the lower intertidal (Brinkhuis and Jones 1976). The frequency of sexual reproduction is greatly reduced in these forms, but a portion of the sexual offspring produced are attached. In addition to this distinction, additional morphological differentiation probably exists. 'There can be no doubt that genetically differentiated races of Ascophyllum exist, and in large numbers' (Baardseth 1970).