title for learn-about section of A SNAIL'S ODYSSEY
  Life in the intertidal zone
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Wave effects


Topics dealing with life in the intertidal zone include wave effects considered here, and TEMPERATURE EFFECTS, HEAT-SHOCK PROTEINS, GAS EXCHANGE, SALINITY EFFECTS, TRANSLOCATION STUDIES, WATER-CHEMISTRY EFFECTS, considered in other sections.

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Research study 1

graph showing times to emerge from burial in gravel for 32 sets of 25 mussels Mytilus trossulusWaves not only scour rocks and remove intertidal organisms forcibly from their attachment sites, but burial by sand and gravel is also a common a feature of intertidal habitats.  Studies on the Santa-Barbara coast of California show that bay mussels Mytilus trossulus, but not sea mussels M. californianus, have the ability to crawl to safety after being buried.  Tests with 3 replicates of 25 mussels each, buried under 5cm of road gravel, for example, result in all M. trossulus appearing safely at the surface within 3d (see graph, each of 3 replicates shown).  This “crawling-out” ability might also ensure that if the smaller species is trapped within mixed clumps of M. californianus, which could lead to death through crushing or smothering, it can move to the safety of an outside position.  The author suggests that the behaviour is a response to some tactile sensation involved with the pressure of objects resting against the mussel. 

Would light receptors be useful to a Mytilus trossulus in finding its way out of mixed clumps?  The author tests this using lights from different directions and shows that direction of illumination is not relevant to the direction the mussels crawl. Harger 1968 Veliger 11: 45.

NOTE  it may be surprising to know that adult mussels have ocelli (the authors refer to them as“eyes” or “cerebral ocelli”, but perhaps the terms eyespots or ocelli would be more suitable). Studies at the Bodega Marine Laboratory, California show that the ocelli are paired and located at the bases of the drawing showing location of right ocellus in a musselclump of bay mussels Mytilus trossulusctenidial filaments of the left and right inner demibranchs (see drawing on Right). Each ocellus is 40-50µm in diameter, reddish-brown in colour, and simple in construction. Several pigment cells enclose a few sensory cells, and a lens is lacking. The adult ocelli apparently derive from the ocelli of the veliger larva, which play a role during settlement.  More interestingly, the authors describe a triangular translucent zone, or “window”, in the lateral anterior part of each shell valve which may permit diffuse light to reach the ocelli.  In simple field and lab experiments, however, the authors fail to show any type of behavioral response indicative of light sensitivity.  As to their function, the authors conjecture that they could be involved in long-term responses such as photoperiod related to the reproductive cycle.  Rosen et al. 1978 Veliger 21: 10.


Clump of bay mussels Mytilus trossulus

Research study 2

graph showing sizes of mussels Mytilus trossulus in relation to degree of wave exposurephotograph of wave-beaten shore on west coast of Vancouver Island, British ColumbiaGrowth, maximum size attained, and population sizes of bay mussels Mytilus trossulus are inversely related to wave exposure.  This is shown in several related studies around Santa Barbara, California at sites differing in degree of seasonal wave impact.  Harger 1970 Veliger 12: 401; Harger & Landenberger 1971 Veliger 14: 195.



Waves beat on a rocky shoreline
on the west coast of Vancouver
Island, British Columbia

Research study 3

Although plants and animals growing on a mussel may be beneficial to the mussel by providing visual and chemical camouflage, they can also be detrimental through effects of smothering, interfering with suspension-feeding, and eroding the shells of their hosts.  Because mussels live in a photograph of sea mussel Mytilus califronianus with sea palm Postelsia palmaeformis growing on itmechanically stressful environment, growths on the shell such as large kelps Postelsia, Laminaria, and Alaria can create frictional drag that can tear a mussel from its bed.   Water flow over a mussel creates forces of lift and drag that, just like air flow over an airplane wing, together produce a resultant force that tends to lift and rotate the shell. This makes the mussel “taller”, extending it out of the relatively slow flow within the bed and into mainstream flow-velocities above the bed. The presence of an attached alga will accentuate this force.  Studies in Tatoosh Island and San Juan Island, Washington show that resultant forces may be 2-6 times greater with an attached alga than without, depending upon flow velocities. It is surprising to learn, however, that there is no significant relationship between surface area of a kelp attached to a mussel and the frictional drag forces it creates.  The authors suggest that this is because longer kelps “flutter”, thus increasing pressure drag and perhaps confounding what should be a straightforward scaling relationship. Witman & Suchanek 1984 Mar Ecol Progr Ser 16: 259.  

NOTE  the authors of the study use the term epizoan for plants growing on mussels, but this term is more confusing than helpful.  As with other similar terms such as epiphyte, epifauna, epiflora, and epizoite there is no intuitive way to know if an epizoan is an animal growing on something, say, a plant, or if it is a plant growing on an animal

NOTE  effects of and differences between forces of pressure drag and frictional drag are considered elsewhere in the ODYSSEY: LEARN ABOUT SPONGES: HABITAT INTERACTIONS

Sea palm Postelsia palmaeformis growing on a sea mussel Mytilus
Note that in addition to increasing drag forces on
the mussel, the plant is occluding its host's shell valves 0.8X

Research study 4

photograph of clear patch within a mussel bed Mytilus californianusThe closely packed array of individuals comprising a mussel bed is regularly disrupted by formation of patches.  These result from battering by logs in storms and feeding activity of predators.  The role of patches in mussel-bed ecology is well studied.  On high wave-energy shores, such as Tatoosh Island, Washington, patches may also form by hydrodynamic forces of lift and drag.  In such areas, the tightly packed nature of the mussel bed provides its own downstream protection from hydrodynamic forces.  However, where this smooth continuum is interrupted, perhaps by an irregularly positioned individual or, in more extreme cases, by the presence of a hummock, then photograph of a clump of mussels Mytilus californianus intermixed with barnacles Balanus glanduladifferential pressure on either side may create sufficient lift to tear away the mussels.  Denny 1987 J Exp Mar Biol Ecol 113: 231.

NOTE  see elsewhere in this learn-about section on MUSSELS: POPULATION & COMMUNITY ECOLOGY: COMMUNITY SUCCESSION

A clump of mussels Mytilus californianus is at greater risk of being removed
by waves owing to the presence of drag-producing barnacles Balanus glandula


Patch in a mixed community of mussels Mytilus
, goose barnacles Pollicipes polymerus,
and acorn barnacles Balanus glandula

Research study 5

map showing study sites in Barkley Sound, British Columbia for research on wave effects on mussels Mytilus trossulusphotographs of mussels Mytilus trossulus showing effects of waves on growth of shellDegree of wave exposure has other effects on growth form in bay mussels, as shown in a detailed study of juvenile Mytilus trossulus of 29-35mm shell length in Barkley Sound, British Columbia (see map).  In comparison with specimens from sheltered sites, ones from wave-exposed sites have a squatter more bulbous shape and thicker shell and ligament (see photographs on Right). Additionally, the wave-exposed individuals have higher more robust dysodont teeth (see photographs lower Left). 

photographs of dysodont teeth of mussels Mytilus trossulus showing effects of wave exposure on their growth formFor a given shell length in adult mussels, mean shell thickness at a typical wave-exposed site is about 60% greater than at a sheltered site.  Comparable difference in ligament thickness is about 50%. Juveniles collected from a wave-exposed shore and a wave-sheltered shore separated by only a few hundred meters display similar differences.  The authors hypothesise that a lower, more cylindrical shell shape in wave-exposed habitats may reduce hydrodynamic forces of drag, thus decreasing the probability of dislodgment and increasing overall fitness.  A thicker shell has obvious survival benefits, both for resisting wave impact and perhaps also predation, although the authors note that the latter factor has yet to be investigated. Although the function of the dysodont teeth in mytilids is not known, a greater degree of interlocking may have selective advantage in keeping the valves aligned and tightly shut during periods of high wave impact.  Proper alignment of the valves during wave exposure may also explain the thicker, and likely stronger, hinge ligament.  The authors note that their findings represent the first correlation between wave exposure and dysodont teeth/ligament morphology in a bivalve.  Akester & Martel 1999 Can J Zool 78: 240.

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