Effects of nitrogen concentrations in turtlegrass Thalassia testudinum on consumption by the bucktooth parrotfish Sparisoma radians

Abstract

Terrestrial herbivores often preferentially consume plants that have higher nitrogen concentrations. In these circumstances, low nitrogen concentrations in exposed tissues would be an effective defense against herbivory. In marine environments, the relationship between nitrogen concentration in plants and herbivore preferences has only recently been explored, and results are inconsistent. We examined preferences of the bucktooth parrotfish Sparisoma radians for the turtlegrass Thalassia testudinum with high or low nitrogen concentrations in both field and laboratory experiments. These species were chosen for study because (1) turtlegrass, the dominant seagrass in the Caribbean, is the primary food resource of the bucktooth parrotfish and (2) S. radians is known to be undeterred by the chemical defenses (i.e. phenolic compounds) and physical defenses (i.e. tough tissues) of T. testudinum. In field choice experiments, we found that S. radians consumed a greater proportion of T. testudinum shoots with high nitrogen concentrations than shoots with low nitrogen concentrations (68 vs 5%). Similarly, in laboratory experiments, S. radians consumed more T. testudinum with high nitrogen than with low nitrogen concentrations (25 vs 5%). To determine whether S. radians used the color or shape of shoots as a basis for its preferences, we repeated laboratory experiments using an agar mixture with powdered T. testudinum leaves of either high or low nitrogen concentration. S. radians again showed a significant preference for the high nitrogen food source. We, therefore, conclude that mechanisms for choice of high nitrogen food were most likely related to gustation and/or olfaction of chemical cues (i.e. nitrogen or phenols).

Seagrass selection by omnivorous and herbivorous consumers: determining factors

“…In macrograzers, such as fish and sea urchins, some studies have already highlighted the role of nutritional characteristics due to physiological constraints and/or the presence of toxins in food preferences and consumption rates (e.g. Lobel & Ogden 1981, Paul & Hay 1986, Goecker et al 2005. The role of secondary metabolites has been indicated for many species of macroalgae (Paul & Hay 1986, Hay & Fenical 1988, but, despite their widespread occurrence in seagrasses (McMillan 1984), they do not seem to cause the avoidance of Thalassia testudinum and Halodule wrightii by macrograzers such as Lytechinus variegatus (Steele & Valentine 2010).…”

“…Epiphytes were removed from the seagrass and leaves were dried at 70°C for 24 h and then ground to a fine powder. Four grams of each seagrass species were placed in a heated mixture of 100 ml of distilled water (as indicated by Goecker et al 2005) and 2 g of agar (Carolina Biological Supply). The mixture was poured into small molds (2 cm diameter) and allowed to cool for 1 h within a refrigerator.…”

“…In some cases, such as those of herbivorous mollusks, the association with plants is so highly evolved that they have developed specialized feeding apparati that are specific for the size and toughness of targeted plants (Steneck & Watling 1982). Yet, there is little evidence that feeding decisions of macro herbivores such as fish and sea urchins are based on the structural and/or color features of plants (but see Goecker et al 2005, Prado et al 2010. These consumers possess distinctive types of buccal apparati such as the Aristotle's lantern of sea urchins or the jaws of fishes (see Watts et al 2001), and may respond to structural aspects of food items such as shape, size, toughness and manipulability of food (Klinger 1982).…”

Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

“…From large-scale comparative studies, we know that grazing intensity can be highly variable, both within and between primary producer groups (Duarte & Cebrián 1996, Cebrián 2002. Many studies show an association between high nutrient content of the primary producers and high consumption rates (Horn 1989, McGlathery 1995, Cebrián 1999 and references therein, Goecker et al 2005); however, other factors such as herbivore abundance, per capita grazing rates of the dominant herbivores, and feeding preferences also play important roles in determining patterns of herbivory (Cebrián 2004 and references therein). Based on these factors, we would expect a unimodal pattern of relative grazing rate in response to elevated nutrient loading (Fig.…”

Can strong consumer and producer effects be reconciled to better forecast ‘catastrophic’ phase-shifts in marine ecosystems?

“…Strong consumer effects may be largely under-appreciated (Valentine and Duffy, 2006), particularly under conditions where the indirect effects of over-harvesting of fish cascades via urchins, past the initial benefits of reducing epiphyte biomass and onto the over-consumption of seagrass. This balance between production and consumption may be altered through nutrient enrichment which can increase the palatability of seagrasses to grazers (Goecker et al, 2005), and thereby strengthen consumption (Power, 1992). Similar to studies of coral and kelp loss, it is difficult to solve the broader problem of seagrass loss through the isolated study of alternate drivers, then hoping that they may be assembled into a coherent theory.…”