Date Published: May 1, 2019
Publisher: Public Library of Science
Author(s): Bruno Welter Giraldes, Alexandra Leitão, David Smyth, Geerat J. Vermeij.
A number of molluscs within the Class Bivalvia are defined by their ability to secrete fine silk like threads known as byssus which are used to anchor themselves to solid substrates. With relatively few exceptions the majority of these species remain in a sedentary state throughout their life attached via their byssal threads. However, observations of adult Pinctada imbricata radiata pearl oysters made during this study revealed this species’ ability to implement active movement. Byssal threads were secreted in a sequence of attachment and detachment phases, which resulted in the active displacement of the oyster. The oyster was observed, in the laboratory over a 9 day period, travelling a distance of 28cm in a horizontal path. After horizontal displacement, a vertical climbing phase was observed until the oyster reached the water surface at which point the byssus was discarded and the animal dropped, drifting in accordance with water current intensity. It is possible that these adaptations of byssal use are a result of environmentally induced evolutionary change within P. i. radiata.
The Arabian-Persian Gulf is renowned globally as one of the most challenging ecological marine environments due it arid conditions [1,2]. Annual sea temperature variations range from 14–36°C with salinities averaging >39 psu and in some coastal regions >60 psu [1–3]. As a result of these harsh environmental conditions numerous unique ecosystems exist such as; the biogenic reef structures created by the pearl oyster Pinctada imbricata radiata (Leach, 1814) [4,5]. These reefs once occurred on an immense scale. Pliny the elder in 38 AD estimated that the oyster beds of the western Gulf covered an area of >1050 Km stretching from Sharjan (in the United Arab Emirates) to Qatif (in Saudi Arabia) [5,6]. However recent research has shown that overexploitation and environmental stressors have resulted in a considerable reduction of the ecotype throughout the region .
P. i. radiata samples were collected by divers during September 2018 from an inshore site close to the coastal port of Semaisma on the eastern coast of Qatar. The oysters were located on a sand/shell substrate at a depth of 5m close to sea grass beds in a water temperature of 32 oC. The specimens were transported from the survey site to the laboratory using an in-situ water sample. There is no specific permission required to collect oysters in this site and this study does not involve endangered or protected species.
Daily Laboratory observations of the Semaisma samples began 24hrs after the oysters had been positioned in the aquarium. The first observational session recorded the visible displacement of one specimen moving towards the other. It was noted that the oyster was secreting a single byssal thread and contracting the muscle in the foot to drag itself along the aquarium. The byssal thread was fixed to the lateral side of the aquarium and discarded after movement had taken place, directly after this event another thread was secreted and attached to repeat a further displacement phase. A recognized sequence of byssal secretion, attachment, drag and discard was observed throughout the sessions. The oyster discarded a linear sequence of horizontally attached byssal threads which followed the path of movement along the aquarium glass (Fig 1A, 1B, 1E and 1F). A total of 28cm was travelled in 9 days (Fig 1A) (S1 Data). The oyster displayed a recognized displacement sequence of secretion, attachment and discard in a determined path of direction (Fig 1E and 1F).
The basic biology, anatomy and attachment-displacement strategies of bivalves, can be directly related to their modes of life . The mussel families Pinnidae, Mytilidae and Dreissenidae  and the false oysters Pteriidae  use byssal threads and are considered as byssaly attached species . The observed active displacement of adult P. i. radiata (Pteriidae) using byssus identifies the species within the recognized locomotion-attachment group of the seventh ecological category of bivalves  and is similar to other byssaly attached species which present benthic movement during adult stages . However, the locomotive behaviour observed by P. radiata during this study can be considered unique as it represents the fastest benthic displacement for a byssaly attached species within the group. Comparable studies have recorded bivalve displacement at maximum speeds of < 25cm in one month therefore 28 cm in 9 days represents a significant increase in maximum locomotive speed . This proactive benthic displacement capacity by a byssally attached species has been undocumented until now. Source: http://doi.org/10.1371/journal.pone.0215865