Nodilittorina peruviana (Gastropoda: Littorinidae) effects on epilitic biofilm abundance and on barnacle settlers survival in a rocky intertidal of central Peru

HIDALGO, FERNANDO J; FIRSTATER, FAUSTO N; FANJUL, EUGENIA; LOMOVASKY, BETINA J; TARAZONA, JUAN; IRIBARNE, OSCAR

Lima, Perú

Conferencia; International Conference The Humboldt Current System: Climate, ocean dynamics, ecosystem processes, and fisheries; 2006

IMARPE (Instituto del Mar del Perú), IRD (Institud de recherche pour le développement), FAO (Food and Agriculture Organization)

Littorinid gastropods are cosmopolitan and ubiquitous in intertidal marine ecosystems. Most high shore Littorinidae feed on the epilithic biofilm of micro-algae, but also on ephemeral filamentous or foliose algae and macro-algae; influencing both primary production and overall community structure. They can also affect directly and indirectly recruitment and survival of sessile organisms, such as barnacles, while grazing. In the Peruvian rocky shores, the high zone is dominated by the littorinid Nodilittorina peruviana (Paredes, 1974). There, the vertical distribution of N. peruviana ranges from the bare substrate in the higher zone, to the barnacle belt of Jehlius cirratus and Notochthamalus scabrosus lower in the shore, overlapping its distribution with that of barnacles. In this study we evaluated the effects of N. peruviana grazing on the abundance of the epilithic biofilm of micro-algae of the mid-high intertidal zone and on the settlement and survival of the barnacles J. cirratus and N. scabrosus. The study was conducted on a rocky intertidal next to the Ancón bay (11º46’S, 77º11’W), in central Peru. Methods- To investigate the grazing effects of N. peruviana on the abundance of micro-algae and barnacle recruitment, 10 x 10 cm plots in which snails were excluded (exclusions) were compared with plots where snails were allowed to enter (controls and experimental controls). Experimental plots were established at two heights of the shore: in the upper limit of distribution of barnacles and within the barnacle belt (herein after the “high” and “low” zones, respectively). The vertical distance between the two heights was ~35 cm. Eight plots were assigned to each treatment. To homogenize initial conditions, each plot was previously scraped and sprayed with oven-cleaner (NaOH, which degrades to NaCl and water when in contact with seawater) to ensure the elimination of all organisms. Exclusions were bordered with a thin coat of epoxy putty and a copper based antifouling paint was applied directly on the epoxy when it was still wet. Experimental controls were also bordered with the epoxy, but only with two opposite sides painted. This allowed testing for possible effects of the epoxy border in retaining water, creating little pools; and for the effects of the antifouling paint on the biofilm and barnacle recruiting and survival. Control plots were marked with epoxy putty in two diagonally opposed corners. Exclusions were controlled at regular intervals, removing any snail that could be entered, and painting if necessary. Experiments were set up on October 2005. Biofilm abundance was assessed indirectly by means of the chlorophyll a (Chl a) content of the rock. Forty-five days after starting the experiments, 1 to 10 rock chips, with a total mean area of 4.69cm2, were extracted from each plot and Chl-a was measured with the cold methanol method (see Thompson et al. 1999). In March 2006, density of barnacle recruits was estimated either visually or from digital pictures taken from four sub-sampled quadrants (2 x 2 cm) of each plot. Results- Chl-a abundance was higher in the low intertidal zone than in the high zone (two-way ANOVA, p < 0.001, log10 transformed data); and was consistently higher in exclusions than in controls and experimental controls for both intertidal zones (ugChla/cm2 (mean, SD); E-L: 1.84, 1.08; EC-L: 1.28, 0.55; CL: 0.84, 0.33; E-H: 0.70, 0.14; EC-H: 0.65, 0.39; C-H: 0.40, 0.18. “H” refers to high zone, “L” to low zone; “E” are the exclusions, “EC” are the experimental controls, and “C” the controls), despite significant differences among treatments within zones were only found between exclusions and controls from the low zone (Two-way ANOVA, Tukey HSD test, p = 0.01). Barnacle recruits density was markedly higher in the exclusion of the low zone, while there were no differences in the rest of the treatments (N recruits/4 cm2 (mean, SD): E-L: 9.47, 11.01; EC-L: 0.22, 0.25; C-L: 0.12, 0.27; E-H: 0.06, 0.11; EC-H: 0.09, 0.13; C-H: 0.03, 0.09; Two-way ANOVA, Tukey HSD test, p < 0.01 for all comparisons). Discussion- The copper based antifouling paint combined with the epoxy putty was a successful method to exclude N. peruviana, and did not have evident negative effects on the biofilm abundance, neither on barnacle recruitment nor survival. N. peruviana grazing reduces the abundance of the epilitic biofilm at both the high and low shore. However, longer emersion times in the high shore might reduce biofilm abundance and therefore littorinids would have little impact on it. Barnacle settlers survival was markedly enhanced in the exclusions in the low shore, and were nearly absent in low shore controls and in all the treatments in the high shore. This shows that settlers’ survival is reduced by grazing in the low shore. Grazing can affect barnacle recruitment either directly by consumption, bulldozing or crushing of new settled individual; or indirectly through the removal of the epilithic biofilm, which in turns may facilitate cyprids settlement (Thompson et al, 1998). Higher in the intertidal, however, desiccation stress or a lower number of larvae, as a result of shorter immersion times, may control recruits settling and survival. Our results suggest that the relative importance of littorinid grazing decreases with increasing tidal height in the high shore, and even a ~35cm vertical height difference may cause a considerable difference in community dynamics, especially given the narrow vertical tidal range in the central coast of Peru.