Differences in refuge use are related to predation risk in estuarine crabs.
Crabs (Environmental aspects)
Hulathduwa, Yasoma D.
Stickle, William B.
Brown, Kenneth M.
|Publication:||Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2011 National Shellfisheries Association, Inc. ISSN: 0730-8000|
|Issue:||Date: Dec, 2011 Source Volume: 30 Source Issue: 3|
|Topic:||Event Code: 310 Science & research|
|Product:||Product Code: 0913040 Crabs NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
ABSTRACT We determined the dominance hierarchy in competition for
refuges by the xanthid crabs Panopeus simpsoni, Eurypanopeus depressus,
Rhithropanopeus harrisii, and juvenile Callinectes sapidus, and used it
to predict predation risk from adult blue crabs in laboratory
experiments at two salinities. Experiments were first run with pairwise
combinations of species with limited refuges. To determine any effects
of diffuse competition on dominance and predation risk, all four species
were then held together with limited refuges. The same process was then
repeated in experiments with a blue crab predator, again with limited
refuges. Juvenile C. sapidus and E. depressus were dominant over P.
simpsoni and R. harrisii in occupying shelters at both salinities, in
both paired- and multiple-species combinations. Dominance in refuge use
increased with salinity in C. sapidus, E. depressus, and P. simpsoni,
but not in R. harrisii. Because they were more dominant, C. sapidus and
E. depressus sustained lower mortality to predation than P. simpsoni,
and R. harrisii. Furthermore, field sampling indicated the least
dominant species. R. harrisii, was common only in low-salinity areas
with few predators. The greater dominance of C. sapidus and E. depressus
may thus decrease their predation risk in estuarine waters and explain
their broader distribution across salinities.
KEY WORDS: refuges, predation, mud crabs
Xanthid or mud crabs are important predators of oyster spat on oyster reefs (Grabowski 2004, Grabowski et al. 2008) and vertically partitioned oyster reefs (Meyer 1994). Brown et al. (2005) argued that dominance hierarchies among mud crab species on oyster reefs could determine their ability to acquire food or shelter resources. Resource quality is an important factor affecting aggressive behavior in many animals (Hack et al. 1997, Yamaguchi & Kawano 2001, Lindstrom & Pampoulie 2005), and in initiating fights (reviewed by Huntingford and Turner (1987) and Gherardi (2006)). In marine systems, shelter is an important resource determining the distribution and abundance of many crustaceans (Dingle 1983, Beck 1995). Survival rates of hermit crabs (Bertness 1981), lobsters (Wahle & Steneck 1991, Eggleston & Lipcius 1992), stone crabs (Beck 1995, Shervette et al. 2004), and blue crabs (Heck & Coen 1995) from predation are known to increase with the availability of refuges.
Although exceptions occur (Elwood et al. 1998, Hernandez & Benson 1998), larger size is usually considered a good predictor of the outcome of contests (Lindstr6m 1992, Bridge et al. 2000, Renison et al. 2002), suggesting that body size is a decisive factor in securing resources (Austad 1983). However, for crabs, relative size of chelae may be a more salient factor (Sneddon et al. 1997).
In this article, we investigate dominance hierarchies in occupation of refuges and their role in determining the mortality rates of three xanthid (mud) crabs and juvenile blue crabs (family Portunidae), all common in estuaries along the northern Gulf of Mexico. First we determine the dominance hierarchy for occupying limited refuges using pairwise interactions, and then we repeat the same experiment with all four species interacting together. This approach was used both because we wanted to determine whether dominance hierarchies were altered with increasing competitor diversity (e.g., by "diffuse" or multispecies competition (MacArthur 1972, Davidson 1985)), and because trophic interactions in oyster reefs are often altered by the complexity of the system (Grabowski et al. 2008). Based on the resulting dominance hierarchy for occupation of refuges, we next determined predation risk to adult blue crabs in laboratory experiments. Our working hypothesis was that superior competitors for refuges should have reduced predation risk.
Connell's (1961a,1961b) classic studies with Semibalanus balanoides and Chthamalus stellatus demonstrated that inferior competitors might coexist because they were adapted to more stressful environments, and we therefore wanted to look at the effect of stressful environments on competition for refuges. For many estuarine organisms, brackish waters can be very stressful, if not lethal (Remane & Schlieper 1971), limiting colonization of lower salinity portions of estuaries. Near the mouth of the estuary, where salinities are higher, competition for shelter could be more important in limiting the abundance of species, for the reasons mentioned here. We were interested in whether our laboratory-determined dominance hierarchies for refuges, paired with knowledge of salinity tolerance, would explain the distribution of mud crabs in the field. We therefore sampled mud crabs at three sites along a salinity gradient in Barataria Bay, LA.
The mud crabs Panopeus simpsoni and Eurypanopeus depressus are common inhabitants of oyster reefs along the southeastern Atlantic and Gulf coastlines of the United States (McDonald 1982). Adult P. simpsoni reach a size of 5 cm, and E. depressus are about 2 cm in carapace width (McDonald 1982). E. depressus extends much farther into brackish water in estuarine bays along the northern Gulf of Mexico coast (May 1974, Hulathduwa & Brown 2006). Our earlier research (Brown et al. 2005, Hulathduwa et al. 2007) suggested that E. depressus was dominant to P. simpsoni, especially at low salinity, and as a consequence, E. depressus had lower predation risk to adult blue crabs in laboratory experiments (Hulathduwa et al. 2007).
Rhithropanopeus harrisii (Gould, 1841) or the Harris mud crab, is a small euryhaline crab that reaches approximately 2 cm in carapace width as an adult (Williams 1984). Sexual maturity occurs at a carapace width of 4.5 mm for males and 4.4-5.5 mm for females (Turoboyski 1973). R. harrisii is even more euryhaline than E. depressus, and has been reported in estuaries and quasi-freshwater lakes with salinities as low as 0.5 PSU (Boyle et al. 2010). This crab is associated with oyster reefs, living and or decaying vegetation, and woody debris to a depth of approximately 37 m (Turoboyski 1973, Williams 1984, Petersen 2006). Although the general biology of R. harrisii has been studied, (Costlow et al. 1966, Turoboyski 1973, Williams 1984), its relative dominance hierarchy to co-occurring xanthid and juvenile blue crabs is unknown.
The euryhaline blue crab (Callinectes sapidus Rathbun, 1896) is common on the Atlantic and Gulf of Mexico coasts of the United States. Juvenile blue crabs (carapace width, <40 mm) are common in seagrass beds and salt marshes (Orth & van Montfrans 1987, Thomas et al. 1990). and salt marshes are important refuges for juveniles (Zimmerman & Minello 1984, Wolcott & Hines 1989, Peterson & Turner 1994, Fantle et al. 1999). Oyster reefs also provide refuge for commensal organisms (Guitierrez et al. 2003, Soniat et al. 2004), including juvenile blue crabs (Galstoff 1964, Coen et al. 1999). Although juvenile blue crabs thus spatially overlap with mud crabs in estuarine vegetation and oyster reefs, the juvenile blue crab's position in the dominance hierarchy of co-occurring salt marsh xanthid crabs is also relatively unknown. However, juvenile blue crabs do compete with mud crabs for oyster spat prey (Eggleston 1990a, Eggleston 1990b). We chose adult blue crabs as predators in our laboratory experiments because they are an important mortality source for both juvenile blue crabs and xanthid crabs in these estuaries (Hines et al. 1990, Heck & Coen 1995).
Collection and Maintenance
Juvenile blue crabs were collected with long-handled nets in channels near the Louisiana Universities Marine Consortium laboratory at Port Fourchon, LA, about 10 km inland from the Gulf of Mexico. Salinities fluctuate at this location annually from 10-30 PSU (Guerin & Stickle 1992). P. simpsoni and E. depressus were collected from Vexar mesh bags deployed at the Louisiana Sea Grant oyster hatchery or the Louisiana Wildlife and Fisheries Marine Laboratory on Grand Isle, LA. Mesh bags (0.67 x 0.33 m: mesh size, 1.6 cm) were filled with oyster shell and set out in the sites for 1-2wk (Stuck & Perry 1992). Bags were retrieved carefully and placed in tubs to avoid loss of smaller crabs. The oyster shell was carefully rinsed with seawater, and crabs were retrieved with a small aquarium net and placed individually in compartments of plastic tackle boxes with holes drilled in them to allow water flow during transport to LSU in ice chests. Ambient salinity and seawater temperature varied between 20.5 PSU and 30.5 PSU, and 19.8[degrees]C and 30.2[degrees]C, respectively, on the collection dates.
In the laboratory, crabs were identified using the methods of Hopkins et al. (1989), and carapace widths (including anterior--lateral spines) were measured to the nearest 0.1 mm with dial calipers. A l-way analysis of variance was performed to determine whether the species differed in carapace width. Tukey's a posteriori tests were used to identify which means differed. Crabs were held in 38-L aquaria equipped with under-gravel filters. Aquaria were maintained at 25 PSU using artificial sea salts (Instant Ocean; Aquarium Systems Inc., Apopka, FL). Crabs were isolated in chambers (10 x 1 x 6 cm) in larger (50 x 40 x 6 cm) plastic boxes to prevent cannibalism.
To determine whether the species differed in ability to use refuges, and whether refuge use was dependent on salinity, we performed a set of laboratory experiments in fall 2009. Eight 19-L aquaria with under-gravel filters were maintained at 20 PSU and 7.5 PSU (4 at each salinity). A salinity of 20 PSU is near average for mouths of Louisiana estuaries, whereas 7.5 PSU is near the lower salinity tolerance limit of P. simpsoni (Hulathduwa et al. 2007). Before the experiments, crabs were acclimated for 1 wk and isolated to prevent interactions. P. simpsoni was marked with white paint on the carapace for ease of identification (to facilitate identifying crabs in refuges, because it is similar in carapace width to E. depressus). Preliminary experiments indicated that marking did not affect the behavior of crabs (Brown et al. 2005). Five PVC pipes (5 cm long x 2.5 cm in diameter) with one end covered with 1-mm Vexar mesh to retain food, were placed in each aquarium as refuges. Small pieces of shrimp were initially placed in the refuges. Each experiment was run with 2 species (with all species combinations) with 5 crabs per species (e.g., 10 crabs total to make refuges a limiting resource). Refuges were checked 3 times daily for 3 days, and the species occupying each shelter noted.
Immediately after these experiments, to determine whether refuge use was altered when additional species were added, experiments were run with 10 refuges per aquarium and 20 crabs (5 from each of the 4 species). Again, refuges were checked 3 times daily for 3 days, and the species occupying each shelter was recorded. To determine whether there was a difference between species in shelter occupancy, 2-way ANOVAs were performed with species and salinity as the main effects. Tukey's a posteriori tests were used to identify which means differed (SAS Institute Inc. 1988).
Shelter Use and Predation Risk
To determine whether differences in refuge use had an influence on predation risk, a third set of experiments was performed immediately after the first 2 sets of experiments. Initially, 2 crab species were held with an adult blue crab predator, again with a limited number of refuges (5 shelters and 5 crabs per species). Adult blue crabs were held individually prior to experiments in 38-L aquaria with under-gravel filters, and acclimated for 1 wk. Five 38-L aquaria were equipped with under-gravel filters at 20 PSU and 5 at 7.5 PSU. A single adult male blue crab (carapace width, 13-15 cm) was introduced 3 h after the prey crabs to allow the smaller mud crabs and juvenile blue crabs to find and occupy shelters. The number of crabs surviving for each species in each tank was recorded after 4, 10, 24, and 48 h from the time of blue crab introduction. This procedure was followed again for all the possible paired species combinations. Differences in survival were tested for by a repeated-measure analysis of variance (2 species x 2 salinities, with the 4 observations of each tank as the repeated measure). Percent survival was arcsin-square root transformed to solve normality and variance homogeneity problems (Sokal & Rohlf 1995). Immediately after this experiment, additional, similar experiments, but with 10 refuges per aquarium and 20 crabs (5 from each species) were conducted to look for any diffuse competition effects on survival (see discussion in Introduction). In both cases, a single adult male blue crab (carapace width, 1315 cm) was again introduced 3 h after the prey crabs, and the number of crabs surviving for each species in each tank was recorded after 4, 10, 24, and 48 h from the time of blue crab introduction. A repeated-measure analysis of variance was again performed (4 species x 2 salinities, with the four observations of each tank as the repeated measure), and percent survival values were again arcsin--square root transformed to solve normality and variance homogeneity problems.
Field Crab Distributions
To determine xanthid crab distributions in the field, we collected mud crabs at three sites in the Barataria Bay estuarine complex along the Louisiana coast, and compared mud crab diversity among sites with varying salinity. Our low-salinity site was in Galliano, LA (29[degrees]27'25.5" N, 90[degrees]21'44.8" W). The average salinity at this site, from YSI measurements, was 6.7 [+ or -] 0.9 PSU (n = 6). The second site was along the shoreline of Bayou LaFourche in Leeville, LA (29[degrees] 16.1' 53" N, 90[degrees] 12.7' 80" W). Mean salinity at this "medium" site was 14.8 + 1.0 PSU (n = 6). The high-salinity site was near Port Fourchon, LA (29[degrees]06'38.9" N, 90[degrees]11'10.8" W), with a mean salinity of 23.5 [+ or -] 1.4 PSU (11 = 6). The mud crabs were recruited to bags made of 2.5-cm Vexar mesh (66 x 30.5 x 7.5 cm) filled with oyster shell and left out for 2 wk for colonization at each site (Stuck & Perry 1992). Sampling for xanthids was repeated 3 times in summer 2009.
Size (Carapace Width) of the Crabs
Carapace width differed between species (F = 2,142.2, P < 0.0001). Based on Tukey and Scheffe a posteriori tests, C. sapidus was significantly larger than all the other three species (Fig. 1). R. harrisii was significantly smaller than all the other three species.
Shelter Use: Pairwise Interactions
When juvenile C. sapidus were paired with E. depressus (Fig. 2), a significant species difference occurred (Table 1), and shelter occupancy increased with salinity. However, a significant species x salinity effect suggested refuge occupancy was fairly similar for both species at 7.5 PSU, but that C. sapidus was clearly dominant at the higher salinity (Fig. 2). In contrast, juvenile C. sapidus appeared clearly dominant to P. simpsoni at both salinities, although the salinity and interaction effects were still significant. For C. sapidus and R. harrisii, differences in dominance were even more clear (Table 1), with juvenile blue crabs 3 times more dominant in refuge use at the lower salinity and 5 times more dominant at the higher salinity (Fig. 2), resulting in both a significant salinity and salinity x species interaction.
E. depressus had a significantly higher refuge occupancy rate than P. simpsoni (Table 1), and refuge occupancy increased with salinity for both species, with an insignificant interaction between species and salinity. E. depressus was also clearly dominant over R. harrisii, with refuge occupancy rates 2 times greater at the lower salinity and 5 times greater at the higher salinity (Fig. 2), resulting in a significant species x salinity interaction.
[FIGURE 1 OMITTED]
Finally, for P. simpsoni and R. harrisii (Table 1 ), a significant interaction effect suggested that refuge occupancy rates were significantly higher in P. simpsoni only at 20 PSU (Fig. 2).
Shelter use: Multiple-Species Experiments
When all four species were held together (Fig. 3), there was a significant species effect (Table 1), with C. sapidus and E. depressus having similar refuge occupancy rates, both being roughly 3.5 times the refuge occupancy rates of P. simpsoni and R. harrisii, which were fairly similar (Fig. 3).
[FIGURE 2 OMITTED]
Predation Risk Experiments with Pairwise Interactions
In the repeated-measures analysis of variance (Table 2), time always had a highly significant effect, indicating clear declines in survival resulting from predation by the adult blue crabs. However, there were no significant 2-way or 3-way interactions, suggesting mortality patterns over time were similar in each species across salinities. Comparing survival between paired C. sapidus and E. depressus (Table 2), there was no difference between the 2 species. However, juvenile C. sapidus did survive better than both P. simpsoni and R. harrisii. E. depressus also survived better than P. simpsoni and R. harrisii (Table 1), but the latter 2 species had similar survival rates when paired with each other.
Predation Risk: Multiple Species
Base on the paired comparison results suggesting similar dominance levels, survival rates might be expected to be similar for juvenile C. sapidus and E. depressus, and both would be greater than survival rates in the 2 less-dominant species, again assuming no higher order interactions, and that is exactly what happened (Fig. 4). When all 4 species were held with an adult blue crab, there was a significant species x time interaction, indicating the different survival trends among species (Table 2).
[FIGURE 3 OMITTED]
Although juvenile C. sapidus are quite common in estuarine environments (see discussion in the Introduction), they did not recruit well to the shell bags used in this study, and these data may not be representative of the abundance of C. sapidus on the reef. P. simpsoni was found at intermediate- and high-salinity sites, whereas E. depressus was found at all sites, and R. harrisii was found at all sites, but reached extremely high abundances at the lowest salinity site (Fig. 5). Its higher abundance at the lowest salinity site probably explains both the significant species (F = 34.9), salinity (F = 31.4) and species x salinity (F = 35.8) effects.
In this study, C. sapidus and E. depressus were clearly dominant over P. simpsoni and R. harrisii in occupying shelters at both salinities, based on either paired comparisons or when all species were held together. There are evidently no effects of "diffuse" competition between the crabs when considered as an assemblage. The relative dominance in occupying refuges of the four species was also a good predictor of vulnerability to predation by adult blue crabs. Dominant species were thus able to sequester refuges and lower their mortality rates to adult blue crabs. Because larger body size is usually a good predictor of dominance hierarchy, C. sapidus, the largest species in this study, was successful in securing refuges and avoiding predation. R. harrisii, the smallest species used in this study, was the least successful in securing refuges. Although P. simpsoni can grow to a carapace width of 5 cm, Brown et al. (2005) reported that for both P. simpsoni and E. depressus, agnostic acts peaked at carapace widths ranging from 15-20 ram, and suggested that interactions for resources may be most intense in these life history stages in the field.
The availability of suitable refuges is known to affect the density and size structure of other marine populations (Caddy & Stamatopoulas 1990, Wahle & Steneck 1991, Beck 1997, Turra & Leite 2003), and competition for shelters is common in co-occurring crustaceans. However, microhabitat use (Gherardi & Nardone 1997, Turra & Denadai 2002), body size differences (Turra & Leite 2002), activity rhythms (Barnes 2002), resource partitioning (Gherardi & Nardone 1997, Turra & Leite 2002), and tolerance for desiccation (Bertness 1981, Turra & Denadai 2001) have also been implicated in allowing co-occurrence of crab species. McDonald (1982) suggested that a combination of trophic and microhabitat factors could explain the co-occurrence of Panopeus herbstii and E. depressus in South Carolina. Although both P. simpsoni and E. depressus could survive a salinity of 6 PSU for at least a week in the laboratory (Hulathduwa et al. 2007), P. simpsoni was not found in the low-salinity site during our field sampling. Other researchers (May 1974, Shumway 1983) also report the occurrence of P. herbstii only in high-salinity areas. Hulathduwa et al. (2007) also found that E. depressus is more tolerant of low salinities than P. simpsoni, explaining the fact that E. depressus is common in brackish waters. Brown et al. (2005) suggested that E. depressus was more aggressive than P. simpsoni, and Hulathduwa et al. (2007), also suggested that E. depressus was dominant and had a higher refuge occupancy rate than P. simpsoni, especially at low salinities.
The refuge occupancy rate of C. sapidus, E. depressus, and P. simpsoni increased at higher salinity, unlike that seen in R. harrisii. Hulathduwa et al. (2007) found that both P. simpsoni and E. depressus had lowered scope for growth and were less active at lower salinities. Guerin and Stickle (1992) reported maximal energy absorption and the highest scope for growth for a Louisiana population of juvenile C. sapidus to be between 10 PSU and 25 PSU, implying that the juveniles of this population grow fastest at these salinities. Thus, although C. sapidus, E. depressus, and P. simpsoni can survive low salinities, these species are more active in higher salinities.
[FIGURE 4 OMITTED]
R. harrisii was least successful in occupying shelters in this study. It was not found in the high-salinity site and was significantly more abundant only at the low-salinity site. Other researchers have also reported R. harrisii in low salinities (Turoboyski 1973, Williams 1984), and Resisser and Forward (1991) suggested that R. harrisii had a reproductive refuge at less than 10 PSU because parasitic Loxothylacus panopaei could not infect the mud crab. However, its smaller body size could also allow R. harrisii to occupy smaller refuges, facilitating niche partitioning with other crab species.
Several studies have suggested refuges reduce predation risk (Holt 1987, Hixon & Menge 1991, Shervette et al. 2004). Crab predation risk is often a function of refuge availability (Heck & Wilson 1987, Orth & van Montfrans 1990, Williams et al. 1990, Beck 1997, Angel 2000). Shervette et al. (2004) found that refuge availability affected both population size structure and density in another xanthid, the juvenile stone crab (Menippe adina), and that predation by the oyster toadfish Opsanus beta led to competition for refuges between M. adina, P. simpsoni and E. depressus. Although at first glance a 10 20% difference in refuge occupancy or predation risk among species may not seem dramatic, over time such differences in survival could certainly affect field distributions.
[FIGURE 5 OMITTED]
Blue crabs are an important factor structuring marine communities in general (Hines et al. 1990, Mascaro et al. 2003). Adult C. sapidus feed both in a tactile mode on sedentary prey, and visually on mobile prey (Hughes & Seed 1995). Crustaceans, bivalve molluscs, and fish are all part of the blue crab's diet (Hsueh et al. 1992). In fact, cannibalism can comprise a major source of predation risk for juvenile blue crabs (Elgar & Crespi 1992, Ruiz et al. 1993, Heck & Coen 1995, Dittel et al. 1995, Hines & Ruiz 1995, Smith 1995, Moksnes et al. 19971. Predation rates in general are reported to be higher in the Gulf of Mexico than on the Atlantic coast (Heck & Wilson 1987), and Heck and Coen (1995) also suggest that there is greater predation on the juvenile blue crabs along the Gulf coast compared with the east coast.
In summary, our results suggest the survival of mud crabs in the presence of the adult blue crab predator corresponded with the relative ability to secure and defend refuges. Subdominant species (P. simpsoni and R. harrisii) were eaten first, and survival of the two dominant species (C. sapidus and E. depressus) was significantly higher. Our field samples did not find high abundances of juvenile blue crabs on oyster reefs, but that may be a bias of our sampling technique, because we certainly observed many juvenile blue crabs crawling on the outside of the bags during sampling. The higher dominance in refuge use of juvenile C. sapidus indicated in this study may thus increase survival during early life history stages when vulnerability to cannibalism and predation is intense. Similarly, the relatively better ability or E. depressus to defend shelters may play a role in determining its broad distribution in estuarine waters.
We thank Mike Whiteside and Sshady Gameledein for recording data during the preliminary study. Earl Wiedner helped in collecting crabs. Louisiana Universities Marine Consortium Port Fourchon Laboratory is acknowledged for providing logistic support.
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YASOMA D. HULATHDUWA, (1)* WILLIAM B. STICKLE, (2) BARRY ARONHIME (2) AND KENNETH M. BROWN (2)
(1) Department of Biology, University of Tampa, 401 W. Kennedy Boulevard, Tampa, FL 33606,"
(2) Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803
* Corresponding author. E-mail: yhulathduwa(a ut.edu
TABLE 1. Significance levels in a 2-way analysis of variance for each pair of mud crab species and for all 4 species combined. C. sapidus x C. sapidus X C. sapidus x Contrast E. depressus P. simpsoni R. harrisii Species ** ** ** Salinity ** ** * Interaction ** ** ** E. depressus X E. depressus X P. simpsoni X Contrast P. simpsoni R. harrisii R. harrisii Species ** ** ** Salinity ** ** ** Interaction NS ** ** All Four Contrast Species Species ** Salinity ** Interaction ** * P < 0.05. ** P < 0.001. NS, not significant. TABLE 2. Significance levels for repeated-measures analysis of variance of percent survival of pairs of mud crabs, and all 4 species together exposed to blue crab predation, measured over 5 time intervals at 2 salinity levels. MANOVA C. sapidus X C. sapidus X Contrasts E. depressus P. simpsoni Time ** ** Time x species NS NS Time x salinity NS NS Time x species X salinity NS NS Between subjects Species NS ** Salinity NS NS Species X salinity NS NS MANOVA C. sapidus X E. depressus X Contrasts R. harrisii P. simpsoni Time ** ** Time x species NS NS Time x salinity NS NS Time x species X salinity NS NS Between subjects Species ** * Salinity NS NS Species X salinity NS NS MANOVA E. depressus X P. simpsoni X All Four Contrasts R. harrisii R. harrisii Species Time ** ** ** Time x species NS NS Time x salinity NS NS NS Time x species X salinity NS NS NS Between subjects Species * NS ** Salinity NS NS NS Species X salinity NS NS NS * P < 0.05. ** P < 0.001. NS, not significant.
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