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You are here:   OldClasses > 2012 > Erosaria erosa | Bartholomew Woodham

 

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Erosaria erosa

Eroded Cowry

Bartholomew Woodham (2012)


 

 

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Summary


Physical Description


Ecology


Life History & Behaviour


Anatomy & Physiology


Evolution & Systematics


Biogeographic Distribution


Conservation & Threats


References & Links

Life History & Behaviour

Feeding

While there is little information regarding the diet of E. erosa the diet of cowries in general is known (Hayes, 1983). In general, cowries can be considered to be omnivorous as they feed largely on invertebrates such as sponges and tunicates (Hayes, 1983) as well as algae that they scrape from the surface of the substrate using their radula (Besley et al., 1998 & Hayes, 1983). They feed during the night and remain in their shells for the majority of the day.

Locomotion

Cowries move by the use of their muscular foot extended from the shell aperture from where it can get traction on the substrate. Through numerous small muscular contractions of the foot surface and assisted by mucous secreted by the foot the cowrie is propelled forwards.

Respiration

Gas exchange takes place over the gills that located in the mantle cavity. Water is drawn in from the anterior end of the shell through the siphon and pumped across the gills (Ruppert et al., 2004).

Reproduction

Cowries have separate sexes. Internal fertilisation is achieved by copulation. There is little information on the reproductive habits of E. erosa but it has been noted in other species that the female will lay a small group of eggs and then brood them before they hatch several weeks later (Wilson, 1985). There are two modes of dispersal that different cowrie species' young rely on, direct development and planktonic dispersal (Wilson, 1985 & Burgess, 1986). Direct developing young are brooded for a longer period of time compaired to their plankton living counterparts (Wilson, 1985).



Below, an experiment conducted at Heron Island Research Station in 2012 regarding the size and burrowing ability of E. erosa is presented.

Size and burrowing ability in E. erosa.

Bartholomew A. Woodham, University of Queensland

Introduction
E. erosa inhabit shallow areas of reefs across the Indian ocean and parts of the West Pacific Ocean, typically in less than 10 meters of water. During the day, these cowries are usually found on the underside of boulders or coral structures where they remain, with their mantle retracted, until nightfall when they move out to feed. There is little information on the diet of different species of cowries, but those that have been studied have been found to consume algae, which they scrape up using their radula, and invertebrates such as sponges and ascidians (Besley et al., 1998). When disturbed on the underside of boulders cowries often drop off and fall to the bottom where they retract into their shell and will remain for extended periods of time. While they remain on the substrate, often sand, they can be covered with sediment that has also been disturbed. This experiment examines the effect of shell size on the ability of E. erosa individuals to burrow their way back to the surface after being buried.

Methods
Specimens of E. erosa were collected from various locations on the reef surrounding Heron Island, Queensland, by hand. The specimens were then returned to the research station and housed in plastic aquaria with a constant flow of seawater. The aquaria had pieces of rubble under which the cowries could reside. The tank also housed a number of other species of cowrie including Cypraea tigris, Cypraea annulus and a number of another species that were not identified. The cowries were left in this tank for several days before the experiment began.

Using sand collected from the inner reef, in an area where E. erosa had been collected, a shallow tray, approximately 15 centimeters deep, was filled to a depth of 10 centimeters.The tray was then filled with seawater to cover the sand to a depth of 5 centimeters. The sand was then mixed to ensure saturation, leveled and then had small divots of sand removed so that the cowries could be placed in later.

Cowries were taken from the aquaria and disturbed manually, by moving about on the sand, until they had retracted completely into their shell to simulate the state in which they would be buried. Once they were completely retracted they were then placed into divots in the sand approximately 6 centimeters deep and the sand originally removed was then sprinkled on top to completely bury the cowrie. At this point a timer was started. The cowries were considered 'out' when at least half of the rim of the shell was above the sand. If a trial lasted more than 25 minutes the trial was ended and the depth to which the cowries were buried was measured using the depth probe of vernier calipers. The cowries were then removed manually. Those removed manually were not recorded as having burrowed out and instead were compared within their own group. This was repeated with all available cowries and at the conclusion of the experiment all individuals were returned to the reef.

Results

Within the group that burrowed to the surface the average time to surface was 512 seconds with a standard deviation of 347 seconds. There was no correlation between shell length of E. erosa and time to surface from a depth of 6 centimeters of sand. The graph below was produced using the data and the lack of any trend is evident.


In those cowries that did not surface after 25 mins the data was analysed and a slight trend was evident but not significant. Below, the data is presented. The depth at which the cowrie was still buried (in millimeters) is shown on the y-axis and the length of the E. erosa individual's shell is shown on the x-axis. A linear fit is also shown. It can been seen that there is a trend towards larger individuals remaining at greater depth. There is also a cluster of individuals that were so close to the surface that the shell was barely visible.

Discussion

In the group of E. erosa that surfaced from the sand there was no trend apparent in the data. It can be seen that the time to surfacing appears fairly random. Reasons for larger cowries not being correspondingly fast at burrowing out may be due to friction acting over the larger surface area of their shell nullifying the extra power of their larger muscular foot. It is also probable that these cowries were not moving at their highest rate of speed. Controlling this experimentally would be extremely difficult.

There was a trend apparent in those cowries that did not surface in that larger individuals tended to remain lower in the sediment. It was noted, in all but one case, that the cowries did have their siphon projecting through the surface of the sand. It would appear that these individuals move upwards until they can secure access to water and then remain there for extended periods of time. The handling that the E. erosa individuals were subjected to prior to testing probably induced defensive behaviour in these cowries. Larger individuals are likely to be remaining lower in the sediment due to the fact that they have a longer siphon and are thus able to access water but remain further out of reach of a perceived predatory threat.

This study had no conclusive findings and further study would be required to support or refute this topic. In general, more experimentation is required in the field of cowrie research, however,the focus should be on abundance, distribution, diet and reproductive behaviour as data regarding these is lacking for most species.

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