Real-time rotation of the multispiral operculum of Phorcus lineatus (da Costa, 1778) (Gastropoda: Trochidae): evidence for a semidiurnal rhythm and its use in growth studies

Shurlock, Barry C., Pepper, Duncan S., Hawkins, Stephen J. and Mieszkowska, Nova (2017) Real-time rotation of the multispiral operculum of Phorcus lineatus (da Costa, 1778) (Gastropoda: Trochidae): evidence for a semidiurnal rhythm and its use in growth studies. Journal of Molluscan Studies, 83 (2), 211-219. (doi:10.1093/mollus/eyx010).

Abstract

The first real-time evidence of the rotation of the operculum of the trochid gastropod Phorcus lineatus has been obtained by marking the operculum of live individuals and recording the angular orientation of the operculum in relation to the apertural tooth using time-lapse microscopy. Short-term monitoring of the rotation of opercula of juvenile P. lineatus in a temperature-controlled tidal aquarium at temperatures of 15 and 19 °C gave mean rates (clockwise) of 5.6 and 7.2°/d (0.097 and 0.126 rad/d), respectively, representing a 30% increase for a 4 °C rise in temperature and a complete turn in 1.7–2.0 months. A daily phase difference of 6 h in the tidal cycle had no effect on rates. Measured rates of opercular rotation were converted to more conventional growth rates, based on weight and physical dimensions, by using biometric data from the study population. In the case of weight, close agreement between theory and direct measurement was demonstrated. Quasi-regular series of imprints of the opercular disc and peripheral striae visible on some opercula are interpreted as evidence of a discontinuous mode of growth, the features acting like the ‘ticks of a clock’. The ratio of the angular separation of these features to rates of opercular rotation is in close accord with a semidiurnal rhythm. The rate of opercular rotation is recommended as a growth parameter in its own right, measurement of which is useful (especially over short periods), for elucidating in fine detail the effects of environmental factors on growth in aquaria and mesocosms and, with more difficulty, on the shore.

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