Predators & defenses
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  In addition to the shell, defenses include swimming, considered here, and SPONGE COATINGS, CAMOUFLAGE, and ANCHORING THREADS considered in other sections.
Research study 1

Scallops swim by forcibly ejecting water from the mantle cavity in 2 streams or jets that exit on either side of the morphological dorsal, or hinge, area of the shells.  Fast clapping of the shell valves creates the jets, and fleshy curtains known as the velum prevent the water from exiting out the valve aperture.  A scallop swims, therefore, with the flapping valves foremost, not the other way round.  Lift is likely generated by the angle of attack of the valves through the water.

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CLICK HERE to see a video of a scallop Chlamys sp. swimming from the perceived threat of a SCUBA-diver.

NOTE the video replays automatically

Research study 2

photograph of cross-section of scallop eye courtesy Land 1965 J Physiol 179: 138photograph of close view of eyes of giant scallop Crassadoma gigantea Land 1965 J Physiol 179: 138Swimming scallops Chlamys spp. are highly sensitive to shadows, water movement, vibrations, and chemical stimuli. All probably play a role in stimulating a scallop to swim. The eyes are interesting, not just for their number, size, colour, and reflectivity, but for the fact they each have a double retina.

diagram of optical system of eye of scallop Pecten sp. courtesy Land 1965 J Physiol 179: 138Eyes of west-coast scallop species have received some research attention, but the optical system is better known for the Atlantic scallop Pecten maximus. Each eye of a scallop consists of a cornea, lens, distal and proximal retinas, and a curved reflective lining at the back known as the argentea pigment layer (see photo above Left). It is this lining that gives the eye its reflectivity.

Interestingly, the lens plays no role in producing an image. In fact, the image reflected from the argentea layer falls on the receptor cells of the distal retina. There is no image on the proximal retina. The author suggests that fine resolution is possible, but is limited to the central region of each distal retina, and this may account, in part, for the presence of many eyes (60 or more, covering about 300 degrees). The absence of a resolved image in the proximal retina suggests that it may function for monitoring light intensity (as involved in a shadow response).

Note that the potential focussing distance can be quite close to the eye (see diagram lower Right). So, what is the eye used for? Because of the potential for close focussing, some authors propose that the scallop can distinguish small particles in the nearby water, including phytoplankton food particles. How this would aid the scallop, however, is unclear. The scallop would certainly be perceiving other types of food signals, including possibly both chemical and physical, and would hardly need to see the food to know that it is around. Land 1965 J Physiol 179: 138; Land 1966 J Exp Biol 45: 433. Photo of
Crassadoma gigantea
courtesy Ron Long, SFU, Burnaby.

NOTE because the image reflects back from this curved surface, the eye is termed to have a curved mirror design

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photograph of a scallop Chlamys hastata swimming from a simulated attack by a sunflower star Pycnopodia helianthoides taken from a video

CLICK HERE to see a video of a scallop Chlamys hastata swimming from a simulated attack by a sunflower star Pycnopodia helianthoides.

NOTE the video replays automatically

Research study 3

Balance when swimming and proper orientation when landing in scallops is enabled through sensory organs, the statocysts, located on either side of the pedal ganglia at the base of the foot.
explanation of how scallops when swimming maintain their proper orientation
In the pediveliger larval stage the statocysts are present in these same positions on either side of the pedal ganglia (see drawing).  The statocysts open by a canal to the mantle cavity, and contain solid concretions known as stataconia.  As the bundles of stataconia move with gravity during positional changes, differential firing of sensory cells (hair cells) lining the capsule wall of each statocyst gives information on body position.  Drawing of pediveliger of the Atlantic scallop Pecten maximus from Cragg & Nott 1977 J Exp Mar Biol Ecol 27: 23.

NOTE lit. "foot sail" G., referring to the fact that this late-stage larva of the scallop has a bivalved shell and a prominent foot

Research study 4

histogram of oxygen consumption of scallops Chlamys sp. when resting and swimming, and with and without a sponge coating on their shellsActive defense in west-coast scallops involves swimming from a predator by jet photograph of scallop Chlamys rubida swimming with a heavy coating of sponge Mycale adhaerenspropulsion.  Swimming bouts in Chamys hastata and C. rubida are usually short, lasting on average about 9sec and carrying the scallop about 1.5m.  While this is ample to allow escape from sea stars, other swimming predators such as sea otters and octopuses should easily be able to chase down and capture the swimming scallop. Do the sponge coatings on the scallops slow them down and/or increase the metabolic costs of swimming? These questions are addressed in experiments at Shannon Point Marine Laboratory, Washington and show that removal of the sponge from test specimens increases the duration of a scallop’s average swimming bout significantly (but less than 10%), but has no significant effect on oxygen uptake of the scallop while swimming (see graph on Right).  Surprisingly, removal of the sponge does not significantly affect elevation or distance of swim.  The authors attribute this to the fact that even a heavy coating of sponge increases the overall mass of the scallop by just 5%.  Donovan et al. 2002 J Mar Biol Assn UK 82: 469.

NOTE the sponges are Mycale adhaerens and Myxilla incrustans

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photograph of scallops Chlamys sp. being stimulated to swim by the appearance of a sunflower star Pycnopodia helianthoides taken from a video

CLICK HERE to see a video of another scallop Chlamys sp. being stimulated to swim by the arrival of a juvenile sunflower star Pycnopodia helianthoides.

NOTE the video replays automatically

Research study 5

photograph of a scallop Chlamys hastata bearing a single large barnacle Balanus sp. courtesy Photo courtesy Deb Dovovan, Western Washington U, Bellinghamhistogram of swimming data for scallops Chlamys hastata with and without barnacles on their shellsBarnacles are sometimes found growing parasitically on the shells of scallops Chlamys hastata, often no more than one in number, especially if the barnacle is large.  Studies at Shannon Point Marine Laboratory, Washington show that, not surprisingly, a scallop removed of its load of barnacles will swim longer, higher, and further than it does with them still attached (histograms on Right). Removal of large barnacles can increase a scallop’s swimming distance up to 9-fold, and there is a significant relationship between mass of barnacles carried and swimming distance attained (graph lower Left). Note that even a decrease as small as 20% in total mass can lead to a doubling of swimming distance.

Although some scallops are prevented from swimming by extra-heavy loads of barnacles, it is notable that even with barnacles weighing in excess of the host1, swimming may still be possible. In the field, even though swimming direction is likely to be random after contact with a graph showing increase in swimming distance of scallops Chlamys hastata when their load of barnacles is removedpredator, any increased time spent in the water column will result in longer-distance swims through increased transport in currents.  Finally, a barnacle-encrusted scallop claps its valves at a 15% greater frequency2 than a non-encrusted one.  Overall, through added mass to carry and increased frictional drag, more energy is required for a barnacle-encrusted scallop to swim than a non-encrusted one, as evidenced by higher anaerobic3 energy expenditure in the former.  Donovan et al. 2003 J Mar Biol Assn UK 83: 813. Photo courtesy Deb Dovovan, Western Washington U, Bellingham.

NOTE1  in the graph these are the 4 individuals whose mass drops by more than 50% on removal of their barnacles

NOTE2  barnacle-encrusted: 150 valve-clappings . min-1; barnacle-removed: 130 valve-clappings . min-1

NOTE3  other studies by these researchers show that Chlamys hastata relies on anaerobic sources for over 80% of the energy expended during escape swimming.  Donovan et al. 2002 J Mar Biol Assn UK 82: 469

Research study 6

Although the function of the double-retina system1 in scallop eyes is unclear, there is new evidence from researchers at Duke University, North Carolina that habitat recognition may be involved in addition to escape from predators.  The scientists compare morphology of eyes in 7 species, including 5 swimming and 2 sessile species2, and conclude, based on differences in receptor angles in the proximal retinas, that swimming species may have better vision than non-swimming ones.  Proximal and distal retinas in the 7 species differ significantly in optical sensitivity3, but correlation with ecological factors such as swimming ability, preferred substratum type, and habitat depth, are lacking.  However, based on differences in variability in receptor angles and optical sensitivities in the 2 retinal types, the authors hypothesise that distal retinas may perform common functions in all species, such as warning of predator approach, while proximal retinas may perform tasks more important to swimming species, such as detection of preferred habitats such as eelgrass beds, sand, or rocky areas.  All in all, the topic seems quite open photograph of scallop Chlamys sp. with sea star Orthasterias koehlerito further interpretation and research.  Speiser & Johnsen 2008 Am Malacol Bull 26 (1/2): 27.diagram of cross-section of scallop eye

NOTE1  in a scallop eye, light passes through a cornea and lens to a concave mirror where it is reflected back through proximal and distal retinas (see drawing on Right). A fluid-filled cavity between the mirror and proximal retina varies in size in different species, being smaller in swimming forms and larger in sessile forms.  The proximal retina in swimming species is more densely packed with photoreceptors than in sessile species, and for that reason is thought to be involved in image formation.  The authors suggest that the eye may alternately focus light onto either of the retinas through slight changes in shape

NOTE2  west-coast representatives among the former are Chlamys hastata and C. rubida, and among the latter, Crassadoma gigantea

NOTE3 optical resolution of a scallop eye measured in equivalent inter-receptor angles (degrees) is 1.6. This is better than the eyes of a wolf spider (1.8), but much less sensitive than those of an octopus (0.01), human (0.007), or eagle (0.004)

Scallop Chlamys sp. prepares to swim on being touched
by arm of sea star Orthasterias koehleri 1X

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