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Echinoderms are major predators of anemones in temperate ecosystems. The fate of two algae, zooxanthellae and zoochlorellae, after their host anemone (Anthopleura elegantissima Brandt) was consumed by the leather star Dermasterias imbricata Grube was determined in experiments conducted in July and August 2004. Productivity, photosynthetic pigments, and mitotic index (percent of cells dividing) were used as indicators of algal health; algae released after leather stars consumed their host were compared with algae freshly isolated from anemones. Two types of waste products contained algae: pellets resulting from extraoral digestion, and feces. Zooxanthellae and zoochlorellae isolated from these waste products were photosynthetic, although to different extents. For algae from feces and pellets, light-saturated photosynthetic rates (Pmax) were 85 and 13%, respectively, of Pmax of freshly isolated zooxanthellae; and were 20 and 46%, respectively, for zoochlorellae. The photosynthetic pigments and mitotic index (percent of dividing cells) were not altered by the feeding activities of the leather star. These results show that algae released by seastar predation on their hosts remain viable, and are hence available for establishing symbioses in A. elegantissima and other potential hosts.

The phaeophyte alga Desmarestia munda (Order Desmarestiales) concentrates sulfuric acid (H2SO4) in cell vacuoles. The primary function of the accumulated acid in the alga's life history is unknown. To investigate if sulfuric acid works as a chemical defense, grazing by the sea urchin herbivore Strongylocentrotus droebachiensis on D. munda was compared to grazing on four common non-acidic phaeophyte algae (Alaria marginata, Laminaria saccharina, Costaria costata, and Nereocystis luetkeana) in multiple-choice feeding preference experiments. Both fed and starved urchins discriminated among algae; N. luetkeana and L. saccharina were consumed preferentially to D. munda and A. marginata. Fed urchins consumed a smaller quantity of A. marginata and D. munda than the starved urchins (15–28% less). The lack of preference for D. munda was not correlated with the presence of polyphenolic compounds or with nitrogen concentration. A. marginata had a high polyphenolic concentration (0.8% DM, dry mass) compared to that of the other four algae (0.08–0.24% DM). The nutritional content (C:N ratio and percent nitrogen) of the five algae ranged from 10.8 to 16.2 and from 2.08 to 3.55, respectively, but was not correlated with food choice by the urchins. Sulfuric acid made up nearly 16% DM of D. munda tissue and the pH of homogenized tissues averaged 1.99 pH units. Sulfuric acid, added to prepared food to provide a range of pH values from 1.9 to 6.2 pH units, deterred feeding by urchins at pH values ≤3.5 pH units. This result provides direct evidence that sulfuric acid at concentrations found in algal tissues deters grazing by S. droebachiensis. Therefore, acid accumulation in D. munda provides an important ecological defense against grazing by this herbivore.

The nutrient status (limitation vs sufficiency) of dinoflagellates (zooxanthellae) symbiotic with reef corals in Bermuda was assessed in 1989 and 1990 by measuring the enhancement of dark carbon fixation with 20 μM ammonium by isolated symbionts. A colony ofMadracis mirabilis was kept in the laboratory and fed daily or starved for one month. Symbionts from fed portions of the colony had ammonium-enhancement ratios (NH 4dark + ; SWdark;SW=seawater without added ammonium) similar to those of the original field population (1.2 to 1.3). Ammonium-enhancement ratios increased with starvation of the host (x≧1.7) as did values forV D′:V L [(ammonium dark rate-seawater dark rate): light rate in seawater]. Both parameters indicated decreasing nitrogen sufficiency of the algae when the host was not fed, but starvation appeared to affect these algae less than symbionts of sea anemones. Field samples of zooxanthellae fromM. mirabilis (Three Hill Shoals and Bailey's Bay Flats) yielded results similar to those for fed corals, but those taken from Bailey's Bay Flats in May 1990 yielded exceptionally high values for enhancement (>3) andV D′:V L indicating pronounced nitrogen limitation at the time of sampling. We sampled zooxanthellae from populations ofMontastrea annularis at 8 m (Three Hill Shoals) and 24 m (Soldier's Point) depths. Enhancement andV D′:V L values for zooxanthellae from the 8 m corals were density-dependent: symbionts from corals with “normal” symbiont densities displayed the most nitrogen limitation (enhancement values=1.4 to 2.0), while those from bleached corals with lower density exhibited enhancement andV D′:V L values typical of nitrogen-sufficient algae. Symbionts isolated from the 25 m corals yielded the highest values, and appeared to exhibit the least nitrogen-sufficiency for this species.