International Journal of Marine Science, 2016, Vol.6, No.11, 1-10
8
branch at the base (Chan et al., 2006) but in the present study this particular burrow type was reported to be made
by the adults of the species.
In Y shape burrows, two types of burrows were reported. First type is the simple Y shape in which burrow opening
is available on primary arm while the second type is YDO in which the opening is available on both arms. In both
types the orientation of the primary arm was reported towards seaward side while the orientation of the secondary
arm was reported towards landward sides. Chan et al. (2006) and Chakrabarti (1981) have reported similar kind of
pattern in terms of arm orientation in Y shape burrow made by
O. ceratophthalmus
on the sandy shore of Hong
Kong and Bay of Bengal of India. In the present study, the secondary arm of the Y shape burrow did not reach the
surface but ended with spherical blunt end. Hill and Hunter (1973) also reported similar pattern in Y shape
burrows made by the individuals of
O. quadrata
on Texas coast. In the present study, the adults of
O.
ceratophthalmus
had created YDO burrow type in which the secondary arm opens on the surface. The diameter of
secondary arm was significantly smaller than the primary arm and inclination angle of primary arm was
significantly smaller than the primary arm. YDO type of the burrow is first time reported for
O. ceratophthalmus
in present study and it has not been reported even for other crab species.
The function of the secondary branch of Y shape burrows is still unknown but it has been proposed that the
secondary arm may provide shelter to the crab individuals from the splash of the waves and predators
(Chakrabarti, 1981). In the present study, the YDO burrows ended with small chamber at the base but the size of
the chamber was smaller as compared to the chamber size reported in Y shape burrows made by individuals of
O.
ceratophthalmus
in Hong Kong (Chan et al., 2006). Such kind of chamber was not observed in the Y shape
burrows of crab species in other studies (Takahashi, 1932; Hill and Hunter, 1973; Chakrabarti, 1981). In fiddler
crab males of the species make such kind of chambers during the mating season (Lim, 2006; Christy, 1982) so it
may be believed that presence of such chambers in
O. ceratophthalmus
burrows may provide protection against
predators or shelter during mating season. In the present study, multi branched burrows (MB) were also recorded.
Such kinds of burrows were also recorded in
O. ceratophthalmus
population of Bay of Bengal but not found in
Hong Kong population (Chakrabarti, 1981; Chan et al., 2006). Chan et al. (2006) have quoted the reason behind
the absence of multi branched burrow in
O. ceratophthalmus
population in Hong Kong. They have stated that the
study had been carried out in maximum population abundance zone of the species but not in the backshore zone
where the individuals may have created multi branch burrows. In the present study U shape burrows had both the
branches opening on the surface. Such kinds of burrows were created by large and older crabs. The U shaped
burrows were not reported in previous studied on the species (Chakrabarti, 1981; Chan et al., 2006).
Specific pattern was observed in the distribution of different burrow shapes from upper to lower part of the sandy
shore. Maximum number of J shape, Y shape, and single tube shaped burrows which are occupied by juveniles
were recorded in the lower part of sandy shore near the water line while burrows containing other shapes like JBB
shape, Bulb shaped, MB shape, YDO and U shape burrows occupied by adult and older individuals were located
on the upper part of the sandy shore away from the water line. The adult individuals of the species have bigger gill
surface area which helps them to tolerate the exposure of air or sunlight for longer period of time. Because of the
above mentioned adaptation, the larger individuals of the species stay in the burrow for entire day and come out
on the surface only in night time for feeding. This adaptation also helped them to reduce the risk of predators. It
was also observed that in the burrow shapes made by adults, the lower part of the burrows contain moisture which
help the individuals to respire without changing their respiratory water.
The burrow temperature variation followed specific pattern. During the study period the sediment surface
temperature recorded was 42-45°C which could be lethal for the species but the sediment temperature dropped
along with the depth of the burrow (sub surface). The temperature recorded in the deepest part of the of the
burrow was near 29°C and the temperature reduced up to 14-15°C which suggests that the burrows can provide
refuge or shelter to the crabs during the sunny days. Similar kind of pattern of temperature gradient was also
observed in burrows of Hong Kong population of
O. ceratophthalmus
(Chan et al., 2006). Ferrow (1971) had
