B . Morton, W.Y. Yuen J. Exp. Mar. Biol. Ecol. 246 2000 1 –29
3
numbers of individuals Ramsay et al., 1997a. Laboratory studies of P . bernhardus and
other epibenthic scavenging invertebrates collected from the Clyde Sea showed that the hermit crab was attracted to bait before the others Nickell and Moore, 1992a,b.
Confrontations in the form of short-term approach–retreat cycles were observed frequently among P
. bernhardus around the bait Nickell and Moore, 1992b. The counterparts of these crabs on intertidal sand flats in Hong Kong are Diogenes
edwardsii and Clibanarius infraspinatus, and both are common at Starfish Bay where they live sympatrically with Nassarius festivus. This study investigated and compared
their food preferences and their sequence of arrival at and departure from bait at this site. The more numerous D
. edwardsii was chosen for further investigations of feeding behaviour, in comparison with that of N
. festivus, in laboratory experiments. It was hypothesized that N
. festivus possesses certain characteristics which confer upon it an advantage in competition with D
. edwardsii for carrion, resulting in the dominance of the former over the latter in feeding clusters on the shore. Laboratory experiments
involving an investigation of the respective abilities of the two species to detect carrion and their consumption profiles were therefore carried out, and the results compared.
Finally, this study also aimed to provide evidence for interspecific competition. This is best studied by altering the densities of both species and monitoring their resource
utilization patterns Underwood, 1986; Begon et al., 1990. The ratio of the numbers of N
. festivus to D. edwardsii in feeding assemblages was manipulated, accordingly, and any changes in feeding patterns monitored.
2. Materials and methods
2.1. Food preferences Preliminary food preference experiments for Nassarius festivus and hermit crabs were
conducted on the sandy shore at Starfish Bay, New Territories, Hong Kong. Fresh carrion, including fish Lateolabrax sp., bivalve Tapes philippinarum, soldier crab
Mictyris longicarpus and mud-shrimp Upogebia major were offered as bait. The four
3
different food items of | 2 cm were placed at 2-m intervals across the lower intertidal shore at a depth of | 5 cm during an ebbing tide for 60 min. Scavengers which came to
the carrion were collected by hand and the numbers of each species recorded. The experiments were repeated five times more on different days in October 1998 at different
times and locations on the shore. N
. festivus, Diogenes edwardsii and Clibanarius infraspinatus were the commonest scavengers attracted to the bait. The results of the six
experiments upon these three species were pooled so as to assess the attractiveness of the four different baits using a chi-square test for goodness of fit. A significance level of
a 5 0.05 was used for this and all the other subsequent statistical analyses.
Fish was selected for subsequent feeding experiments because the results of the food preference experiments suggested that this and bivalve carrion attracted the most
individuals and use of the former made bait standardization easier.
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. Morton, W.Y. Yuen J. Exp. Mar. Biol. Ecol. 246 2000 1 –29
2.2. Sequence of arrival and departure
3
Fifteen pieces of | 1 cm of fish carrion were placed on the lower intertidal shore at 1.5-m intervals during an ebbing tide. After 5 min, one piece of fish carrion was
recovered and the numbers of Nassarius festivus, Diogenes edwardsii and Clibanarius infraspinatus which approached it, recorded. Other pieces of fish carrion and feeding
clusters were recovered, and the numbers of individuals of each species recorded every 5 min throughout the 75-min experiment. The experiment was repeated six times more on
different days in October 1998 and at different times and locations on the shore. The mean numbers of individuals of each species which approached the bait in every 5-min
interval were calculated using the pooled data from the seven experiments.
Since the two hermit crabs, Diogenes edwardsii and Clibanarius infraspinatus, were observed to behave similarly in the field, only the former was chosen for further
investigations of feeding behaviour in the laboratory.
2.3. Maintenance of animals Nassarius festivus and Diogenes edwardsii collected from Starfish Bay were kept
separately in aquaria with flow-through seawater at The Swire Institute of Marine Science. Ambient seawater was used to ensure that salinity 29‰ and temperature
22628C matched those values in the field. Prior to each laboratory experiment, the hunger levels of the animals were standardized by allowing them to feed on fish carrion
to satiation and then starving them for 2 weeks.
2.4. Food detection distance A long aquarium 90 3 30 3 30 cm was used to investigate the chemoreception
ability of Nassarius festivus and Diogenes edwardsii. Seawater in the aquarium flowed
21
unidirectionally at a speed of 3 mm s . Thirty-two similar-sized N
. festivus shell length 5 12.0–15.0 mm were selected and divided randomly into eight groups of four
individuals. Spots of different coloured rapid-drying enamel paints unique to each of the eight groups were marked on the shells facilitating easy identification. Individuals
belonging to the same group were arrayed in a line at distances of 10, 20, 30, 40, 50, 60,
3
70 and 80 cm from a piece of fish carrion of | 2 cm placed at one end of the aquarium where the seawater flowed in. The times at which each individual reached the bait and
began feeding were recorded. Individuals were, however, restrained from eating the bait by removing them immediately and placing them in a tray without food. Animals that
could not locate the bait within 30 min were discounted. Five replicate experiments were conducted using the 32 individuals. The distance of each group of animals from the bait
was, however, selected randomly for each trial. The aquarium was also cleaned between each trial. The whole procedure was repeated using Diogenes edwardsii home shell
length 5 12.0–15.0 mm.
Individuals were considered to be within detection distance of the food if, 1 50 or more from a certain distance reached the bait and began feeding, and 2 all groups
B . Morton, W.Y. Yuen J. Exp. Mar. Biol. Ecol. 246 2000 1 –29
5
positioned closer to the bait were also found to be within detection distance Britton and Morton, 1994b.
The data collected from the five trials were pooled for each species. The cumulative numbers of individuals arriving at food from each distance were compared by a
chi-square test for goodness of fit for each species. The arrival times to food for individuals at each distance were compared by one-way analysis of variance ANOVA
for each species using the computer software, JMP Sall and Lehman, 1996.
2.5. Time spent feeding and consumption Twenty individuals of Nassarius festivus shell length 5 12.0–15.0 mm were used in
the consumption experiments. Their shells were also marked using different-coloured paints. They were then placed in a 17 3 14 3 7 cm plastic tray with running seawater at
21
a flow rate of 3 mm s and allowed to feed upon a piece of fish carrion of | 0.6 g. The
time each individual spent feeding was recorded. N . festivus began feeding when its
proboscis was extended to the bait, i.e. the proboscis search reaction Kohn, 1983. Feeding ended when the individual abandoned the bait. The experiment was terminated
when all individuals abandoned the bait. Prior to each experiment, the bait and similar-sized controls were immersed in
seawater for 40 min in order to standardize the amount of contained tissue water. The controls, which were placed in a tray with running seawater in the absence of Nassarius
festivus, were used to determine the error due to natural weight loss to or gain from the water. They were blotted dry, using standard procedures and weighed to the nearest
0.00001 g. The bait and the controls were also blotted and weighed immediately after the experiment. Shell lengths of those individuals which fed on the bait were measured
to the nearest 0.5 mm. Their wet body weights were obtained after removal from the shells and standard blotting. The dry weights of the food and the dead N
. festivus were obtained after drying in an oven at 808C for 2 days. This experiment was repeated four
times. The initial wet and dry body weights of Nassarius festivus were obtained from
calibration graphs of wet and dry body weights log against shell length. Similarly, the
e
initial dry tissue weights of the fish carrion were determined from a calibration graph of dry tissue weight against wet tissue weight. The former were constructed from the wet
and dry body weights of 23 N . festivus of known shell lengths between 9.7 and 15.4
mm whereas the latter was constructed from the wet and dry weights of 39 pieces of fish carrion wet weight between 0.01 and 1.97 g.
The difference in the weights of the food before and after feeding by Nassarius festivus, after adjustment for either weight loss or gain, represented consumption. The
consumption rate was then calculated and expressed in terms of mg wet or dry weight
21 21
of food consumed individual min
. The amount of food eaten relative to wet or dry body weight was expressed as the percentage wet or dry body weight consumed
21
min .
The procedure was repeated using Diogenes edwardsii home shell length 5 12.0– 15.0 mm, but feeding duration was considered to be the time between when an
individual used its chelipeds to tear pieces of fish tissue and when it left the bait. After 2
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. Morton, W.Y. Yuen J. Exp. Mar. Biol. Ecol. 246 2000 1 –29
h, the experiment was terminated even though some of the individuals had not abandoned the bait but continued to eat. Instead of shell length, the shield length of each
individual was measured to the nearest 0.1 mm with an ocular micrometer under a stereomicroscope, after removal from its shell. Shield length is defined as the carapace
from the rostrum tip to the cervical suture which separates it from the abdomen Gherardi et al., 1994. The initial body weights of D
. edwardsii were deduced from the calibration graphs of wet and dry body weights log against shield length. Such graphs
e
were plotted by recording shield lengths and the wet and dry body weights of 20 D .
edwardsii shield length 5 1.27–2.23 mm. 2.6. Behaviour when feeding together
Twenty individuals of Nassarius festivus to Diogenes edwardsii shell length 5 12.0– 15.0 mm were used in each trial of this experiment. The ratio of the numbers of the two
species varied in each trial. The ratios used were 20:0, 19:1, 15:5, 10:10, 5:15, 1:19 and 0:20, and the shells of the animals marked with different colours using paint. They were
placed at the end of a 17 3 14 3 7 cm plastic tray with running seawater maintained at 3
21 3
mm s and allowed to feed upon | 1 cm of fish carrion which was placed on the other
side of the tray at a distance of 12 cm. The seawater flowed in the direction from the carrion to the scavengers. After 5 min, the numbers of individuals which showed any of
the behaviours and the numbers of intra- and interspecific interactions described in Tables 1–3 at that particular instance were recorded. Behaviours continued to be
recorded at 5-min intervals for 30 min. Three replicate experiments were undertaken for each ratio, but each individual was used only once.
The percentages of the total numbers of individuals of both species showing a certain behaviour at each 5-min interval were calculated for each ratio, using the pooled data
from the three replicate experiments. A bar chart showing the changes in the behaviour of the animals over time for each species at each ratio was plotted. Graphs showing the
variation in the total numbers of intraspecific interactions among Diogenes edwardsii and interspecific interactions between the two species with changes in the ratios, using
the pooled data, were constructed.
2.7. Percentage feeding, arrival time and time spent feeding when together The experimental procedure identified in Section 2.6 above was repeated. Instead of
recording the numbers of individuals showing a particular behaviour and the numbers of
Table 1 Types of behaviour shown by Nassarius festivus
Behaviour Description
Moving in Moving towards the food
Feeding Proboscis extends and penetrates the food
Moving out Abandons the food and moves away from it , 5 cm
Moving around Moves around at a distance of . 5 cm from the food
Stationary away from food Remains stationary away from the food
B . Morton, W.Y. Yuen J. Exp. Mar. Biol. Ecol. 246 2000 1 –29
7 Table 2
Types of behaviour shown by Diogenes edwardsii Behaviour
Description Moving in
Moving towards the food Feeding
It uses its left larger cheliped to hold the food and right smaller cheliped occasionally left cheliped to tear pieces of it and convey
them to its mouth Grasping food
Grasps the food with its chelipeds without feeding Moving out
Abandons the food and moves away from it , 5 cm Stationary near food
Remains stationary at a distance of , 2 cm from the food Moving around
Moves around Stationary away from food
Remains stationary at a distance of . 2 cm from the food Intraspecific interactions
a
Manipulation The aggressor holds the shell of the other crab, the recipient, with its
chelipeds and removes it from blocking its way to the food. Recipients are usually individuals which are feeding, grasping the food or near the food
a
Cheliped strike Two individuals extend their chelipeds towards each other and make physical
contact. Such aggressive behaviour usually occurs between two individuals around the food, between two individuals feeding or grasping the food, or
between two individuals feeding grasping and around the food
a
Ambulatory display Two individuals extend their legs towards each other and make physical
contact. Such behaviour occurs between two feeding individuals or between a feeding individual and an individual around the food. This
usually results in one of them being driven away
a
Descriptions modified from Ramsay et al. 1997a.
interactions at 5-min intervals, however, the total numbers of individuals of each species feeding upon the fish carrion, the arrival time at food and the time spent feeding by each
individual were recorded. The experiment was terminated after 30 min. The time spent feeding by each individual was calculated by adding up all its feeding times during the
30-min trial. A total of five replicate experiments was conducted for each of the seven ratios.
The percentages of the total numbers of individuals of each species feeding was calculated when the frequency data for the experiments corresponding to each ratio were
pooled. One-way ANOVA was carried out to test for any significant difference in the percentages of the total number of Diogenes edwardsii feeding upon the food among the
Table 3 Types of behaviour associated with the interspecific interactions between Nassarius festivus and Diogenes
edwardsii Behaviour
Description Cheliped contact
A D . edwardsii attacks a feeding or moving-out
N . festivus from the front with its chelipeds
Grasping A D
. edwardsii climbs over and grasps the back of a feeding or moving-out N
. festivus Manipulation
A D . edwardsii holds and lifts up the shell of a feeding
or moving-out N . festivus, usually causing the feeding
gastropod to leave the food
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trials using the ratios of 19:1, 15:5, 10:10, 5:15, 1:19 and 0:20. The Tukey–Kramer test for multiple comparisons of mean percentages was performed because a significant
difference was indicated by the ANOVA test Sokal and Rohlf, 1995. Mean arrival and mean feeding times were calculated for each species at each ratio
and were plotted on graphs showing their variation with the changing ratio of the numbers of Nassarius festivus to Diogenes edwardsii. A Kruskal–Wallis test was
performed to compare the arrival times among the different ratios for each species. A nonparametric test was used because of the non-normal distribution of the data Sokal
and Rohlf, 1995. Feeding times were also analyzed in the same way. Tests that indicated significant differences were further analyzed by a Mann–Whitney U-test for
pairwise comparisons Sokal and Rohlf, 1995. The computer software JMP was used for all statistical analyses except the Mann–Whitney U-test Sall and Lehman, 1996.
3. Results
