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Burrowing capacity of the Opisthobranch Philinopsis Pilsbryi depending on grain size of sand on Heron Island


Philinopsis pilsbryi is a species of Opisthobranch of the order Cephalaspidea. This organism inhabits coral reefs, and is commonly find in reef flat zones. As burrowed in the sand during most of its life, it is a species particularly hard to study and only a little is known about this species. Specimens that are observed and collected are most of the time collected by chance when they are crawling on the sand or on corals instead of being hidden in the sand. The aim of this project is to identify the preferred size of grain size of Philinopsis Pilsbryi, by comparing the burrowing capacity in different situations. It will allow to characterize the kind of sand where Philinopsis pilsbryi is more likely to be found. My hypothesis is that (i) Smaller individuals can burrow easier in the sand, and (ii) Philinopsis Pilsbryi will prefer the medium grain size sand, as it is the sand that is find in its habitat.

Materials and Methods

Three specimens of Philinopsis pilsbryi were collected on Heron Island (23°26S, 151°51E), a vegetated coral cay in the Capricorn-Bunker Island group at the southern end of the Great Barrier Reef, Australia. There were collected in the between the rocky beach and the beach crest.

They were placed in plastic vials, transported back to shore, and then held in tanks with running sea water. After experiment was made, opisthobranchs were released.

The respective size of the 3 specimens were :

  • Individual 1: 1,6 cm
  • Individual 2: 4,2 cm
  • Individual 3: 6,1 cm

Three different types of sand were collected:

For each type of sand, 3 trials were made with each individuals. 
For each trial, the time of burrowing was recorded with a stopwatch, from the beginning of burrowing to when the animal was entirely covered in sand. (See the video in the Behavior section as an example of a trial).


Time of Burrowing depending on grain size of sand

From the results of this experiment, we can observe that for the individual 2 and individual 3 (respectively medium and large size), it is faster to burrow in the medium grain size sand than in the small or large grain size sand.
However, the individual 1 (small) seems to prefer the small grain size sand.

Moreover, the size of individuals is inversely proportional to the time of burrowing.


As expected, the smaller is the individual, the easier it is for him to burrow in the sand. The individual 1 (the youngest one) probably has better burrowing capacity in order to protect himself from predators.
Secondly, the two adults individual seemed to prefer the medium grain size sand, which suggests that they are more likely to be found in the reef crest zone.