Saccharina sessile (C. Agardh, 1824) Kuntze, 1891
formerly Laminaria sessilis (C. Agardh, 1824)
Hedophyllum sessile (C. Agardh, 1824) Setchell, 1901
Link to AlgaeBase: http://www.algaebase.org/search/species/detail/?species_id=73746
Type locality: Whidbey Island, WA
Common Name: Sea cabbage
Taxonomy: Phaeophyceae: Laminariales: Laminariaceae: Saccharina
Identification: Saccharina sessilis is a medium to dark brown alga that appears as large blades originating from a large hapteroid holdfast without a stipe. On San Juan Island, the blades may be more than 20 cm in length and 5 – 10 cm wide. However, depending on the wave energy of the alga’s site, an individual may have different numbers of blades, bullate or flat blades, and intact or tattered blades. It is appropriately called the Sea Cabbage due to its ‘leafy’ appearance that grows out of the holdfast with large, cabbage-shaped blades.
Like other brown algae, S. sessilis has a two-phase hetermorphic life history. The sporophyte is the phase of the life history visible to the naked eye.
Distribution: Saccharina sessilis can be found on protected and exposed rocky shores, from the mid to low intertidal zones. The range of S. sessilis extends from Monterey, California to the Aleutian Islands, Alaska.
Vouchers: The FHL Herbarium houses many pressed specimens of S. sessilis from sites all over San Juan Island (both outer and inner coasts) dating back to 1914. Sites on San Juan Island include American Camp Beach, Deadman Bay, Cattle Point, Mt. Dallas Beach, and Eagle Cove.
Research Notes: Saccharina sessilis is able to live in a wide range of flow conditions due to morphological plasticity. In slower flow habitats, like those of protected shorelines, S. sessilis will become more bullate and may be shorter or have fewer blades. Conversely, in faster flow habitats, like those of exposed shorelines, S. sessilis will have smoother blades and each individual will have more blades, though these blades can appear tattered due to abrasion and damage (Milligan and Dewreede 2004). As with other macroalgae, having bullate blades in slower flow conditions may help mix water near the surface of the alga, thereby promoting the absorption of dissolved nutrients and exchange of gases (Chen et al. 1988; Schuepp 1993; Albayrak et al. 2012). Normally, a consequence of having bullate blades is increased drag force per unit of thallus surface area, which can normally increase risk of dislodgment or thallus damage (Koehl et al. 2008). However, for S. sessilis, the blade morphology does not significantly affect the drag forces on the alga’s holdfast (Milligan and Dewreede 2004). This phenomenon is likely due to the ability of both bullate and planar forms of S. sessilis blades to clump together and reconfigure in flow to form a smaller and more streamlined shape (Milligan and Dewreede 2004).
One plastic response to flow conditions is the attachment strength of the holdfast to the substratum. Surveys of S. sessilis attachment strength in Barkeley Sound, British Columbia, Canada showed that juveniles attach differentially based on substratum-type and wave-exposure, with higher attachment forces on coralline algal turfs and in exposed habitats (Milligan and Dewreede 2000). On the other hand, the attachment strength of adult specimens did not vary with site (Milligan and Dewreede 2000). Seasonal storms are thought to dislodge all of the weakly-attached adults across all substratum-types and wave-exposures, establishing populations of S. sessilis with similar attachment strengths. Dislodgment or breakage may also be enhanced by grazing damage that weakens the holdfast, but seems to kill juveniles more than adults (Markel and Dewreede 1998).
Other plastic responses to flow conditions are the material properties of the thallus. For S. sessilis on San Juan Island, Armstrong (1987) found no significant differences in the breaking force and maximum extensibility of blades across wave-exposures, but she did find that blades are stiffer in exposed habitats, and that the bases of the blades (toward the holdfast) are stronger and more extensible (i.e. stretchy) than the middle or tips of the blades.
Despite the ability to modify blade surface texture or material properties, the most effective means to reduce drag and limit the probability of death due to dislodgment is to remain small (Carrington 1990; Milligan and Dewreede 2004), which limits the surface area of the alga that can be exposed to flow. However, if the alga is good at reconfiguring and becoming streamlined, it may be able to grow numerous or larger blades without necessarily experiencing any strong selective pressure to be small.
All of these morphologically plastic traits work together to enable populations of S. sessilis to survive on shores over a wide range of flow conditions, tidal heights, and even grazing pressures. For example, blades of individuals in wave-exposed habitats move more rigidly back and forth with crashing waves. This movement exposes different parts of the thallus to light periodically (a process called lightflecking), and leads to higher rates of photosynthesis (Wing and Patterson 1993). Higher photosynthetic rates may enable the alga to produce tougher blades or grow faster (e.g. producing more or longer blades) to compensate for any damage to the thallus caused by the higher wave-exposure or even grazer damage. As with many other species of marine algae, having a wide capacity for morphological plasticity allows this species to survive on many different types of shores (e.g. wave-exposed and wave-protected shores) and also on shores that are extremely variable (e.g. seasonal storms). For this reason, S. sessilis may be found on a variety of sites on San Juan Island with both the bullate and the planar morphology.
Albayrak, I., V. Nikora, O. Miler, and M. O’hare. 2012. Flow-plant interactions at a leaf scale: effects of leaf shape, serration, roughness and flexural rigidity. Aquat. Sci. 74: 267-286.
Armstrong, S. L. 1987. Mechanical properties of the tissues of the brown alga Hedophyllum sessile (C. Ag.) Setchell: variability with habitat. J. Exp. Mar. Biol. Ecol. 114: 143-151.
Carrington, E. 1990. Drag and dislodgment of an intertidal macroalga: consequences of morphological variation in Mastocarpus papillatus Kutzing. J. Exp. Mar. Biol. Ecol. 139: 185-200.
Chen, J. M., A. Ibbetson, and J. R. Milford. 1988. Boundary-layer resistances of artificial leaves in turbulent air II: leaves inclined to the mean flow. Bound.-Layer Meteor. 45: 371-390.
Koehl, M. a. R., W. K. Silk, H. Liang, and L. Mahadevan. 2008. How kelp produce blade shapes suited to different flow regimes: A new wrinkle. Integr. Comp. Biol. 48: 834-851.
Markel, R. W., and R. E. Dewreede. 1998. Mechanisms underlying the effect of the chiton Katharina tunicata on the kelp Hedophyllum sessile: size escapes and indirect effects. Mar. Ecol.-Prog. Ser. 166: 151-161.
Milligan, K. L. D., and R. E. Dewreede. 2000. Variations in holdfast attachment mechanics with developmental stage, substratum-type, season, and wave-exposure for the intertidal kelp species Hedophyllum sessile (C. Agardh) Setchell. J. Exp. Mar. Biol. Ecol. 254: 189-209.
—. 2004. Morphological variations do not effectively reduce drag forces at high wave-exposure for the macroalgal species, Hedophyllum sessile (Laminariales, Phaeophyta). Phycologia 43: 236-244.
Schuepp, P. H. 1993. Tansley review No. 59. Leaf boundary layers. New Phytol. 125: 477-507.
Wing, S. R., and M. R. Patterson. 1993. Effects of wave-induced lightflecks in the intertidal zone on photosynthesis in the macroalgae Postelsia palmaeformis and Hedophyllum sessile (Phaeophyceae). Mar. Biol. 116: 519-525.
Author: Nicholas Burnett is a graduate student in the Department of Integrative Biology at the University of California – Berkeley.