HETEROMORPHIC LIFE‐HISTORY STRATEGIES IN THE BROWN ALGA SCYTOSIPHON LOMENTARIA (LYNGB.) LINK1

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Abstract
The adaptive significance of a life‐history strategy, expressed as divergent morphological forms, was examined for the heteromorphic alga Scytosiphon lomentaria. Successional studies were performed by physically clearing mature, temporally‐constant intertidal communities on San Clemente I., California. At week six, after clearing, complanate thalli dominated the successional plots (mean cover = 23.5%) and began to decline as the cylindrical form became abundant. The latter attained its peak cover (82.3%) at week 13, whereupon it too began a precipitous reduction. The crustose ralfsioid form appeared surprisingly early (4–13 wks) in trace amounts but did not achieve its greatest cover (85.0%) until week 43. The ranking from high to low primary productivity (cylindrical form = 8.1 mg C · g dry. wt‐1·h‐1, complanate form = 6.5 mg, crustose form = 0.5 mg) corresponded closely with the data for photosynthetic vs. structural components (cylindrical = 92.3% pigmented, complanate = 65.3%, crustose = 32.0%). This finding indicates that selection in the crust form, which is more readily accessible to epilithic grazers, has tended to increase allocation of materials to nonpigmented structural tissue at the expense of photosynthetic tissue and reduced production rates. The results for thallus losses to urchin grazing over 48 h were complanate form = 82.7% lost, cylindrical = 81.4% and crustose = 16.2%, which correlates with the calorific contents of the three forms (i.e. complanate = 4.97 kcal · ash‐free g dry wt.‐1, cylindrical = 4.46 kcal and crustose = 3.55 kcal). The crustose form had tougher thalli (26 g · mm‐2 to penetrate thallus) than either the complanate form (12 g) or the cylindrical form (15 g). It is likely that opposing selective factors have resulted in the evolutionary divergences observed in algae with heteromorphic life histories. Previous work may have overemphasized the selective role of grazing because the crustose form is also adapted to withstand physical forces (sand‐scouring, burial and wave‐shearing) or as an overwintering stage under physiologically stressful conditions.