A novel adaptation facilitates seed establishment under marine turbulent flows

Seeds of Australian species of the seagrass genus Posidonia are covered by a membranous wing that we hypothesize plays a fundamental role in seed establishment in sandy, wave swept marine environments. Dimensions of the seed and membrane were quantified under electron microscopy and micro-CT scans, and used to model rotational, drag and lift forces. Seeds maintain contact with the seabed in the presence of strong turbulence: the larger the wing, the more stable the seed. Wing surface area increases from P. sinuosa < P. australis < P.coriacea correlating with their ability to establish in increasingly energetic environments. This unique seed trait in a marine angiosperm corresponds to adaptive pressures imposed on seagrass species along 7,500 km of Australia’s coastline, from open, high energy coasts to calmer environments in bays and estuaries.


Rotation, Drag and Lift
A seed will rotate on the seabed when the rotational moment ( ! ) is not in balance ! ≠ 0 . ! is non-zero when a force applied to the seed at a distance from the seed's centre of rotation is not balanced by an equivalent opposing force. The corresponding coefficient of rotation can then be determined from this moment balance: where ! is the is the rotational moment coefficient about the axis specified by the centre of rotation, ! is a reference area for the coefficient, ! is the water density and is the velocity of the water to which the seed is exposed.
A seed will begin to slip along the bed when the drag force ! exceeds the friction between the bed and the submerged gravitational weight of the seed ! less the lift ! that has developed: where is the static friction coefficient (which was calculated using a friction angle of 30°). The drag acting on an object is defined as: where ! is the coefficient of drag, ! is a reference area for the coefficient, ! is the water density and is the velocity of the water to which the seed is exposed.
A seed positioned on the bed and subjected to flow is subjected to a lift force, which develops due to the acceleration of the flow near the seed and results in a vertical pressure gradient develops. This lift force can be both positive (acting to lift an object off the bed) or negative (acting to press an object onto the bed). The lift force can be defined as: where ! is the coefficient of lift and ! is a reference area for the coefficient.
The submerged weight of the seed was calculated as: where is the seed's submerged mass and is the gravitational acceleration constant.
In our analysis, we calculated the drag forces, lift forces and rotational moments directly in the numerical model. We then calculated the respective coefficients for each case and normalised this coefficient using projected plan area (for ! , ! and ! ) for each seed species and form (with / without a wing). This enabled comparison of the coefficients for each position on the bed. The projected plan area was obtained from the numerical model post-processing and verified with the area calculated from from the µCT analysis. For Reynolds numbers above 10 4 (i.e., for cases analyzed in this study), seeds of same species and form are expected to differ only in dimensions but not form, thus we expected that cofficents values of will be approximately constant. This was verified in our analysis by comparing the coefficients for a single velocity applied to all cases as well as the velocity that initiated movement for each individual case based on laboratory experiment results.
. To simplify (homogenise) the image intensities to facilitate seagrass seed feature extraction.

Edit new label field
Threshold tool and magic wand tool -applied to all slices To generate label fields defining the specific regions of interest -including: whole seed, seed kernel, pedicel, major keel, minor keel, whole seed (minus keel, pedicel, seed and minor keel, respectively).

Label analysis 3D, basis measures
To calculate the surface area of each label field. Table S2. Avizo workflow for calculation of maximum and minimum projected areas -based on filtered data from step 2 in Table S1.
Step Avizo module Sub-settings Description To generate projections of the seed profile against the nominated planes and to then extract an image from each projection.

Edit new label field
Threshold tool To create a label field for the projected image of the seed.

6.
Label analysis xy planes To calculate the surface area for each label (seed projection)