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Fig. 1. Photograph of Palaemonetes pugio with Probopyrus parasite on its right side.
and activity of the grass shrimp. We hypothesized that since the parasite forms a large obvious deformation on the side of the host Fig. 1, it might reduce the host’s activity
level, coordination, and ability to capture live food. Since we planned prey capture experiments with different prey species that required different salinities, we also tested
for effects of salinity itself on the activity level of the grass shrimp. We also hypothesized that the large, opaque, parasite would make the transparent host more
conspicuous to predators. In view of prior work showing that stressed prey are more vulnerable to predators, we hypothesized that the parasite would reduce the host’s ability
to escape from predators.
2. Materials and methods
A tidal marsh system located in Tuckerton, New Jersey TK, was the study site. TK is a high-salinity mean, 28 ppt salt marsh that is tidally inundated daily. Parasitized P
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.S. Bass, J.S. Weis J. Exp. Mar. Biol. Ecol. 241 1999 223 –233
and unparasitized U grass shrimp 24–33 mm were collected from tide pools using
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umbrella nets side , 106.68 cm; 5 mm mesh and transferred to the lab where they were allowed to acclimate for several days. All shrimp were measured to the closest mm from
the tip of the rostrum to end of the telson.
2.1. Shrimp activity level and salinity A grid of lines 2 cm apart was drawn on the bottom of opaque plastic containers
length, 10 cm; height, 19 cm which were filled with 200 ml water at salinity of 5, 15, or 26 ppt using commercial sea salts. One shrimp, either P 28 mm60.43 SE or U 28
mm60.60 SE, was randomly selected, measured, placed in the container, and allowed to acclimate for 5 min. The number of lines crossed each line crossed, 2 cm was noted
after 1 and 3 min. Trials were run with 20 P and 20 U at 15 ppt, and 10 P and 10 U at both 5 and 26 ppt. These salinities were chosen to check whether salinity itself could
alter activity levels of the shrimp, since prey capture experiments see below were run at these different salinities. Two-sample t-tests were used to compare the activity level
number of lines crossed of P vs. U shrimp at each salinity 5, 15 and 26 ppt after 1 and 3 min. One-way ANOVA compared P and U shrimp separately at each salinity.
2.2. Shrimp activity and hunger A 9.5-l clear glass tank filled with 4 l of 28 ppt sea water was used as an observation
tank. Three sides of the tank were covered with paper. One shrimp, either P 26 mm60.33 SE or U 27 mm60.34 SE, was randomly selected and measured, placed in
the tank and allowed to acclimate for 5 min. Three variables walking, swimming, and resting were measured concurrently over a 5-min period using three stopwatches. The
role of hunger was investigated by testing shrimp: a after being fed 1 h prior to test; b after being starved for 1 day; and c after being starved for 2 days. Trials were run with
13 P and 13 U under condition a and 10 P and 10 U for both b and c. Data were analyzed by t-tests and ANOVA.
2.3. Shrimp responses to an observer In order to ascertain the reaction of the shrimp to the observer, a 9.5-l clear glass tank
filled with 4 l of 15 ppt sea water was used. All four sides of the tank were covered with paper. One shrimp P or U was randomly selected and allowed to acclimate for 5 min.
Once the shrimp was deemed active either swimming or walking, the observer would stand up and look down directly into the tank to mimic a potential predator above. All
shrimp ‘froze’ and ceased activity immediately. The time taken to resume activity in the presence of the ‘predator’ was then recorded. Observations were made on 20 P and 20 U
shrimp. Data were analyzed by t-tests.
2.4. Prey capture Grass shrimp mean, 29 mm were starved for 2 days, and placed in opaque
observation tanks length, 10 cm; height, 19 cm filled with 200 ml water. One grass
C .S. Bass, J.S. Weis J. Exp. Mar. Biol. Ecol. 241 1999 223 –233
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shrimp P or U was placed in the container and allowed to acclimate for 5 min. Ten adult D
. magna Straus were then added as prey. Salinity was 4 ppt since Daphnia are basically freshwater species but can tolerate low salinity Schuytema et al., 1997. The
number of prey eaten was recorded after 5 and 10 min. Trials were run with 13 P and 13 U grass shrimp. The experiment was repeated with 10 adult Artemia salina L. as prey,
with the salinity at 28 ppt. Trials were run with 20 P and 20 U grass shrimp. Another set of trials was done using 10 1-week-old Cyprinodon variegatus Lacepede larvae as
prey, with salinity at 15 ppt. These fish larvae are a more natural food item, which grass shrimp are likely to find in their normal habitat, unlike Daphnia or Artemia. Trials were
run with 20 P and 20 U shrimp. Since fish larvae exhibit predator avoidance behavior and are more difficult to catch than the two other species, trials lasted 2 h, and the test
tanks were left alone not watched until the 2 h had elapsed.
2.5. Predator avoidance Adult mummichogs, F
. heteroclitus L. from TK 70–80 mm SL were starved for 2 days and groups of three fish were allowed to acclimate in an 80-l test tank for 1 h prior
to testing. The test tank had sand on the bottom, and three large rocks for a prey refuge. The experimental procedures used in Smith and Weis 1997 were followed with minor
modifications. Six P . pugio 3 P and 3 U, mean 28 mm were introduced into the test
tank. The number and order in which the shrimp were captured was recorded over 30
2
min. Eight trials were run. Data were analyzed by x . 2.6. Statistical procedures
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All statistical analyses ANOVA, t-tests and x were executed using Statistix for
Windows analytical software.
3. Results
