Dietary preference for leaves of the red mangrove (Rhizophora mangle) as measured by leaf damage and by mangrove tree crab (Aratus pisonii) gut contents

Dietary preference for leaves of the red mangrove (Rhizophora mangle) as measured by leaf damage and by mangrove tree crab (Aratus pisonii) gut contents

Amy A. Erickson,* Mark Saltis, Susan S. Bell, and Clinton J. Dawes. Department of Biology, University of South Florida, Tampa, FL 33620-5200, USA.

Herbivore consumption and diet preference often are assessed by determining the percent of damaged leaves in a given area. Another useful tool for the same analysis is examination of herbivore gut contents. Both techniques have been used in determining mangrove crab diets. While percent leaf damage indicates the extent to which plant populations are damaged or preferred, gut content analysis has distinct advantages. First, it allows an instantaneous versus a cumulative measure of consumption and preference. Second, it can be used to determine trophic status. Mangrove crabs considered as herbivores often supplement their diets with animal material, suggesting they are omnivores. This study compares the utility of one method against the other and examines whether comparable results are found for each. Mangrove tree crabs, Aratus pisonii, were sampled from stands of different mangrove species, around Tampa Bay, FL, during summer 2001. Stomach contents were dissected and classified to a number of categories. Species consumption and preference were determined by comparing relative numbers of mangrove stomata in gut contents and by determining the percent of damaged leaves in crab collection areas. Results suggest that both methods provide a relatively comparable estimate of preference. The instantaneous measures based on crab gut contents correspond to values of leaf damage found in the field, and the crab appears to consume predominantly mangrove but other food items as well.

Molecular phylogenetics of verongid sponges: utility of ITS-2 and 28S ribosomal DNA sequences

Genomic DNA was extracted from 5 species representing 3 genera of sponges in the Order Verongida. The second internal transcribed spacer region (ITS-2) and a portion of the 28S ribosomal subunit of Poriferan nuclear ribosomal DNA (rDNA) were amplified by PCR from each species, yielding an approximately 600 bp product. After sequencing, 226 bp ITS-2 and 352 bp 28S were aligned and analyzed using PAUP*. Phylograms constructed from ITS-2, 28S, and the entire dataset were compared using maximum likelihood models to determine the phylogenetic resolution of each region of rDNA. More parsimony-informative characters were present in ITS-2 (n=73) than in 28S (n=26); thus, ITS-2 is a useful marker for intra-order comparisons. However, comparisons of ITS-2 to sponges in other orders were complicated by the great difficulty of alignment. The 28S subunit was more easily aligned among orders, indicating a greater number of conserved bases, but less useful within genera.

Kristine E. Faloon¹* and C. E. Siddon.^{2 1}Boston University Marine Program, Boston, MA, USA; ²Dept. of Ecology and Evolutionary Biology, Brown University, Providence, RI, USA.

The movements of individuals and populations are significant components of community structure and dynamics. Understanding factors that modify movement, especially in important consumer species, have implications for community composition, succession and species interactions. Habitat characteristics such as availability of refuges and alternative food sources may modify movement. We investigated the movement patterns of subtidal populations of the green sea urchin, Strongylocentrotus droebachiensis, within mussel beds, urchin barrens and algal dominated habitats of the Gulf of Maine, USA, to assess the impact of habitat characteristics on movement. A diffusion coefficient for tagged urchins was calculated and averaged across multiple sites within each habitat to quantify movement rates. Movement was significantly different in each habitat type with the highest rate of diffusion within mussel beds, followed by urchin barrens and algal beds. In barrens sites, with and without neighbor urchins present, movement was not affected by urchin density. These results demonstrate the effect habitat characteristics have on urchin movement and suggest that the movement of consumers may play an important role in structuring subtidal communities.


Diadema antillarum effects on benthic community structure in the Florida Keys National Marine Sanctuary (FKNMS): preliminary results

Charles Fasano,¹* Margaret Miller,¹ Alina Szmant,² and Nicole Fogarty.^{2 1}NOAA-Fisheries, Southeast Fisheries Science Center, 75 Virginia Beach Dr., Miami, FL 33129, USA; ²University of North Carolina at Wilmington, CMS, 1 Marvin K. Moss Ln, Wilmington, NC 28409, USA.

Following the die off of the sea urchin, Diadema antillarum, in the 1980’s thick turf and fleshy algae have proliferated on Caribbean reefs. We studied the effects of re-introducing moderate Diadema density on benthic community structure at Little Grecian Reef (FKNMS). Wild-caught adult Diadema were transplanted into fenced enclosures at a density of 7 m^-2 (background Diadema density ~0 m^-2). The density of Diadema in the enclosures was monitored periodically and declined over time. Four other areas served as controls (no Diadema, no enclosures). We estimated percent cover of bare surface, macroalgae (predominantly Dictyota sp.), Crustose Coralline Algae (CCA), and invertebrates (hard corals, sponges, Palythoa sp., and octocorals) using point counts of photographs taken at ca. bimonthly intervals. Initial conditions were quantified in May 2001, urchins were transplanted in July, and experimental effects were assessed after three, five, and seven months of urchin grazing. Biostatistical methods involved Multivariate Repeated Measures ANOVA. Initially, urchin and control treatments did not differ for all cover groups. After seven months, total macroalgae was lower in urchin treatments than controls but this predicted response was not evident for the first five months (Time*Treatment p=0.0057). CCA and bare substrate increased slightly but not significantly under Diadema grazing. Urchin grazing significantly depressed sponge cover and this effect increased over time (Time*Treatment p=0.033). Other invertebrate groups did not show significant treatment effect. Overall, grazing by a moderate density of Diadema antillarum had complex effects on benthic community structure.


Nutrient limitation of benthic macrophytes in the upper Florida Keys: an in-situ nutrient enrichment experiment

Meredith Ferdie* and James W. Fourqurean. Southeast Environmental Research Center and Department of Biological Sciences, Florida International University, Miami, FL 33199, USA.

South Florida contains some of the most expansive documented seagrass communities in the world. Stoichiometry of seagrass tissue from the upper Florida Keys suggest that benthic coastal communities are phosphorus (P) limited inshore and nitrogen (N) limited offshore. This spatial variability hypothesis of nutrient limitation is presently being tested with a one year in-situ sediment fertilization experiment. The objectives of this investigation are to determine the limiting nutrient for Thalassia testudinum growth and to assess the responses of inshore and offshore benthic communities to nutrient addition. Nutrient loading rates are based on current estimates of dominant nutrient sources in the Florida Keys. Each site (3 inshore, 3 offshore) consists of 4 treatments (N, P, N+P, control) with six replicates, for a total of 144 experimental plots. The following data from 6 months of nutrient enrichment are presented: seagrass productivity and leaf tissue C:N:P, sediment C:N:P, sediment chlorophyll a, and total epiphyte load. Results may be used to model changes that anthropogenic eutrophication may cause in the subtropical coastal marine waters of the upper Florida Keys.


Can whelks find food in flowing fluid?

Matthew C. Ferner* and Marc J. Weissburg. School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA.

Mobile consumers often forage by tracking chemical signals. Successful tracking behavior is particularly important in estuarine habitats where turbidity is high and hydrodynamic conditions can be variable. Compared to other abundant predators and scavengers in these habitats (e.g., blue crabs), whelks move relatively slowly and thus may employ sampling strategies and tracking behaviors that are different than those of faster foragers. Here we present results from a flume study of the tracking behavior of two sympatric species of whelks, Busycon carica and Busycotypus canaliculatus, which differ in morphology, activity level, and small-scale distribution. Each animal was randomly assigned to one of four flow velocities (1.5, 5, 10, or 15 cm/s) and exposed to either a control (seawater) or treatment (mussels soaked in seawater and then filtered). Significant differences were observed between species in terms of search behavior and frequency of tracking success. Busycotypus canaliculatus tracked faster and had a higher success rate at all velocities except 5 cm/s, at which both species were equally successful. The differential ability to follow chemical plumes in a range of flow velocities may cause these two species of whelks to forage more efficiently in different hydrodynamic environments. This suggests the potential for habitat partitioning that could reduce competition for food and may help to regulate community structure.


Fitness consequences of allorecognition-mediated agonistic interactions in the colonial hydroid Hydractinia [GM]

David L. Ferrell.* Department of Biological Science, Florida State University, Tallahassee, FL 32306-1100, USA.

Many clonal marine invertebrates possess competitor-induced structural defenses used for overgrowth or territorial purposes. The colonial hydroid Hydractinia [GM] utilizes an inducible defense when encountering conspecifics. My goal was to ascertain the consequences of agonistic interactions in H. [GM] for three primary fitness components: colony survival, growth rate, and reproductive effort. I investigated numerous encounters involving pairs of colonies differing in competitive ability established near or far from each other (resulting in interactions between small or large colonies, respectively). I collected five large colonies from St. Joseph Bay, FL, and established competitive contests between all pairwise combinations of colonies by attaching tissue explants from different colonies near (4 mm) or far (12 mm) from one another. All ten pairings were replicated five times at both distances, and five replicates of each colony were also established in the absence of competitors, as controls. Hydroid cultures were maintained in a single aquarium, and the survival, area covered (growth rate), and number of reproductive zooids (reproductive effort) of all colonies were recorded at 15, 30, 60, 80, and 90 days after establishment.

Not surprisingly, interactions between small colonies generally imposed greater costs in terms of colony survival, growth rate, and reproductive effort. Also, competitors differed in colony survival, reproductive effort, and growth rate (though to a lesser degree). Superior competitors typically eliminated competitors in small-colony encounters, but in larger-colony interactions, only the most dominant competitors competitively excluded others. In such encounters between less dominant competitors, however, colonies exhibited increased reproductive effort and low mortality, although reduced growth was evident. In fact, in these instances reproductive effort was greater than that of control colonies established in the absence of competitors. I will discuss several implications of these results with regard to the life history evolution and ecology of H. [GM].

Janna Fierst,* Janet Kübler, and Steve Dudgeon. California State University, Northridge

We investigated small scale spatial structure in Mastocarpus papillatus, a red alga with sexual and apomictic life history variants. Spatial structure and habitat partitioning can facilitate the coexistence of ecologically similar organisms, and in M. papillatus a large scale geographic spatial division is well documented. Variants of M. papillatus display geographic parthenogenesis where sexual fronds are common south of San Francisco Bay and apomicts dominate north of San Francisco Bay. Although geographic parthenogenesis in M. papillatus is well documented small scale spatial structure was not previously known due to the similar appearance of sexual and apomictic upright fronds. We investigated whether sexual and apomictic fronds differed in their spatial distribution by sampling for vertical differences in intertidal height and horizontal patchiness. Fronds were sampled when reproductive and cultured for life history. We found sexual variants were significantly aggregated at lower intertidal heights while apomicts were found throughout the intertidal range. Sexual fronds were strongly seasonal and reproduced October-December whereas apomictic fronds reproduced throughout the year. These results parallel the large scale geographic distribution with apomicts inhabiting marginal areas, and suggest dispersal capability may also facilitate geographic parthenogenesis.

Thalassinid shrimp are common members of soft-sediment intertidal habitats along the Northern Gulf of Mexico where densities can reach 700 burrows m^-2. Water contained within the burrow is enriched in inorganic nutrients (e.g. nitrogen, phosphorous, and silicate), and these nutrients may play an important role in benthic production if delivered to the surface sediment or overlying water column. However, very little is known about the temporal patterns of burrow ventilation or the fate of nutrients as they are pumped into the surface water. To address these issues, we are examining the biomechanics of burrow ventilation and effluent plume formation for several coastal species in the lab and field. The first phase of this research examines burrow morphology and temporal patterns of ventilation to parameterize later studies of effluent dispersal. Resin castings of burrows from two species, Callichirus major and C. islagrande, reveal elongated, narrow constrictions (chimneys) at the burrow outlet that are 2 to 3 times smaller in diameter than the burrow proper. These constrictions may act as a nozzle to increase exit velocity 4 to 9 times, and push effluent higher into the benthic boundary layer. Lab measurements show highly variable pumping patterns with bouts of burrow ventilation lasting seconds to minutes. Effluent velocities at the burrow exit can reach 12 cm s^-1, with volume flows of 2 to 3 ml s^-1. Shrimp in the lab were capable of pumping over 2 liters per day, even though ventilation was sporadic. Given observed burrow densities and estimated volume flow rates through the burrows, it is possible that 1400 L m^-2 d^-1 of burrow water is pumped into the overlying water column. We are currently investigating seasonal patterns of ventilation, the source of burrow water (i.e. porewater vs. surface water), and the effects of burrow effluent on benthic primary production.

Geremea P. Fioravanti* and John F. Valentine. Dauphin Island Sea Lab and the University of South Alabama, 101 Bienville Blvd. Dauphin Island, AL 36528, USA.

Scientists and conservationists are increasingly concerned that the overfishing of marine consumers has caused significant alterations in the structure and function of marine ecosystems. Consequently, marine reserves are increasingly being utilized in an effort to restore commercially important higher-order consumers in tropical marine ecosystems. While it is true that the establishment of some marine reserves has led to the recovery of marine consumers, not all have been successful. Here I hypothesize that considering measures of reef structural complexity or landscape geometry can explain why marine reserves are not always successful. In this study, I used a randomly selected set of patch reefs located in the northern Florida Keys National Marine Sanctuary to determine which of these patch reef features were most important in controlling fish community structure. Based on visual censuses of fishes conducted on these patch reefs, community similarity varied greatly among sites. Using the chain method, we found a significant proportion of the variance in fish density and species richness could be explained by differences in reef structural complexity. In addition, I found that reef perimeter also played an important role, although it explained less of the observed variance than structural complexity, in determining fish community structure. These results indicate that landscape geometry is a potential factor in confounding the estimate of the recovery of higher order consumers in marine reserves.


RIDGE 2000: the next decade of innovative studies and discoveries at the mid-ocean ridge