Diurnal rhythms of activity, sediment egestion and cryptic behaviour of Holothuria whitmaei (sea cucumber) at Ningaloo Reef, Western Australia

Brief description

The diurnal rates of activity and sediment egestion on three seperate occasions in January, April and August, 2003, and the cryptic behaviour on one occasion in August, 2003 were measured to determine diurnal patterns of holothurian behaviour.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: On Ningaloo Reef, Western Australia, densities of H. whitmaei are greatest on the reef crest and reef flat, where they are commonly observed upon sand among coral heads and scattered coral rubble. Within a section of reef flat of approximately 10000 m2, thirty adult black teatfish were collected by SCUBA and divided equally among five study sites. For identification purposes, animals were marked with a number (see File Id: b30d05b0-3369-11dc-a676-00188b4c0af8). For ease of diver navigation, the boundaries of each of the study sites were connected with submerged string lines, and the positions of animals indicated by coloured plastic tape attached to the nearest hard object. Prior to the beginning of monitoring, numbered indicator plates were placed directly posterior to each animal so that any subsequent movement could be determined (following Uthicke 2001a). Using a flexible measuring tape, divers measured the path indicated by the faecal trail left behind the animal to record the total distance travelled. In addition, each animal was observed to determine whether the feeding tentacles were extended or retracted. To elucidate rates of feeding, egested faecal material was carefully collected and transferred to plastic vials. Care was taken not to make contact with the animal during collection, as disturbances resulted typically in retraction of the feeding tentacles and a short period of inactivity. Following completion of these tasks, marker plates were returned carefully to the substrate immediately posterior to the animal. Rates of activity and sediment egestion were subsequently determined based upon the total distance (cm) / dry weight volume (g) diveded by the total time between monitoring periods (h). Diurnal trends in activity and egestion were measured over five consecutive days on each of the three occasions: August 2002, January 2003 and April 2003. To ensure that activity and sediment egestion levels were monitored constantly throughout day light hours, two teams of divers worked in alternating shifts. Under ideal circumstances, this meant that rates of activity were measured at approximately two hourly intervals throughout the day. However, due to seasonal changes in day length, the number of monitoring intervals varied between months, with more dives being possible in January and April. Night time rates of activity were calculated as the distance travelled between the final monitoring period for the day, and the initial montioring period next mornin, divided by the number of hours between measurements. The volume of faeces egested was considered roughly proportional to the volume consumed by the animals, given that calcareous dissolution in the gut of holothurians is negligible.

Notes

Purpose
To i) evaluate the possible biological advantages of diurnal activity/feeding rhythms and ii) discuss the possible ramifications of diurnal sheltering behaviour from the perspective of the surface based census technique, manta towing.