“igloo” construction by the ocypodid crab, dotilla myctiroides (milne-edwards) (crustacea;...

E L S E V I E R Journal of ExpefimentalMafine Biology and Ecology,

198 (1996) 237-247

JOURNALOF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

"Igloo" construction by the ocypodid crab, Dotilla myctiroides (Milne-Edwards) (Crustacea; Brachyura): the role

of an air chamber when burrowing in a saturated sandy substratum

Satoshi Takeda"'*, Masatoshi Matsumasa b, Hoi-Sen Yong c, Minoru Murai I ,a

aMarine Biological Station, Tohoku University, Asamushi, Aomori 039-35, Japan bDepartment of Biology, School of Liberal Arts and Sciences, lwate Medical University, Morioka 020,

Japan ~Department of Zoology, Faculty of Science, University of Malaya, 59100, Kuala Lumpur, Malaysia

dDepartment of Biology, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812, Japan

Received 27 June 1995; revised 4 January 1996; accepted 10 January 1996

Abstract

Dotilla myctiroides, an inhabitant of tropical sandy shores, builds a unique burrow structure in semi-fluid, unstable sand, in addition to a vertical burrow constructed in well-drained and firm sand. The crab rotates in the sand, forming a circular wall of sand pellets around it and subsequently roofs the wall with pellets. The resulting burrow structure, termed an " ig loo" , encloses a small quantity of air together with the crab itself.

In this study, the function of the igloo was examined and compared to that of the vertical burrow structure. In the air-filled chamber beneath the sand surface, which results from the construction of the igloo, the crab continued to scoop up the sand from the lower surface of the chamber, and attached the scooped pellets to the ceiling of the chamber. Consequently, the crab, together with the chamber, continually moved deeper into the sand until it was below the water level. In contrast, 40% of the crabs artificially buried in sand without a surrounding air chamber remained where they had been buried 12 h later. This indicated that the air chamber greatly facilitates vertical movement of the crab. The making of the igloo is considered to be an adaptation that enables the construction of an air chamber in semi-fluid sand, where a vertical burrow cannot be constructed, because the sand is not sufficiently firm to prevent it from collapsing. The descent of the crab into the sand through the construction of an igloo may prevent an encounter with dangers such as predators arriving with the incoming tide, a role played by the construction of a vertical burrow when the sandy substratum is firm and well-drained.

Corresponding author. ~Present address: Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-02, Japan.

0022-0981/96l$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0022-0981 (96)00007-X

238 S. Takeda et al. / J. Exp. Mar. Biol. Ecol. 198 (1996) 237-247

Keywords : Dotil la myctiroides; Burrowing behaviour; Burrow structure; Ocypodidae; Physical condition of substratum

1. Introduction

Crabs of the genus Dotilla are found on tropical sandy shores of the Indo-Pacific region (Hartnoll, 1973). Generally, they occupy the lower part of the shore (MacNae and Kalk, 1962; Silas and Sankarankutty, 1967; Mclntyre, 1968; Hartnoll, 1973; Hails and Yaziz, 1982; Matsumasa et al., 1992).

When their habitat is submerged, crabs reside in the sand. When the tide exposes their habitat, the crabs emerge onto the sand surface to feed and court (Tweedie, 1950; Altevogt, 1957; Hartnoll, 1973; Hails and Yaziz, 1982). Before the rising tide reaches them, they cease their surface activities and retreat into the sand (Tweedie, 1950; Altevogt, 1957; Hartnoll, 1973; Hails and Yaziz, 1982). Two species, D. myctiroides and D. fenestrata, have been reported to adopt two types of burrowing behaviour, resulting in the construction of two forms of burrows, in response to the physical condition of the sandy substratum (Tweedie, 1950; Altevogt, 1957; Hartnoll, 1973). One is a vertical tube-like burrow, constructed by digging and throwing out the sand in the form of pellets around the burrow entrance. Crabs dig these burrows in a well-drained and firm sand, in which the water table falls well below the surface at low tide. The other form has been termed the "igloo". In a moist, semi-fluid and unstable sand, from which the water drains inadequately, the crab rotates in the sand beneath the surface, forming a circular wall of sand pellets around itself, and then roofs the cavity with pellets. The structure encloses a small quantity of air due to the impervious nature of its wall; an air-filled chamber is thus formed in the sand (Tweedie, 1950; Altevogt, 1957; Hartnoll, 1973). Consequently, a dome-shaped structure like an Eskimo's igloo is formed on the sand surface. The crabs which construct igloos are considered to use them only temporarily, compared with the vertical burrows (Tweedie, 1950; Altevogt, 1957; Hartnoll, 1973; Hails and Yaziz, 1982).

In addition, it is frequently observed that crabs which have left their vertical burrows or igloos bury their bodies directly in the semi-fluid or fluid sand beneath the surface, to conceal themselves, immediately after perceiving danger (Tweedie, 1950). After some minutes the crabs emerge from the sand and then re-engage in surface activities. This direct burial is different from the construction of an igloo, in which substratum is carried up to the surface. The latter type of burial behaviour requires more time and labour than direct burial. Nevertheless, no one has investigated why the crabs possess two types of behaviour to bury themselves beneath the surface of the sand, or why they have evolved the igloo construction behaviour in addition to the construction of a vertical burrow.

In the present study, we examined the possibility that the air chamber, resulting from the construction of the igloo in the semi-fluid sand, relates to the vertical movement of the crab in the sand. For this purpose, we initially examined the relationship between the burrowing behaviour, or the shape of the burrow constructed, and the depth of the water level, which affects the physical condition of sandy substratum. The depths at which the

S. Takeda et al. / J. Exp. Mar. Biol. Ecol. 198 (1996) 237-247 239

crabs constructed burrow structures of both types were also measured. In addition, the depths to which crabs moved in the sand in the presence and absence of an air-filled chamber were compared. Finally, the significance of the possession of two types of burrowing behaviour by the crab is discussed in relation to their ecology.

2. Materials and methods

To clarify the relationship between the depth of the water table and burrowing behaviour, or the types of burrow constructed, the following observations were carded out under artificial conditions. Sand was collected from the tidal fiat at Port Dickson (Pantai), Negeri Sembilan, Malaysia (2 ° 30' N; 101 ° 51' E). The crabs there were observed to dig vertical burrows or igloos in response to the change in water level caused by the tides. The sand collected was packed to a depth of 18 cm into five transparent vessels (8 X 8 cm and 24 cm deep) with many small holes in their walls to a height of 6 cm from the bottom, allowing free percolation of water. The vessels were placed in larger containers and the water level of the inner vessels was maintained 0.5 cm above the sand surface, level with it, and 0.5, 2 and 13 cm below the surface by pouring sea water into the outer containers. The sand in the first two vessels was entirely fluid. The sand above the water level was semi-fluid when the level was 0.5 or 2 cm below the surface, and was firm when the water level was 13 cm below the surface. The sand below the water level was always fluid. Crabs with a maximum carapace length of between 7.3 and 10.1 mm were released onto the sand in the inner vessels, and their burrowing and /o r concealing behaviour, and the type of burrow constructed, were observed. In addition, the presence or absence of air around the crabs in the sand was recorded by digging up the crabs. In the vessels with the water level at, or below, the surface of the substratum, the depth at which the crabs were found below the surface was measured 30 rain after they had retreated into the sand.

To indicate the difference in the construction processes between vertical burrows and igloos, the diameter of the burrow and the carapace length of the crab which constructed it were measured in the field at Port Dickson, because in the experimental vessels the shape of the burrow was sometimes affected by the wall of the vessel. The diameter of the vertical burrows was measured with calipers; the diameter of the igloos was measured at the crater-shaped stage of their construction (see Section 3). After measuring the diameter, the crab was carefully taken out of its burrow and its carapace length was measured.

The importance for the crab of the air enclosed in the igloo was examined as follows. The crabs were released in vessels with the water level at 0.5 cm below the sand surface. After the crab had made an igloo and covered it with a dome, formed by sand pellets carded from its base, the upper part of the dome was removed with a spoon and the subsequent behaviour of the crab was observed continuously for 30 min.

To clarify the role that the air chamber plays while the crab is burrowing in the sand, the depths to which crabs went in the absence or presence of an air chamber were compared. To produce the condition of no air chamber, a crab was put into a hole about 5 cm deep made artificially in the sand of a vessel in which the water level was 13 cm


below the sand surface, and immediately buried. Subsequently, a small amount of sea water was poured on to the sand surface to homogenize the sand in which the hole had been artificially made, because the sand that is disturbed by the burrowing of the crab is homogenized by the water as the tide rises.

For the alternative condition with the presence of an air chamber, a crab was released onto the sand of a vessel with a water level that was 0.5 cm below the sand surface. When the crab had made an igloo about 5 cm below the sand surface, including an air chamber, a small amount of sea water was poured onto the sand surface to homogenize the sand. Subsequently, the water level was quickly lowered to 13 cm below the sand surface, by removing the sea water from the outer container, to equalize the sand firmness with that in the previous experiment. After 12 h, the emergence of the crab on to the sand surface, the shape of burrow that it had made, and the depth of the crab were recorded, by digging up the crab in both sets of experiments.

3. Results

When the water level was 0.5 cm above, or level with, the sand surface, all or almost all individuals (96%) did not make burrow structures (Table 1), but directly entered and buried themselves in the fluid sand without an air chamber (Fig. 1A), as reported by Tweedie (1950). They never scooped up the fluid sand to make a burrow structure. In contrast, all individuals made igloos when the water level was 0.5 or 2 cm below the sand surface, that is, when the sand near the surface was semi-fluid and in an insufficiently firm condition. When the water level was 13 cm below the surface, the crabs made vertical burrows in the well-drained and firm sand (Table 1). When the water level was 0.5 or 13 cm below the surface, the crabs made either igloos or vertical burrows in response to the depth of the water level, indicating that the shape of burrow constructed in the sand is closely related to the depth of the water table, i.e., the physical condition of the substratum. In the sand in which the vertical burrows and/or igloos were made, air ~hambers enclosing the crabs were present.

Table 1

Relationships between water table depth, the numbers of the two types of burrow structures constructed and

the depth (means-+SD) of the crabs with or without a burrow structure 30 min after retreating into the sand

Water table depth (cm) Burrow structure Crab depth (cm)

Direct burial Igloo Vertical burrow

+ 0.5 26 0 0 nd

0 24 1 0 3.1 _+ 1.0 (n = 20)

- 0 .5 0 2 6 0 15.1 -+4.6 (n = 20) - 2 0 2 6 0 nd

- 13 0 0 23 12.5-+5.7 (n = 21)

Numbers in parentheses indicate numbers of individuals measured. In the sand with the same water level to the

sand surface, the depth of the igloo was excluded. "nd" = not determined.

All crabs that constructed vertical burrows and igloos were present in the air chambers in the substratum.


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ ~ ; ~:~,,~ "2

: : ) . . . . :, .......... :? . . . . . ?

Fig. 1. (A) Concealment behaviour in saturated, fluid sand; (B) a vertical burrow and the spherical sand pellets thrown out on the left side of its entrance in well-drained, firm, sand; (C-F) stages in the construction of an igloo in semi-fluid sand. (C) An early stage of the igloo comprising a shallow depression and sand pellets encircling it. (D) A crater-shaped stage of the igloo with a circular wall of sand pellets. (E) A dome-shaped stage, i.e., an igloo after roofing the crater-shaped structure. (F) the crater-shaped stage of an igloo constructed by a juvenile crab.

Crabs in sand with the water table level at the surface buried themselves (Fig. 1A) at an average depth of 3.1 cm. When the water level was 0.5 cm below the surface, the crabs constructed igloos that descended below the water level, and were on average 15.1


cm below the surface. When the water level was 13 cm below the surface, the crabs constructed vertical burrows and were found at an average depth of 12.5 cm (Table 1). Eight of the 21 individuals that constructed vertical burrows (38%) and 12 of the 20 individuals that constructed igloos (60%) were found at the bottom of the vessels, together with their air chamber, indicating that they had descended below the water level, while still maintaining the air chamber even in the water-saturated, fluid sand.

The field investigations showed that the diameter of the igloo constructed by the crab was twice that of the vertical burrow (Fig. 2), indicating that the igloo-building crabs initially scooped out an area of sand surface four times as large as that excavated by the crabs that constructed vertical burrows. This supports the idea that igloos and vertical burrows were constructed by quite different processes.

The process of construction of both vertical burrows and igloos in the vessels corresponded to the description by Tweedie (1950). Vertical burrows and igloos were clearly distinguished by the following characteristics in the process of construction. In the early stage of construction of an igloo, the crab scooped up the semi-fluid sand and formed pellets, with which it then encircled the shallow depression from which it had scooped the sand (Fig. 1C), producing a crater-shaped structure with a circular wall (Fig.

A

E U

._c r 0

o Igloo ~ / n=46 o o Y= 0.49.2.54X

r = 0.820

o

o o o

° °~ *

o p o o g o . Vertical burrow S %.0 • n=60

== • l e ~ . v =-0.19° 1.65x

== . r = 0.904

/ - 'X." • ° o-.~_ ,;

I I I i I | I

0 0.5 1.0

Carapace length (cm)

Fig. 2. Relationship between carapace length (x) and diameter (y) of vertical burrows and igloos.

s. Takeda et al. / J. Exp. Mar. Biol. Ecol. 198 (1996) 237-247 243

1D). Subsequently, the crab attached sand pellets to the wall from the inside and roofed the crater-shaped structure, gradually reducing the opening and finally closed it entirely with an arch formed of semi-fluid sand pellets; the resulting dome-shaped structure enclosed the crab and a quantity of air (Fig. 1E). The semi-fluid sand used by the crab coalesced, forming an impermeable wall. Thus, the air chamber of the igloo, constructed on the surface of the sand, was isolated from the outside environment. Igloos were constructed by both large and small individuals (Fig. IF).

During the construction of a vertical burrow, the crab irregularly threw out the firmer sand it had scooped up in the form of spherical pellets, near the entrance (Fig. 1B). When the pellets were heaped up around the entrance to the vertical burrow, the shaft of the burrow remained connected with the outside through the heap of pellets, until the burrow was finally plugged. In the construction of a vertical burrow, the crab plugged the burrow near, or below, the sand surface with sand pellets carried from the bottom of the burrow and constructed an air chamber enclosing itself, as in the construction of an igloo. As mentioned above, whether the crab had made a vertical burrow or an igloo was judged by the shape of the sand pellets and the presence or absence of a pile of pellets on the sand surface (Fig. 1B and 1E), after the crab had retreated into the sand.

When the upper part of the dome of the igloo was removed, the enclosed crabs always immediately buried itself beneath the surface of the water-saturated sand. Subsequently, nine of the 46 individuals went on to conceal themselves beneath the sand surface within 30 min (Table 2). On the other hand, 29 individuals scooped out the sand from the bottom of the damaged igloos and then repaired the dome, and eight individuals made new igloos somewhere else. These crabs then descended into the water-saturated sand. These results suggest that descent into the fluid sand below the water level required the physical presence of the dome structure of the igloo, rather than being something that occurred independently, after completion of the construction of the igloo.

When the crabs were directly buried in the sand without an air chamber, 12 of the 30 individuals (40%) remained at the depth at which they had been buried (Table 3). The remaining 18 individuals ascended to the sand surface. Sixteen of them made vertical burrows elsewhere and descended into the sand. In other words, the buried crabs did not descend into the sand in the absence of an air chamber. In contrast, almost all individuals (83%) that were buried in the sand with an air chamber descended without prior emergence to the sand surface (Table 3). Only five of the 30 individuals (17%) ascended to the sand surface, made vertical burrows and descended into the sand. This

Table 2 Percentage of three types of behaviour by crabs after removal of the dome of the igloo

Behaviour Percentage

Concealed beneath the bottom surface of the broken igloo 19.6 (n = 9) Repaired the broken igloo 63.0 (n = 29) Made a new igloo 17.4 (n = 8)

Numbers in parentheses indicate numbers of individuals.


Table 3 Percentage of three types of behaviour by crabs after being buried in the sand with or without an air chamber

Behaviour Percentage

Without chamber With chamber

Remained in the position where buried Ascended to the sand surface Descended without emergence

40(n=12) 0 (n=0) 60 (n = 18) 16.7 (n =5) 0 (n =0) 83.3 (n = 25)

Numbers in parentheses indicate numbers of individuals.

suggests that the air chamber in the igloo or the vertical burrow structure is indispensable for the crab to move vertically in the firm sand, as well as in the fluid sand (Table 1).

4. Discussion

4.1. The term " ig loo"

Tweedie (1950) reported that D. myctiroides constructs an air chamber beneath the surface of semi-fluid and unstable sandy substrata, through the construction of the structure termed " ig loo" , and subsequently descends into the sand. The igloo was distinguished from the vertical burrow constructed in well-drained and firm sand by the shape of the sand pellets and the presence of a pile of pellets on the sand surface, although the vertical burrow is covered by a pile of sand pellets beside the entrance (Fig. 1).

Since the term igloo was first used by Tweedie (1950), it has been used by a few investigators. Altevogt (1957), as well as Tweedie (1950), described in detail the process of igloo construction by D. myctiroides in the semi-fluid sand. Altevogt (1957) also investigated the distribution of igloos, based only on the presence of the pile of sand pellets on the sand surface, and reported that they were constructed in both semi-fluid and unstable sand and in well-drained and firm sand. Consequently, he did not recognize any ecological function for the igloo.

The construction of an igloo was closely related to the semi-fluid condition of the sandy substratum and was never carried out in firm sand with a low water table (Table 1). In the field, however, it was observed that the sand in which an igloo had been constructed while it was semi-fluid, immediately after the recession of tide, gradually became well-drained and firm as the tide receded. The igloo formed on a firm sand observed b y Altevogt (1957) may have been formed under these conditions.

Fielder (1970) reported that Scopimera inflata made igloos, judging by the presence of piles of sand pellets at the opening of the burrow structure. However, he also reported that S. inflata inhabited the firm sand of the upper part of the tidal fiat, in which it was initially difficult to make a burrow. These descriptions strongly suggest that the pellets piled on the opening of the burrow structure were brought out from the wet, soft bottom of the burrow, when the vertical burrow was repaired, rather than that the pellets were


piled up when an igloo was made in the firm sand. Nevertheless, it was observed that S. g]obosa, that inhabits the lower part of sandy tidal fiats (Ono, 1965), constructed an igloo in the semi-fluid sand near the shore line (personal communications by T. Yamaguchi and T. Koga), similarly to Dotilla myctiroides.

In addition, Hartnoll (1973) described in detail the construction of an igloo by D. fenestrata by the same process as D. myctiroides, but he did not mention the ecological function of the igloo. In general, the ecological function of the igloo has not yet been described scientifically, except for the brief description by Tweedie (1950).

4.2. Ecological function of the igloo

When the sandy substratum was submerged, D. myctiroides buried itself beneath the surface of the fluid sand (Table 1). In addition, if the dome of the igloo was removed immediately after its construction, the crab buried itself beneath the lower surface of the igloo, which extended down to the water level (Table 1 and 2). In this case, the crab never descended further than necessary to conceal itself beneath the surface of the sand, in the igloo from which the dome had been removed. This may be related to the fact that the crab cannot construct an open space in the fluid sand below the water level, such as the shaft of a vertical burrow or the air chamber of an igloo, because the sand does not provide sufficient support to prevent it from collapsing (Takeda and Kurihara, 1987).

When the water level was below the sand surface, the crab made either an igloo or a vertical burrow, in response to the depth of the water level (Table 1). When the water level was near the surface, the crab made an igloo, whereas it made a vertical burrow when the water level was lower. The vertical burrow was plugged by sand pellets near, or below, the sand surface and an air chamber was constructed in the sand as well as an igloo. The crab in the chamber continued to scoop the sand from the lower surface of the chamber and subsequently to attach the scooped pellets to the ceiling of the chamber. Consequently, the chamber containing the air space gradually descended downwards, and the crab also descended together with the chamber. The crab that constructed either a vertical burrow or an igloo extended below the water level together with a chamber (Table 1). This indicates that the air chamber resulting from the construction of a vertical burrow or an igloo was closely related to the descent of the crab into the fluid sand below the water level and also into the firm sand above the water table (Table 3). Moreover, Tweedie (1950) observed that a bubble emerged from the semi-fluid sand, immediately before D. myctiroides appeared on the sand surface after recession of the tide. This means that D. myctiroides ascends and/or descends together with the air chamber in both firm and fluid sand. The soldier crab, Mictyris longicarpus (Mictyridae) descends and ascends together with the air chamber (Maitland and Maitland, 1992), which is constructed by "corkscrew"-style digging in the sandy substratum above the water table (Cowles, 1915; McNeill, 1926; Maitland and Maitland, 1992). However, the mechanism by which it encloses the air in the sandy substratum is unclear. The chamber is considered to provide the crab with space to work freely and move vertically in the sand and to prevent it from being hindered by the weight of the surrounding sand. This hypothesis is supported by the fact that crabs with an air chamber in the sand ascended


or descended freely, in contrast to 40% of the crabs buried directly in the sand without an air chamber, which remained at the position where they had been buried (Table 3).

When a crab constructs an igloo, it scoops four times the area of sand surface than a crab which constructs a vertical burrow does, during the early stage of construction (Fig. 2). In the construction of an igloo, the crabs probably need a larger horizontal space to retain sufficient air in the sand, allowing them working space to descend and/or ascend in the sand, because of the shallow space remaining between the sand surface and the water table.

The crab scoops and throws the surface sand using the same motion regardless of whether it is making an igloo or a vertical burrow, except for the dimension of the scooped area and the arrangement of the sand pellets thrown (Fig. 1). This suggests that the behaviour for constructing the igloo changes from that for constructing the vertical burrow, because ocypodid crabs inhabiting tidal fiats generally construct vertical burrows.

The burrow structures of terrestrial and semi-terrestrial crabs protect them from predators, high temperature and desiccation (Altevogt, 1957; Goshima et al., 1978). The vertical burrows may also protect Dotilla myctiroides from these stresses. Large males of D. myctiroides frequently leave their burrows and feed on the sandy substratum just above the shoreline while droving (Tweedie, 1950; Altevogt, 1957; MacNae and Kalk, 1962; Hails and Yaziz, 1982). When they perceive danger, they immediately bury themselves beneath the semi-fluid and soft sand and thus avoid dangers such as predation. Subsequently, before the sand is submerged by the incoming tide, the large males construct igloos and descend into the sand as they do in vertical burrows. The descent of the crab into the sand, through the construction of an igloo, may effectively avoid in advance the disturbance of the sandy substratum caused by wave action and any encounter with predators arriving with the rising tide, such as the moon crab Matuta lunaris and the swimming crab Portunus sanguinolentus (Yong, unpubl, obs.).

Tweedie (1950) observed that, when male crabs possessing vertical burrows encounter smaller individuals of the same species, they capture them and push the females only into their burrows. The vertical burrows of Dotilla myctiroides may be related to their mating behaviour. On the other hand, the habit of constructing igloos may allow the crabs to inhabit the lower parts of the sandy tidal fiats where other crabs cannot survive.

Acknowledgments

We thank Dr. T. Yamaguchi, Kumamoto University and Dr. T. Koga, Kyushu University, for useful comments. This study was partially supported by a Grant-in-Aid for International Scientific Research from the Japanese Ministry of Education, Science and Culture (No. 06041082).

References

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Cowles, R.E, 1915. The habits of some tropical Crustacea: II. Philippine J. Sci., Vol. 10, pp. 11-18. Fielder, D.R., 1970. The feeding behaviour of the sand crab Scopimera inflata (Decapoda, Ocypodidae). J.

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“igloo” construction by the ocypodid crab, dotilla myctiroides (milne-edwards) (crustacea;...

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