Download Habitat and Feeding Behavior of the Ghost Crab

Habitat and Feeding Behavior of the Ghost Crab
Ocypode gaudichaudii
Ashley Jones, Mike Kowalski, Evan Kuras
Abstract
Painted ghost crabs (Ocypode gaudichaudii) are distributed along the intertidal zone based on their ability to
physiologically tolerate changes in temperature and water availability. Meanwhile, access to food resources,
predation, and intraspecific competition also play a role in the crabs’ distribution. The present study
examines O. gaudichaudii burrow distribution, size and underground microclimate. In addition, this study
analyzes diurnal feeding behavior in juvenile and adult crabs when presented with a high-nutrient fish
carcass. We found that burrows higher in the intertidal zone experienced the greatest temperature
variability as well as the greatest density of crab burrows. Moreover, we found that adult ghost crabs are
dominant over juveniles when feeding, often engaging in fights or territorial displays.
Introduction
Painted ghost crabs (Ocypode gaudichaudii) dominate Pacific shores on the Western coast of South
America and the Galapagos Islands, digging deep burrows along the intertidal zone of sandy beaches.
Painted ghost crabs vary greatly in their eating patterns; being opportunistic feeders, they alternate between
scavenging for organic material and decomposing detritus while preying upon smaller species including
mole crabs and turtle hatchlings (Strachan et al. 1999). Ghost crabs have been observed to feed primarily at
night, however they will frequently leave their burrowing activities during the day should a food source
become available nearby (Strachan et al. 1999). Abandoning the burrow subjects the feeding ghost crab to
the tides, wind, and predators, thus there is likely a tradeoff between choosing food over protection (Tureli
et al. 2009). Past research has hypothesized that larger adult crabs cannot subsist solely on the organic
minerals within the sand, thus scavenging and hunting behavior is more prominent among older individuals.
Likewise, juveniles may benefit from spending more time in the burrow, as they are more vulnerable to
predation (Strachan et al. 1999).
The location of the burrow may be based on the resources within the immediate environment.
Microclimates along sandy beaches vary greatly based on tide cycles and time of day. When selecting a
burrow site, ghost crabs must consider both temperature and tide level in order to limit the chances of
desiccation, inundation, and predation. Burrow architecture shifts with age based on behavior. Juveniles
dig shallow J-shaped burrows near the lower intertidal zone since they leave their burrows regularly to
renew their respiratory water. Adults dig deeper spiral- and Y-shaped burrows in the upper intertidal
zones since they are larger and do not need to inundate their gills as frequently. These deeper burrows are
used for reproductive purposes and as more of a refuge than the more temporary, shallow burrows of
juveniles (Kan Chan et al. 2006).
Due to the previously mentioned factors of tide cycle and temperature, there is a specific zone that
produces optimal burrowing conditions for ghost crabs, in terms of general beach distribution and density.
According to Outland (2004), greatest burrow densities of ghost crabs on the beaches Makapu and Kailua in
Oahu, Hawaii were recorded in the upper intertidal zone with an increase in burrow size up the beach.
Few burrows were found in the lower sections of the beaches with high wave action or the higher, dry
sections of the beach where waves never reached. Most ghost crab species follow this general pattern; this
area of the intertidal zone provides the lowest risks for both inundation by wave action and desiccation from
high temperatures.
One area of this study focused on the distribution of painted ghost crabs burrows along the
intertidal zone of a sandy beach. We anticipated larger burrows, and by default individuals, would be
located in the upper range of the intertidal zone while smaller burrows would be found closer to the wetter
lower intertidal zone. Another aspect of this study investigated the microclimates and temperature
fluctuations of O. gaudichaudii burrows during various times of day. We projected that the burrows would
have a lower internal temperature in comparison to beach surface temperature and that temperature would
peak in the early afternoon. In addition, we observed the diurnal feeding behavior of juvenile and adult
painted ghost crabs when presented with a fish carcass within the burrowing area. We expected to see a
reorganization of O. gaudichaudii burrows around the carcass with the largest individuals located in the
epicenter of the feeding area. Adults were expected to colonize the carcass first and outcompete juveniles
for the food source.
Methods
Habitat
Burrow density and size were measured according to the methods of Barros (2001). A 35 meter
transect was established during low tide starting at the waterline and extended into the high tidal zone.
Seven 5x5m quadrats were established and O. gaudichaudii burrows were measured in alternate quadrats.
The first transect measured was 0-5 meters from the waterline, the second 10-15 meters, the third 20-25
meters and the fourth 30-35 meters. The diameters of burrows within each quadrat were measured using
centimeter calipers. Diameters were used to approximate body size via carapace width (Wolcott 1978).
Internal burrow temperatures were measured using Thermochron iButtons (DS1921G-F5#),
mobile temperature sensors that operate within the range of -40°C to 85°C. iButtons were placed roughly
4.5 cm deep into ghost crab burrows, two in the high tidal zone and two in the mid tidal zone.
Temperatures were recorded every minute for 3 hrs and 45 mins (17:00-20:45 February 22, 2013) during
low tide.
Feeding Behavior
We obtained two decomposing fish carcasses, one which we placed at the middle intertidal zone,
and the other in the higher sand dunes. We then monitored the carcasses for an hour from about ten meters
away, and observed the activity of the feeding crabs with binoculars. When a crab approached the carcass,
we established its direction and time of arrival, its age, (based on coloration and size) and whether it waited,
fed upon, fought for, abandoned, or dragged the fish carcass. Each time the carcass was dragged, we
measured the distance it had been moved, as well as the estimated size of the crab that had dragged it
(according to the burrow diameter). During the last ten minutes of the observation, we subjected the crabs
to the threat of predation by approaching them, and measured the distance between the closest crab and
ourselves.
We compared the proportion of the adult crabs displaying each behavior with the proportion of the
juvenile population displaying the same behavior. We then calculated the total proportion of crabs in the
mixed population demonstrating each behavior, and ran Fisher's exact test using total population
proportions as the expected values
Results and Discussion
Habitat
Burrow density was significantly higher at the 7th transect box than in any other box [Figure 1,
one-way ANOVA, effect of beach height F = 9.61 with P = 0.005 (df = 3), Tukey post hoc significant for
7th both versus all other boxes]. There was no significant difference between burrow diameters in any
range (Figure 2). However, one can observe a slight trend that burrow diameters are smaller in the higher
intertidal range. This observation most likely reflects the greater number of smaller burrows found in this
range and that smaller burrows were more common than larger burrows.
The average temperature difference between burrows in the high and low tidal zone was 3.29˚C
(Figure 3, n = 226, SD = 0.336, variance = 0.113). This consistent difference indicates that burrows
higher in the intertidal zone have elevated temperatures, at least during low tide in the afternoon. Indeed,
we may expect that burrows higher in the intertidal zone experience greater temperature variability due to
the lack of water’s stabilizing influence, compared to burrows closer to the waterline. We can therefore
infer that burrows higher in the intertidal zone are more stressful places to live since they experience
greater extremes in temperatures and that crabs in this zone must be eurythermal. In fact, we found a
greater concentration of crab burrows in the higher tidal zone, which could indicate that crabs are able to
deal with great variability in temperature. However, this result also reflects the difficulty of building
burrows in the lower tidal zone. Namely, the rising tide does not give crabs adequate time to establish a
territory and scavenge. In addition, since we did not find that burrow size changed with tidal zonation, we
can suppose that ghost crabs of all sizes are eurythemal.
However, past research has found that juvenile Ocypode ceratophthalma in India and Hong Kong dug
their burrows closer to the waterline, most likely because they have smaller gill areas and require more
moisture (Chakrabarti 1981, Kan Chan et al. 2006). Bigger crabs have greater gill surface area and can
therefore tolerate longer periods exposed to dry air (Kan Chan et al. 2006). Thus, O. ceratophthalma burrow
distribution may be a function of temperature tolerance and we would expect bigger burrows in higher tidal
zone. Indeed Kan Chan et al. (2006) found that daytime sand surface temperatures were ~16˚C hotter than
internal burrow temperatures and concluded that crabs use the burrows to escape heat and desiccation
stress and are more stenothermal than our study found. This discrepancy may reflect that O. gaudichaudii
and O. ceratophthalma have different physiological responses to stressful temperature extremes.
Feeding Behavior
We observed that the carcass was not fed upon immediately; instead, it was at first randomly found
by a juvenile crab twelve minutes after its placement. The first adult to feed arrived 19 minutes after
placement of the fish, which was then followed by the arrival of small groups of crabs, both adult and
juvenile. In total, 39 crabs fed on the fish over an hour period, consisting of 14 adults and 25 juveniles. The
difference in frequency of eating, waiting, returning, and abandoning the fish were not shown to be
significant in adults versus juveniles (Figure 4, Fisher´s exact test, p = 0.7597, p = 0.2059, p = 1.0, and p
= 0.5671, respectively). However, that adult crabs initiate fights over food more frequently than juveniles
do (Fisher´s exact test, p = .0498), and were the only crabs to attempt to drag the fish carcass to their
original home burrow.
Crabs did not dig new burrows nearer to the food but simply moved the fish carcass closer to their
own burrow. Since past research and the current study found that desiccation stress is an important
consideration for burrow distribution, the observation that crabs did not build new burrows suggests that
crab burrow location is well suited for that individual’s physiology. Construction of a new burrow closer to
a high-energy food source may not be worth the cost of adjusting to a new microclimate or of the
investment needed to dig a new burrow. Only two adult crabs successfully dragged the fish to their own
holes during the feeding. The first crab, an adult estimated 4.5cm wide, dragged the fish 160cm from its
initial placement to his own hole. A second crab, estimated at 2.5cm wide, dragged the fish another 190cm.
Juveniles appeared to be less cautious of potential predators. The adult crab abandoned the carcass when
we approached at a distance of 240cm, yet seven juveniles continued to feed. The last juvenile crab
abandoned the carcass when we were 150cm away.
Our observations suggest that feeding behavior in juvenile versus adult ghost crabs may be due to
intraspecific competition rather than external factors such as water loss. Despite predictions that larger
crabs are more dependent on scavenging behavior to obtain proper nutrition, we observed more juvenile
crabs feeding on the carcass than adults. Additionally, rather than seeing juveniles opt for protection within
their burrows, juveniles tended to approach the carcass more often and were more reluctant to leave even
when faced with a human threat. However, juveniles were willing to abandon the carcass when it was under
an aggressive adult crab´s possession. We may postulate that the juveniles fear confrontation with adults of
their own species more than potential predators. In order to examine this new hypothesis, we would need
to observe feeding behavior in a situation where a natural predator (such as a shorebird) is also present.
Conclusions
Ghost crab burrows were distributed such that the greatest density was found in the high tidal zone.
Burrow diameter and therefore size of crab did not differ between tidal zone. A consistent difference in
internal burrow temperature was found between burrows in the mid and high tidal zone. Because crabs
living in the higher tidal zone are exposed to greater temperature extremes, we concluded that O.
gaudichaudii were more eurythermal than other species, in which burrow distribution reflected
microclimate gradients. Ghost crabs also appear to have a social hierarchy, with older or larger individuals
dominating the use of burrows and food resources. Although all are opportunistic scavengers, juvenile crabs
appear willing to abandon a food source when challenged by a larger adult. However, juveniles may also be
at lower risk for predation and thus more likely to feed despite the presence of an interspecific threat,
including a human. Because ghost crabs are useful indicators of human disturbance, studies such as these are
important to better understand the nature of ghost crab burrow distribution and feeding behavior,
especially under the threat of predation whether natural or not (Barros 2001).
Literature Cited
Barros F. 2001. Ghost crabs as a tool for rapid assessment of human impacts on exposed sandy beaches.
Biological Conservation 97:399-404.
Chakrabarti A. 1981. Burrow patterns of Ocypode ceratophthalma and their environmental significance.
Journal of Paleontology 55(2): 431-441.
Joaquim, OB et al. 2010. Bioecology of the ghost crab Ocypode quadrata (Crustacea: Brachyura) compared
with other intertidal crabs in the Southwestern Atlantic. Journal of Shellfish Research, 29:2, 503512.
Kan Chan BK, Yu Chan KK, Man Leung PC. 2006. Burrow architecture of the ghost crab Ocypode
ceratophthalma on a sandy shore in Hong Kong. Hydrobiologia 560: 43-49.
Outland, Katrina. "Macrofaunal distribution and density on sandy intertidal habitats: ghost crabs in Oahu,
Hawaii." Katrina Outland Writing Portfolio. N.p., 1 Nov. 2004. Web. 8 Apr. 2013.
<kainani.hpu.edu/pfrus/outland/resume.html>.
Strachan PHR, Smith RC, Hamilton DAB, Taylor AC, and Atkinson RJA. "Studies on the ecology and
behaviour of the ghost crab, Ocypode cursor (L.) in northern Cyprus.." Scientia Marina 63.1
(1999): 51-60.
Tureli C, Duysak O, Akamca E, and Kiyagi V. "Spatial Distribution and Activity Pattern of the Ghost Crab,
Ocypode cursor (L., 1758) in Yumurtalik Bay, North-Eastern Mediterranean-Turkey." Journal of
Animal and Veterinary Advances 8.1 (2009): 165-171.
Wolcott TG. 1978. Ecological Role of Ghost Crabs, Ocypode quadrata (Fabricius) on an ocean beach:
scavengers or predators? Journal of Experimental Marine Biology and Ecology 31:67-82.
Burrows Density (burrows/25 m2)
Appendix
27
24
21
18
15
12
9
6
3
0
0-5
10-15
20-25
Quadrat Distance from Low Tide
30-35
Figure 1: Average ghost crab burrow density along the intertidal zone. Bars designate standard error.
3
Burrow Diameter (cm)
2.5
2
1.5
1
0.5
0
10-15
20-25
Quadrat Distance from Low Tide (m)
30-35
Figure 2: Average ghost crab burrow diameter along the intertidal zone. We did not observe any burrows
in the 0-5 meter range. Error bars designate variance.
Burrow Temperature (˚C)
40
37.5
35
Mid
32.5
Mid
High
30
High
27.5
25
16:48
18:00
19:12
Time
20:24
21:36
Figure 3: Internal ghost crab burrow temperatures in the high and mid tidal zone (data obtained during low
tide).
Figure 4. A comparison of feeding behaviors observed in juvenile versus adult crabs. There was no
significant difference in the number of juvenile versus adult crabs that fed upon a fish carcass, nor in the
number that abandoned or returned to the detritus. However, fights were more common among adult
crabs than juveniles (p=0.04) and only juveniles were willing to delay feeding until other crabs had
abandoned the carcass.
Figure 5. Adult and juvenile crabs abandoned their burrows to feed upon a fish carcass, the direction of the
burrow relative to the fish displayed above. While there was no significant difference between number of
adults versus juveniles in middle or dune habitats; only juveniles were found in burrows closest to the sea (p
<0.1).