Investigating the role of the spotted spiny lobster (Panulirus guttatus) in the recovery of the long spined sea urchin (Diadema antillarum) on the coral reefs of the Florida Keys Protect Our Reefs Grant Interim Report (October 1, 2008 March 31, 2009) Principal investigators: Donald C. Behringer and Mark J. Butler IV Project Justification Coral reefs are some of the most diverse and productive ecosystems throughout the world (Knowlton 2001). However, coral reefs worldwide are in a state of decline (Hughes et al. 1999) and as detailed in Florida s Comprehensive Wildlife Conservation Strategy, the reefs of the Florida Keys are no exception. Reefs of the Caribbean, including those of Florida, have been negatively impacted by many factors including global climate change, pollution, eutrophication, and overfishing (Hughes et al. 1999, Lapointe 1997, Jackson et al. 2001), but none as calamitous as the Diadema antillarum mass mortality of the early 1980s (Lessios 1988). Within days of the D. antillarum die-off, macro-algal cover had increased and reef health began a decline from which most reefs have yet to recover (Carpenter 1988). While several areas have seen recovery of D. antillarum populations and subsequent increases in coral cover (Edmunds and Carpenter 2006) the Florida Keys D. antillarum populations are still orders of magnitude below historical densities (Chiappone et al. 2002). Several possible explanations for the heterogeneous recovery of D. antillarum populations include patchy larval supply, inappropriate settlement habitat, and post-settlement mortality. However, the recruitment pulses observed by Chiappone et al. (2002) indicate that post-settlement mortality is probably the most parsimonious explanation for the slow recovery of D. antillarum throughout Florida. Concurrent with the precipitous decline of D. antillarum, has been the negative impact of overfishing on reefs. Fishing effort is typically concentrated on large apex predators resulting in significant declines in their abundance and often their functional extinction (Dulvy et al. 2004). An important consequence of the removal of these top predators is a shift in trophic structure to community assemblages dominated by large invertebrates freed from predation. Where invertebrates have been well studied, they tend to exert strong top-down influences on community structure. For example, several studies have demonstrated the importance of lobster in shaping communities via predation, but these studies have been carried out exclusively in temperate systems (Robles 1987). Despite evidence that changes in lobster populations can spark trophic cascades in temperate systems, similar studies have not been conducted on tropical reefs where the focus has been on impacts attributable to fishes. Furthermore, most studies of coral reefs are conducted during the day when fish are the most abundant predators, but at night a whole different suite of organisms become active, including lobsters and urchins. Could the spiny lobster, via predation, be inhibiting recovery of D. antillarum on the reefs of the Florida Keys? Several species of lobster inhabit the coral reefs of the Florida Keys, with the two most abundant being the Caribbean Spiny Lobster (Panulirus argus) and the Spotted Spiny Lobster (Panulirus guttatus). Panulirus argus supports the most economically important fishery in the Caribbean (Hunt 1994) and is thus the focus of most research. However, P. argus may have
little impact on reefs because it forages off the reef at night. In contrast, P. guttatus is an obligate reef dweller, it preys on echinoderms, mollusks, and crustaceans (pers. obs.), and it is thus more likely to have an impact on reef communities than P. argus. Although there is a fishery for P. guttatus in many areas of the Caribbean, none exists in the Florida Keys where high densities of P. guttatus correspond with documented poor urchin population recovery. The role of D. antillarum as a keystone species on the coral reefs of the Caribbean is well established (Carpenter 1988, Lessios 1988, Carpenter and Edmunds 2001). Therefore, if P. guttatus consumes significant numbers of urchins, then one would expect an associated response at lower trophic levels (e.g., increased algal growth). Thus, the focus of this research will be on the role of P. guttatus in shaping the coral reef communities of the Florida Keys, with special emphasis on its predator-prey interactions with D. antillarum. Goals The objective of this project is to determine if the spotted spiny lobster (P. guttatus) has a demonstrable impact on reef community structure and, in particular, the recovery of long spined sea urchin (D. antillarum) on the reefs of the Florida Keys. The questions we hope to address include: Do changes in P. guttatus density on patch reefs result in quantifiable changes in (a) prey abundance (especially D. antillarum); or (b) D. antillarum behavior, habitat use, or growth? Objective a Densities of both P. guttatus and the long spined sea urchin were manipulated on 12 patch reefs off of Long Key Florida (Florida Keys, Florida U.S.A.) in a 2 factor fully-crossed design. Regular surveys of sessile benthic cover in permanent quadrats, coral recruitment onto artificial substrates, and the abundance of cryptic macroinvertebrates collected via suction sampling and rubble-filled trays serve as our metrics for reef community assessment. Regular counts of grazing fish abundance, size, and bite rates are also conducted to monitor their potential effect and any differences among patch reefs. Analysis of P. guttatus gut content from lobsters captured on patch reefs will provide in situ data on lobster foraging. Thus far twenty five lobsters were hand caught by divers at night on coral reefs in the Florida Keys, Florida (USA), then put on ice and later dissected for stomach content analysis. The stomach contents (preserved in 70% ethanol) were later visually identified under a dissecting microscope and quantified. While crabs comprised the largest percentage of the stomach contents sea urchins were an important component as well. We plan to increase the sample size of this study and compare it with lab prey selectivity studies. The laboratory preference experiments will be conducted in flow-through seawater tanks following standard cafeteria-style prey selectivity experiments (Krebs 1999) in which we will offer a single lobster per trial a suite of potential prey (of similar size). Lobster size or sex and prey size may influence selectivity, so we will conduct separate experiments to evaluate those factors and to address whether P. guttatus is gape limited, potentially offering a size refuge to prey. Another set of experiments will focus specifically on P. guttatus consumption of D. antillarum as a function of lobster size, D. antillarum size, D. antillarum aggregation (alone or in groups), and the presence of shelter for D. antillarum. Additionally, we will offer individual lobsters
unlimited quantity of their preferred prey to calculate the biomass of prey consumed per unit lobster biomass for a given unit of time as an estimate of the impact that P. guttatus could be exerting on reef community structure. Objective b In addition to direct numerical reduction via predation, the presence of lobster predators could indirectly be reducing the functional significance of Diadema on coral reef communities. To test the behavioral response of D. antillarum to lobster chemical cues the long spined sea urchin was placed in tubs ~ 50cm x 30cm x 40cm and allowed to acclimate for approximately 20 minutes. The urchins then had either 2 gallons of untreated seawater or water that had contained a P. guttatus added to their tub. The movement of the D. antillarum was monitored for five minutes after addition of the water. Additional experiments are planned to monitor D. antillarum grazing in response to the introduction of lobster chemical cues. Findings to Date Panulirus guttatus consume a variety of benthic invertebrates, which leads to significant differences in invertebrate rubble communities as evidenced by our tray data (Figures 1, 2). This pattern held, with the low density lobster patches having significantly more invertebrates during all three sampling periods. However, our algal samples showed no effect of lobster on the invertebrate communities (Figure 3, 4). We believe two mechanisms could be responsible for this pattern. The most abundant algae on our sites is Halimeda sp. which is both chemically and structurally defended. Several studies have demonstrated that chemically defended macroalgae deter feeding and provide refuge for organisms that utilize the algae for shelter. Additionally, the structural complexity of the algae could be providing a spatial refuge from lobster predation. Our laboratory feeding trials indicate that P. guttatus feeds on several species of urchins including D. antillarum (Figure 5). Breaking down the D. antillarum consumption into size classes shows that the smallest individuals are those most susceptible to P. guttatus predation (Figure 6). Our findings indicate that P. guttatus can have significant impacts on invertebrate rubble communities on tropical patch reefs. Additionally, P. guttatus could be an important predator on D. antillarum, and could contribute to the slow recovery of D. antillarum throughout the Caribbean. Presentations resulting from this work Investigating the role of the spotted spiny lobster (Panulirus guttatus) on the coral reef communities of the Florida Keys. 37 th Annual Benthic Ecology Meeting, Providence, R.I.
Assessing the ecological significance of interactions between the spotted spiny lobster (Panulirus guttatus) and the long spined sea urchin (Diadema antillarum) in shaping coral reef communities in the Florida Keys. 11 th International Coral Reef Symposium, Fort Lauderdale, FL. Impacts of the spotted spiny lobster (Panulirus guttatus) and the long-spined sea urchin (Diadema antillarum) on the patch reef communities of the Florida Keys. 38 th Annual Benthic Ecology Meeting, Corpus Christi, TX. Figure 1 Mean number of invertebrates in rubble filled trays on patch reefs with high P. guttatus density and low P. guttatus density from the summer of 2008.
Figure 2 Mean number of invertebrates in rubble filled trays from the winter of 2008. Figure 3 Mean number of invertebrates in algal quadrats from summer 2008
Figure 4 Mean number of invertebrates in algal quadrats from the winter of 2008 Figure 5 Number and species of urchin consumed by Panulirus guttatus during feeding trials.
Figure 6 Percent of Diadema size class (mm test diameter) consumed by P. guttatus.