Hydatina physis (Linnaeus, 1758)

Hydatina physis lives on intertidal and shallow subtidal lagoon reefs in the Marshalls. Several small specimens have been found in man-made reef quarries that range in depth from a few centimeters to about 3 meters. The animals are usually seen at night, when they emerge from their hiding places under the sand. The figured specimen in the two shots immediately below is a young one, measuring only 20mm in length. This specimen was observed on 20 November 1981 on sand at a depth of 8m on Enewetak Atoll's eastern lagoon reef. We also have this from Hawaii.

The specimen figured below had a shell that measured 29mm. It was found at the base of a Halimeda plant in a Kwajalein lagoon algae patch at a depth of 8m on 27 July 2008. A fragment of a shell from the same area measured over 40mm; the full sized shell would have been well over 50mm, so these animals can get pretty large.

The specimen in the three photos below was found at a depth of 8m at night in a Halimeda patch on a Kwajalein lagoon reef on 20 June 2009. The third image was captured from video, so it appears to have a bit different color.

Two more specimens, measuring 42.3 and 38.7 (shell length) were observed mating and laying eggs in a Kwajalein lagoon Halimeda at a depth of about 8m on the night of 18 July 2009. These are shown in the two photos below.

Created 27 January 2007
Updated 23 July 2009

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