LOCOMOTION IN JUVENILES OF THE WINGED PEARL OYSTER Pteria penguin: EFFECT OF LIGHT AND OYSTER SIZE.
The winged pearl oyster, Pteria penguin, is a tropical bivalve found from the Indian Ocean to Japan and throughout the Pacific islands. It inhabits a vast geographic range from Madagascar to Australia. It is also the most widespread cultured winged oyster for the production of large and valuable mabé pearls. Despite their economic and biological significance, surprisingly little is known about their behavior and ecology. The present study examines locomotion and attachment activity of P. penguin juveniles as a function of oyster size and illumination intensity.
In order to assess oyster movements, both shell height (SH) and hinge length (HL) of sixteen detached oysters (7.2~35.4 mm, SH) were measured. These oysters were then placed alternately large, then small, according to size, with 5cm spacing, in a 30L plastic tank containing 10L FSW at 25oC, 30 PSU. A lamp equipped with a timer was set approximately 50cm above the tank providing illumination measuring approximately 3500 lux. The experiment started with 6h of light, followed by 6h in complete darkness, repeated by another period of 6h of light to complete 18h of observation. The experiment was repeated three times under identical conditions. Juveniles were filmed using a surveillance camera equipped with an infrared illuminator. A total of 1080 still images were extracted from the raw video and merged using Adobe Premier CC 2014 software. Oyster movement was analyzed using the software application Tracker 4.95 (Open Source Physics Project, htttp://www.opensourcephysics.org/) to estimate the distance traveled by a moving object. Byssal attachment at the end of the experiment was confirmed by gently touching the juveniles with a pair of tweezers.
All oysters exhibited locomotion or rotation movements facilitated by foot. Locomotion was significantly higher in darkness (Tukey HSD test, p<0.01) (Fig. 1). The average distance traveled was 3.3 ± 5.1 cm under lighted conditions, and 32.8±31.2 cm in darkness respectively. The maximum distance traveled by a single individual was 104.6 cm in darkness. Moreover, movements were not observed to be significantly related to the oysters SH and HL (P>0.1). Byssal attachment percentage was 62.5±6.5% in all replicates. Results suggest that P. penguin juveniles undertake locomotion mostly in darkness, possibly an adaptation to reduce the risk of visual predation. Moreover, increased locomotive behavior in darkness may enable juveniles to search more successfully for environments that would augment survival rates, such as those providing more stable substrates for attachment.