Interspecific Variation in Anemone Shrimp Distribution and Host Selection in the Florida Keys (USA): Implications for Marine Conservation

Home , Actinia bermudensis, Bartholomea annulata, Cassiopea, Condylactis gigantea, Periclimenes rathbunae, Periclimenes sagittifer

BULLETIN OF MARINE SCIENCE, 83(2): 329–345, 2008

Interspecific variation in anemone shrimp distribution and host selection in the Florida Keys (USA): implications for marine conservation

Nyssa J. Silbiger and Michael J. Childress

Abstract Host and symbiont abundances often depend on the degree of host specificity and differences in costs and benefits of these associations. In this study, we investigated the distribution of anemone shrimps and their hosts along with the shrimps’ host selection behavior to evaluate the degree of host specificity in a nearshore benthic community undergoing rapid environmental change. We compared the distribution of two shrimp species, Periclimenes pedersoni Chace, 1958, and Periclimenes yucatanicus (Ives, 1891), with their three anemone hosts, Condylactis gigantea (Wein- land, 1860), Stichodactyla helianthus (Ellis, 1767), and Bartholomea annulata (Le- sueur, 1817). The proportion of host anemones occupied by each shrimp species was significantly non-random considering abundance of each species per location and significantly different from a previous study conducted 18 yrs earlier. This was due to a decline in S. helianthus abundance. Periclimenes yucatanicus abundance was correlated with C. gigantea abundance, but P. pedersoni abundance was correlated with B. annulata abundance and all anemone species combined. A dichotomous choice test between C. gigantea and B. annulata revealed that P. yucatanicus selected C. gigantea regardless of its original host species. However, P. pedersoni selected its original host species. These results suggest that P. yucatanicus retains a strong host specificity for C. gigantea, whereas P. pedersoni appears to show no particular host specificity. Species with strong host specificity, such as P. yucatanicus, may be less likely to switch host species, and therefore, may be more susceptible to a decline in host abundance.

Understanding the patterns and mechanisms of host specificity in symbiotic relationships is important to marine conservation. Symbionts may actually increase the health of corals and anemones by providing oxygen and nutrients (Spotte, 1996; Porat and Chadwick-Furman, 2004), protecting hosts from predators (Smith, 1977; Prachett et al., 2000; Prachett, 2001), and even cleaning tissues (Stewart et al., 2006). Understanding how these associations are formed and maintained especially during times of rapid environmental change is essential to our goal of maintaining healthy coral reef ecosystems. Symbiotic relationships, while prominent in marine ecosystems, present some unique challenges to marine conservation (Castro, 1988). What will happen to a symbiont if its host species declines or goes extinct? Does the degree of host specificity indicate something about the probability of persistence when multiple host species are in decline? Does the mechanism of host selection determine which symbionts are most resistant to changes in host abundance? In this study, we explored the host specificity and host selection behavior of two species of anemone shrimps in the genus Periclimenes with three species of sea anemones in the shallow waters of Florida Bay within the Florida Keys National Marine Sanctuary (USA). Our goal was to understand how differences in host selection behavior, which we defined as an anemone shrimp preferentially choosing one anemone host species over another,

Bulletin of Marine Science 329 © 2008 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 330 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008 might influence the distribution of shrimps in the field and their resistance to changes in host abundance. A wide variety of decapod crustaceans, especially shrimps and crabs, are symbionts on mollusks, echinoderms, corals, and sea anemones (Wirtz, 1997; Baeza and Thiel, 2003; Bauer, 2004; Kahn et al., 2004). Coral and sea anemone hosts often benefit directly by protection against predators (Smith, 1977;P ratchett, 2001), decreased sedimentation (Stewart et al., 2006), or increased nitrogen from symbiont waste (Spotte, 1996). Shrimp and crab symbionts often benefit by protection from predators (Vytopil and Willis, 2001) and feeding directly on the host tissue (Fautin et al., 1995). Some species occupy a wide range of different hosts Periclimenes( rathbunae Schmitt, 1924—Spotte et al., 1991; Allopetrolisthes spinifrons (H. Milne-Edwards, 1837)—Baeza and Stotz, 2001) while others tend to be found exclusively on a single host species (Periclimenes ornatus Bruce, 1969—Omori et al., 1994; Guo et al., 1996; Pinnotheres halingi Hamel, Ng and Mercier, 1999—Hamel et al., 1999). If the costs and benefits to host and symbiont vary by association, this would presumably influence the degree of host specificity. Our study focuses on two anemone shrimp species, Periclimenes pedersoni and Periclimenes yucatanicus that are known to associate with multiple species of sea anemones, false corals, jellyfish, and clams (Table 1). Judging from the taxonomic diversity of their hosts, these species might be assumed to have relatively low host specificity, however, this varies geographically. For example, Nizinski (1989) reported that high host specificity occurs in Periclimenes anthophilus Holthius and Eibl-Eibesfeldt, 1964 [P. pedersoni; Wicksten (1995b) and Spotte (1999)] in Bermuda, because shrimps were only found in the giant anemone Condylactis gigantea even though other suitable anemones, such as Bartholomea annulata were available. Mahnken (1972) found nearly the opposite pattern of association in St. John, US Vir- gin Islands, with nearly all the P. pedersoni and a majority of P. yucatanicus found in association with B. annulata. Only a few studies have addressed the specific mechanisms of host selection and acclimation that might lead to host specificity (Table 2). Levine and Blanchard (1980) found that Periclimenes spp. obtain protection from their host anemones through a period of toxin acclimation where the shrimp picks up mucus from the anemone, thereby reducing its susceptibility to being stung by the host’s nematocysts. If a shrimp is separated from its host for > 3 d, it gradually loses this resistance (Crawford, 1992). Mihalik (1989) and Gwaltney and Brooks (1994) studied whether familiarity with a particular host species influenced host selection. They collected P. pedersoni and P. yucatanicus from three anemone hosts (B. annulata, C. gigantea, and Stichodactyla helianthus) at six locations in the Florida Keys and found that P. yucatanicus associated mostly with S. helianthus and selected S. helianthus over either C. gigantea or B. annulata in dichotomous choice tests. In the past 18 yrs since the studies by Mihalik and Gwaltney and Brooks, the nearshore communities of Florida Bay have experienced a cascade of environmental per- turbations. Massive seagrass die-offs in the central portion of Florida Bay have led to increased nutrient levels and corresponding blooms of cyanobacteria (Roblee et al., 1991; Fourqurean et al., 1993). These dense algal blooms resulted in widespread decline of macroalgae, sponges, cnidarians, and juvenile lobsters (Chiappone and Sul- livan, 1994; Butler et al., 1995; Herrnkind et al., 1997). silbiger and childress: anemone shrimp distribution and host selection in florida bay 331 Williams and Williams, 2000 Williams, and Williams Criales and Corredor, 1977 Criales and Corredor, Spotte et al., 1991 Mihalik, 1989; Gwaltney and Brooks, 1994 1995a Wicksten, Criales, 1984 Criales, 1984 Criales, 1984 Spotte, 1995 Limbaugh et al., 1961 Mahnken, 1972 Chase, 1972 1977 Criales and Corredor, Criales, 1984 Spotte et al., 1991 Spotte et al., 1991 Mihalik, 1989; Gwaltney and Brooks, 1994 1995a Wicksten, Criales, 1984 Holthuis and Eibl-Eibesfeldt, 1964; - Wagen and Chase, 1972; Sargent bach, 1975; Nizinski, 1989 Citation Holthuis and Eibl-Eibesfeldt, 1964; 1964; Eibl-Eibesfeldt, and Holthuis Chase, 1972 Lee Stocking Island, Bahamas St. Vincent St. and Caicos Turks Pine Cay, Florida Long Key, Antilles Bonaire, Netherlands Santa Marta, Colombia Santa Marta, Colombia Santa Marta, Colombia Bonaire, Netherlands Antilles Bonaire, Netherlands Bahamas Islands Virgin St. John, US Antigua Vincent St. Santa Marta, Colombia and Caicos Turks Pine Cay, Islands Virgin Thomas, US St. Florida Summerland Key, Antillies Bonaire, Netherlands Santa Marta, Colombia Bermuda Location Bermuda

(Lesueur, 1817) (Lesueur, (Weinland, 1860) (Weinland, (Weinland, 1860) (Weinland, Pallas, 1766 McMurrich, 1889 (Verrill, 1864) (Verrill, Discosoma ( Rhodactis ) sanctithomae (Duchassiang and Michelotti, 1860) Condylactis gigantea Cerianthus sp. Bigelow, 1892 Cassiopea xamachana Bigelow, Bartholomea annulata 1817) Bunodosoma granulifer (Leseur, Antipathes pennacea Aiptasia pallida Actinia bermudensis Condylactis gigantea Host species

Chace, 1958 Periclimenes anthophilus Periclimenes pedersoni Holthuis and Eibl-Eibesfeldt, 1964 Table 1. Summary of known hosts Caribbean anemone shrimps. Table Shrimp species 1 332 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008 Ritson-Williams and Paul, 2007 Ritson-Williams Manning, 1970 Mercado and Capriles, 1982 Criales, 1984 Spotte et al., 1991 Herrnkind et al., 1976 Mercado and Capriles, 1982 Criales, 1980 Criales, 1984 Spotte et al., 1991 Spotte et al., 1991 Criales, 1984 Williams and Williams, 2000 Williams, and Williams Mahnken, 1972 Spotte, 1995 Herrnkind et al., 1976 Criales, 1984 Spotte et al., 1991 Criales and Corredor, 1977 Criales and Corredor, Criales and Corredor, 1977 Criales and Corredor, Limbaugh et al., 1961 Mahnken, 1972 Citation Belize Dominica Puerto Rico Santa Marta, Colombia and Caicos Turks Pine Cay, Grand Bahama, Bahamas Puerto Rico Curacao, Netherlands Antilles Curacao, Netherlands Santa Marta, Colombia and Caicos Turks Caicos Bank, Antilles Bonaire, Netherlands Santa Marta, Colombia La Parguera, Puerto Rico La Parguera, Islands Virgin St. John, US Exuma Cays, Bahamas Grand Bahama, Bahamas Santa Marta, Colombia and Caicos Turks Providenciales, St. Vincent St. Santa Marta, Colombia Bahamas Islands Virgin St John, US Location (Elus, 1767) (Lesueur, 1817) (Lesueur, (Leseur, 1817) (Leseur, (Weinland, 1860) (Weinland,

(Verrill, 1864) (Verrill, Stichodactyla helianthus Ricordea florida Ricordea (Duchassaing and Michelotti, 1860) Lebrunia danae (Duchassaing and Michelotti, 1860) Homostichanthus duerdeni Carlgren, 1900 Eunicea tourneforti Milne-Edwards and Haime, 1857 Condylactis gigantea Bartholomea annulata 1817) Bunodosoma granulifer (Leseur, Ricordea florida Ricordea (Duchassaing and Michelotti, 1860) Lima scabra (Born, 1778) Lebrunia danae (Duchassaing and Michelotti, 1860) Bartholomea annulata Heteractis lucida Duchassaing and Michelotti, 1864 Aiptasia pallida Host species (Ives, 1891) Schmitt, 1924 Periclimenes yucatanicus Periclimenes rathbunae Table 1. Continued. Table Shrimp species 2 silbiger and childress: anemone shrimp distribution and host selection in florida bay 333 Limbaugh et al., 1961 1977 Criales and Corredor, Criales, 1984 Spotte et al., 1991 Mihalik, 1989; Gwaltney and Brooks, 1994 1995a Wicksten, Criales, 1984 Wicksten, 1995 Wicksten, Spotte et al., 1991 Mihalik, 1989; Gwaltney and Brooks, 1994 Mahnken, 1972 Herrnkind et al., 1976 Criales, 1984 Citation Chase, 1972 1977 Criales and Corredor, 1977 Criales and Corredor, Spotte et al., 1991 Mihalik, 1989; Gwaltney and Brooks, 1994 1995a Wicksten, Williams and Williams, 1982, 2000 Williams, and Williams and Paul, 2007 Ritson-Williams et al., 1983 Williams Spotte et al., 1991 Bahamas Vincent St. Santa Marta, Colombia and Caicos Turks Pine Cay, Florida Long Key, Antilles Bonaire, Netherlands Santa Marta, Colombia Bonaire, Netherlands Antilles Bonaire, Netherlands and Caicos Turks Caicos Bank, Florida Conch Key, St. John, US Virgin Islands Virgin St. John, US Grand Bahama, Bahamas Santa Marta, Colombia Location Antilles St. Kitts, Netherlands Santa Marta, Colombia Vincent St. and Caicos Turks Pine Cay, Florida Summerland Key, Antilles Bonaire, Netherlands Dominican Republic Martinique St. Croix, US Virgin Islands Virgin St. Croix, US Panama Toro, Bocas del (Ellis, 1767) (Weinland, 1860) (Weinland, by Spotte et al., 1991 and Williams and Williams, 1982 Williams, and Williams by Spotte et al., 1991 and Chace, 1958 by Wicksten, 1995b and Spotte, 1999. Wicksten, Chace, 1958 by Discosoma (Rhodactis) sanctithomae Condylactis gigantea Stichodactyla helianthus Bigelow, 1892 Cassiopea xamachana Bigelow, Ricordea florida Ricordea (Duchassaing and Michelotti, 1860) Cassiopea sp. Lebrunia danae (Duchassaing and Michelotti, 1860) Host species 3 (Duchassiang and Michelotti, 1860) Periclimenes pedersoni Actinotryx sanctithomae Considered a synonym of Considered a single species throughout the Caribbean by Spotte, 1997. Considered a synonym of Table 1. Continued. Table Shrimp species 1 2 3 334 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008 Nizinski, 1989 Levine and Blanchard, 1980 Levine and Blanchard, 1980 Crawford, 1992 Crawford, 1992 Mihalik, 1989 Mihalik, 1989 Mihalik, 1989 Mahnken, 1972 Mihalik, 1989 Citation Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 Mihalik, 1989 Levin and Blanchard, 1980 Spotte et al., 1991 Mihalik, 1989 Mihalik, 1989 Mihalik, 1989 Mihalik, 1989 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 2000 Williams, and Williams Mihalik, 1989 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 Gwaltney and Brooks, 1994 in lab B. annulata and C. gigantea and B. annulata and B. annulata and C. gigantea and C. gigantea and B. annulata and S. helianthus and S. helianthus in lab in lab in lab in lab in lab in lab in lab gained in hours (1–3) lost in days (3–5) in lab over C. gigantea over B. annulata over B. annulata S. helianthus S. helianthus C. gigantea B. annulata B. annulata C. gigantea B. annulata C. gigantea over C. gigantea over B. annulata over C. gigantea over B. annulata C. gigantea B. annulata S. helianthus Actinia bermudensis Discosoma sanctithomae in field C. gigantea S. helianthus S. helianthus C. gigantea C. gigantea C. gigantea C. gigantea B. annulata C. gigantea C. gigantea 100% of shrimp found on C. gigantea No preference between Greater than 90% of shrimp found on Acclimation to No acclimation to Acclimation to Acclimation to No preference between No preference between No preference between Preference for Type of evidence Type Reacclimation to Preference for Lose acclimation to Preferred S. helianthus Acclimation to Preferred S. helianthus No preference between No preference between No preference between Reacclimation to Preference for No acclimation to No preference between Reacclimation to Preferred S. helianthus Preferred S. helianthus No reacclimation to No reacclimation to by Wicksten (1995b) and Spotte (1999). Wicksten by Condylactis gigantea Host unknown Bartholomea annulata Host species Condylactis gigantea Stichodactyla helianthus Host unknown Bartholomea annulata Condylactis gigantea Stichodactyla helianthus Periclimenes pedersoni Periclimenes anthophilus Considered a synonym of Periclimenes pedersoni Table 2. Summary of evidence for host acclimation and preference in Caribbean anemone shrimps. Table Shrimp species 1 1 Periclimenes rathbunae Periclimenes yucatanicus silbiger and childress: anemone shrimp distribution and host selection in florida bay 335

To evaluate the impact of environmental change on the distribution of anemones and host specificity of anemone shrimps, we conducted a systematic field survey across eight locations in the nearshore hard-bottom communities of Florida Bay. We also conducted host selection tests in the laboratory to determine whether the distribution of shrimps among host anemones could be predicted on the basis of host selection preferences. Our study addressed three questions: (1) What is the distribution pattern of anemones and anemone shrimps in Florida Bay today? (2) Is the distribution pattern of host use by shrimps the result of differences in host selection preferences or local host availability? (3) Has the distribution pattern of host use by shrimps changed with changes in anemone abundance?

Methods

Field surveys were conducted at eight locations on the bayside of Lower Matecumbe Key, Long Key, Grassy Key, Crawl Key, and Fat Deer Key in Florida Bay, Florida, USA (Fig. 1A). At each location, two permanent 25 m × 25 m plots were established haphazardly within the band of nearshore limestone substratum at depths from 1.5–2.0 m. The corners of these plots were marked with a concrete corner block to which temporary buoys were attached and GPS coordinates recorded. The location of all sponges, corals, soft corals and sea anemones within each plot were mapped by divers, which was facilitated by a grid of weighted lines that subdivided the plot into 25, 5 m × 5 m subplots (Fig. 1B). Every sea anemone on the plot was marked with a colored cable tie attached to the adjacent substratum. The four anemone species on our plots were the elegant anemone—Epicystis crucifer (Le Sueur, 1817), the corkscrew anemone—B. annulata, the giant anemone—C. gigantea, and the sun anemone—S. helianthus. Only the last three species were host to anemone shrimps. We observed two species of anemone shrimps residing exclusively in sea anemones, Pederson’s anemone shrimp—P. pedersoni and the spotted anemone shrimp—P. yucatanicus. All other shrimp species, such as Periclimenes americanus (Kingsley, 1878), Thor floridanus Kingsley, 1878, Stenopus hispi- dus (Olivier, 1811), and Lysmata spp. that were observed near anemones but were not found exclusively with anemones, were not recorded. To determine the abundance and distribution of shrimps and sea anemones, we conducted a systematic visual census with a detailed plot map while on SCUBA. Each plot was censused monthly during the months of May, June, and July 2006. By July, the number of new anemones identified on each plot had reached an asymptote and was assumed to be the best estimate of true anemone abundance. Since shrimp abundance also varied slightly from month to month, we used the month with the maximum abundance as our estimate of shrimp abundance and distribution. This number was only slightly higher than the 3 mo mean abundance and did not significantly influence the outcome of our statistical tests. The relative frequency of anemone use by both shrimp species was compared to the relative frequency of all available anemones by log-linear G-test. Similarly, the relative frequency of anemone use ~18 yrs ago as reported by Mihalik (1989) was compared to relative frequency of anemone use in this study by log-linear G-test. We had to use the relative frequency because Mihalik (1989) did not report the size of the area surveyed. We also compared the local anemone abundance to local shrimp abundances by summing the abundance of the two plots at each of the eight locations and then examining their correlation using linear regression. To test the shrimps’ host selection, we conducted a laboratory dichotomous choice test in a Y-maze. Four anemones of each species (B. annulata and C. gigantea) were collected near, but not on, our mapped plots and brought back to a laboratory in Long Key, FL. We decided not to include S. helianthus in this host selection test, since this host species was absent from the majority of our field plots and was never occupied by P. pedersoni. Each sea anemone was placed in a 2 l perforated plastic basket. Two baskets were floated inside of a 35 l glass aquari- um containing filtered, aerated, ambient temperature sea water. Tanks were on outdoor racks 336 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008

Figure 1. (A) Map of the eight census locations in the bayside hardbottom habitat of the Florida Keys National Marine Sanctuary. Each location had two replicate 25 m × 25 m mapped plots with all anemones identified and marked. (B) Each month, the number and type of anemone shrimps present in each anemone was noted on the map. PY—Periclimenes yucatanicus; PP— Periclimenes pedersoni. underneath a thatched roof and thus had natural photoperiod but only indirect sunlight. The anemones would attach to the floating baskets and thus could be easily transferred to the Y- maze choice tank without disturbance. The perforations in the basket were large enough to allow access by the anemone shrimps and water to flow through the basket when placed in the Y-maze. We collected 19 P. yucatanicus (8 from C. gigantea and 11 from B. annulata) and 20 P. pedersoni (10 from B. annulata and 10 from C. gigantea) to be subjects in our host selection experiment. A single shrimp was placed in the floating perforated basket containing a host anemone of the same species from which it was collected. Thus, each housing tank contained two anemones and two anemone shrimps acclimated to that particular host. The shrimp were allowed a period of 24–72 hrs of acclimation before being used in the dichotomous host se- silbiger and childress: anemone shrimp distribution and host selection in florida bay 337 lection test. Once a shrimp was used in the selection test it was released and replaced with a recently-collected naïve shrimp. We used a Y-maze dichotomous choice test to assess host selection. The Y-maze tank was 40 cm across and 50 cm deep, with a 25 cm-long divider. Filtered, aerated, ambient temperature seawater entered the Y-maze through a head tank and flowed linearly down each arm of the maze to a central standpipe drain at a flow rate of 1–2 cm s–1. Before the start of each trial, two anemones, one B. annulata and one C. gigantea, were randomly assigned to either the right or left arm of the maze. By transferring the anemones in their plastic perforated baskets, the anemones experienced very little disturbance and quickly returned to a fully inflated condition. A randomly chosen focal shrimp (either P. yucatanicus or P. pedersoni) was introduced into a 10 cm ring of PVC pipe at the base of the Y-maze and allowed a 5 min acclimation period. Shrimps were always selected from a housing tank other than the test anemones’ tanks to ensure that the focal animal was unfamiliar with both anemones used in the trial. The focal shrimp was released from thePV C pipe ring and allowed to freely move about the Y-maze for 2 hrs. Each trial was videotaped so we could note the exact time of first contact, the behavioral response of the shrimp at first contact, and the final choice of host. The shrimp was considered to have chosen an anemone when it remained on its tentacles for more than 1 min. If the shrimp did not enter the anemone tentacles during the 2-hr period, it was noted as having made no choice, even if the shrimp was in close proximity to the anemone. Shrimps that did not choose an anemone within the 2-hr period were re-tested. If after three trials on consecutive days the shrimp still did not choose, it was noted as a no-choice shrimp. All of the laboratory trials were conducted between 1000–1800 with natural indirect light. We used a binomial probability test to assess preference of one host species over the other. We also tested for, but did not find, any side bias (binomial probability: P = 0.321).

Results

We found a total of 362 anemones at all sixteen sites: 282 B. annulata, 76 C. gigantea, and 4 S. helianthus (Table 3). Periclimenes yucatanicus was found on 41 anemones: 13 B. annulata, 25 C. gigantea, and 3 S. helianthus. Periclimenes pedersoni was found on 23 anemones: 11 B. annulata, 12 C. gigantea, and 0 S. helianthus.

Both P. yucatanicus (G-test: G2 = 35.8, P < 0.001) and P. pedersoni (G-test: G2 = 10.3, P = 0.006) did not randomly occupy anemones based on abundance of each host species. However, the proportion of all anemones occupied in the field did not differ significantly between the two shrimp species (G-test: 2G = 4.93, P = 0.085). A majority of anemones were occupied by only a single shrimp (80.0% of B. annulata, 71.4% of C. gigantea, 100% of S. heliathus), while the remainder were occupied by a pair of conspecific shrimps. The shrimps did not inhabit hosts solitarily by chance alone (Poisson: λ = 1.17, P = 0.019). A comparison between our shrimp host-use patterns to those from Mihalik (1989)

(Table 3) revealed significantly different patterns for both P. yucatanicus (G-test: G2

= 21.1, P < 0.001) and P. pedersoni (G-test: G2 = 4.66, P = 0.031). We attributed these dissimilarities to differences in the frequency of anemone hosts (G-test: 2G = 152.4, P < 0.001). The most dramatic difference was in the frequency of sun anemones, S. helianthus, dropping from 53.4% of all anemones in 1989 to 1.1% of all anemones on our sites in 2006. Unfortunately, a direct comparison of anemone density (number per 100 m2) was not possible because Mihalik (1989) did not report the area censused. The density of anemones and cleaner shrimps varied dramatically across our eight locations (Fig. 2). Our location with the lowest anemone density (ODR—2.72 338 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008

Table 3. Summary of numbers of anemone host and anemone shrimps as recorded in this study and Mihalik (1989) from sites near Long and Conch Keys. Percentage of host anemones given in parentheses.

Occupied by Occupied by Periclimenes Periclimenes Study Host species Unoccupied yucatanicus pedersoni This study Bartholomea annulata 258 (91.4%) 13 (4.6%) 11 (3.9%) Condylactis gigantea 39 (51.3%) 25 (32.9%) 12 (15.7%) Stichodactyla helianthus 1 (25.0%) 3 (75.0%) 0 (0.0%) Mihalik (1989) B. annulata 34 (100%) 0 (0.0%) 0 (0.0%) C. gigantea 7 (41.2%) 6 (54.5%) 4 (23.5%) S. helianthus 35 (79.5%) 9 (20.5%) 0 (0.0%) anemones 100 m–2) had no anemone shrimps. Our location with the second highest anemone density (MAT—12.64 anemones 100 m–2) had sixteen anemone shrimps. Periclimenes yucatanicus always occupied S. helianthus if they were found at that location and usually occupied C. gigantea over B. annulata if they were abundant (Fig. 2B). Periclimenes pedersoni would often occupy only one host type per location, either C. gigantea or B. annulata, despite the fact that both anemones were available at all locations (Fig. 2C). The abundance of P. yucatanicus was significantly related to the abundance of C. 2 gigantea (linear regression: r = 0.680, F1,6 = 15.92, P = 0.007) (Fig. 3A), but not B. annulata (linear regression: r² = 0.069, F1,6 = 0.546, P = 0.488) nor with the total abundance of all three anemones (linear regression: r² = 0.186, F1,6 = 2.60, P = 0.158). The abundance of P. pedersoni was not significantly related to the abundance of C. gigantea (linear regression: r² = 0.096, F1,6 = 1.75, P = 0.234) (Fig. 3B), but was significantly related to the abundance of B. annulata (linear regression: r² = 0.805, F1,6 = 29.87, P = 0.002) and the total abundance of all three anemones (linear regression: r²

= 0.775, F1,6 = 25.11, P = 0.002). In the laboratory host selection experiment (Table 4), P. yucatanicus collected from C. gigantea chose C. gigantea 100% of the time (binomial probability: P = 0.018) and P. yucatanicus collected from B. annulata chose C. gigantea 72.7% of the time (binomial probability: P = 0.081). This suggests that all P. yucatanicus individuals had a strong preference for C. gigantea regardless of their current acclimation status. Periclimenes pedersoni collected from C. gigantea chose C. gigantea 80% of the time (binomial probability: P = 0.044) but P. pedersoni collected from B. annulata chose B. annulata 80% of the time (binomial probability: P = 0.044). This suggests that P. pedersoni individuals had a strong preference for the host anemone species they were acclimated to.

Discussion

In Florida Bay, Florida, the distribution of anemone shrimps among available hosts is significantly non-random, with some anemone species being occupied at higher frequencies than expected by chance. This pattern appears to be driven by two fac- tors: the availability of anemone hosts at a local spatial scale and the host selection preference of the individual shrimp. For P. yucatanicus, local host availability may be an important factor that limits their abundance. This is driven by a strong species- specific host selection preference for the giant anemone,C. gigantea. As a result, the silbiger and childress: anemone shrimp distribution and host selection in florida bay 339

Figure 2. Number of anemones 100 m–2 by location for (A) the total number of anemones, (B) anemones occupied by Periclimenes yucatanicus, and (C) anemones occupied by Periclimenes pedersoni. Black is sun anemone, Stichodactyla helianthus, white is the corkscrew anemone, Bartholomea annulata, and gray is the number of the giant anemone, Condylactis gigantea. See Figure 1 for abbreviations of Florida Keys locations. abundance of P. yucatanicus is best predicted by the local abundance of C. gigantea. The occasions where aP. yucatanicus was found in B. annulata were usually restrict- ed to locations with very low densities of C. gigantea. In contrast, P. pedersoni appears to preferentially colonize the numerically-dominant corkscrew anemone, B. annulata. However, the results of the dichotomous choice test suggest that P. pedersoni does not always have a significant preference for B. annulata over C. gigantea. Instead the choice tests suggest that once P. pedersoni 340 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008

Figure 3. Abundance of anemone shrimps in the Florida Keys as predicted by the number of anemones (Condylactis gigantea—gray diamonds with dashed line; Bartholomea annulata— white squares with dotted line; all three anemone species combined—black circles with solid line) for (A) Periclimenes yucatanicus and (B) Periclimenes pedersoni. Significant regressions (P < 0.002) indicated with lines. has selected a host, it retains a preference for that particular species. This explana- tion is also consistent with the observation that the abundance of P. pedersoni is also predicted by the local abundance of all anemones regardless of species. Once settled, an individual shrimp acclimates to a particular host anemone species and this deter- mines its host selection preference if displaced. These conclusions help us understand the wide variation in previous studies that have examined host use in these species. In both Mahnken (1972) and Nizinski (1989), the Pederson’s anemone shrimp, P. pedersoni, occupied the most abundant host anemone (B. annulata in St. John and C. gigantea in Bermuda). This could easily be the result of acclimation to the first host species encountered. For locations where one anemone host is not numerically dominant, we might expect to find the shrimp silbiger and childress: anemone shrimp distribution and host selection in florida bay 341

Table 4. Results from Y-maze choice test.

Choice during dichotomous choice test Collected from anemone Shrimp species species C. gigantea B. annulata neither Periclimenes yucatanicus Condylactis gigantea 8 0 1 Bartholomea annulata 8 3 0 Periclimenes pedersoni C. gigantea 8 2 0 B. annulata 2 8 0 occupying many different hosts. For example, Criales (1984) found P. pedersoni in Santa Marta, Colombia to occupy six different host species. One important caveat to this hypothesis is that P. pedersoni has never been observed to occupy the sun anemone, S. helianthus, even where they were abundant (Mihalik, 1989). A comparison of our results with those of previous studies near Long and Conch Keys (Mihalik, 1989; Gwaltney and Brooks, 1994) reveals some interesting patterns. First, the relative abundance of sun anemones, S. helianthus, appears to have declined dramatically from 52.3% in 1989 to only 1.1% of all anemones on our sites in 2006. Second, a decline in the preferred host did not influence P. yucatanicus occupation rates in the alternative host species C. gigantea (35.3% in 1989 and 32.9% in 2006) or B. annulata (0.0% in 1989 and 4.6% in 2006). Finally, the relative abundance of P. yucatanicus (PY) over P. pedersoni (PP) has declined with the loss of the preferred host anemone (PY = 0.17 shrimp anemone–1 and PP = 0.05 shrimp anemone–1 in 1989; and PY = 0.06 shrimp anemone–1 and PP = 0.04 shrimp anemone–1 in 2006). The cause of this apparent decline inS. helianthus abundance is unknown. During this time period many species (i.e., macroalgae, octocorals, sponges) in the nearshore hardbottom community declined from low light levels and dense cyanobacteria blooms (Chiappone and Sullivan, 1994; Butler et al., 1995). Sediment movement and re-suspension during Hurricane Wilma in October 2005 may have been a more recent cause; however, this would not necessarily explain why only S. helianthus declined and not the other anemone species. The cause of decline in P. yucatanicus abundance may have been due to either changes in water quality or overharvesting. However, a more likely scenario is that the strong species-specific preference for S. helianthus over either of the other two anemone species led to fewer individuals settling or dispersing to a suitable anemone host (Gwaltney and Brooks, 1994). Periclimenes yucatanicus appears to be surviving by utilizing their second choice anemone host, C. gigantea, which is still preferred over the more abundant B. annulata (Mihalik, 1989; this study). Surprisingly, the proportional use of the three anemone hosts remained essentially unchanged for P. yucatanicus despite the dramatic decrease in the preferred host species. This could occur by differential settlement of postlarvae or by differential -sur vival post-settlement due to predation or anemone toxicity (Crawford, 1992). Both of these hypotheses might be influenced by the size of the host anemone which tends to decrease in mean size from the preferred S. helianthus to the least preferred B. annulata. Larger anemones may be easier to locate and/or may provide increased protection from predators. Host anemone size, color and abundance strongly influences the host use and dispersal of some crab symbionts (Baeza and Stolz, 2001, 2003; Thiel et al., 2003). dos Santos et al. (2004) found that when introducing Periclimenes sagittifer (Norman, 1861) shrimp larvae to their symbiotic host anemone that 90% died after 342 BULLETIN OF MARINE SCIENCE, VOL. 83, NO. 2, 2008 introduction, suggesting that only juveniles colonize a host anemone. Unfortunately, almost nothing is known about the pattern of larval settlement for P. yucatanicus and P. pedersoni, but it has been noted that some tropical anemone shrimps have long range dispersal (Goy, 1990). If P. yucatanicus shows high host specificity, why is it found in association with so many different hosts throughout the Caribbean (Table 1)? It is impossible to know from the literature whether these associations are common or uncommon relative to the population as a whole. For example, Williams and Williams (2000) attempted to introduce P. yucatanicus to the warty false coral Discosoma sanctithomae (Du- chassing and Michelotti, 1860) 20 times but established no long-term associations. In their review of the literature, they suggest that P. yucatanicus is common to only two hosts, C. gigantea and B. annulata, and these associations must be formed at the time of settlement. Future studies should directly measure the fitness costs and benefits of each shrimp-anemone combination. Fautin et al. (1995) demonstrated an increase in growth, survival, and reproduction for Periclimenes brevicarpalis (Schenkel, 1902) when associated with Entacmaea quadricolor (Rüppel and Leuckart, 1828). This anemone host species has some facultative (P. brevicarpalis) and some obligate (P. ornatus Bruce, 1969) shrimp symbionts that show host preference based on familiarity (Guo et al., 1996). What is unknown is whether the differences in benefits to these two species are sufficient to explain the shift from facultative to obligate association.

Acknowledgments

We thank T. Mayer, S. Rider, and A. Zito for assistance in the field and J. Fajans for assistance with shrimp identification. We thank W. Herrnkind and F. Coleman for their advice and encouragement throughout the project. This manuscript has benefited greatly from the helpful suggestions of P. Bouwma, F. Coleman, T. Jordan, K. Parmenter, M. Ptacek, A. Rhyne, J. Seda, S. Sponaugle, M. Thiel, A. Zito, and two anonymous reviewers. We thank the Florida Fish and Wildlife Conservation Commission, Florida Keys National Marine Sanctuary, Flor- ida State University Certificate Program in Marine Biology, FSU Coastal and Marine Labo- ratory’s Academic Diving Program, PADI Foundation, and the Clemson University Faculty Research Grant program for logistical and financial support.

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Addresses: (N.J.S.) Department of Marine Science, University of North Carolina, 340 Chap- man Hall CB 3300, Chapel Hill, North Carolina 27599-3300. (M.J.C.) Department of Biologi- cal Sciences, Clemson University, Clemson, South Carolina 29634. CorresPonding Author: (N.J.S.) Telephone: (252) 726-6841 *219, E-mail: .