A novel adaptation facilitates seed establishment under marine turbulent flows

Abstract

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.

Figure 1

(A) Micro-CT scans Posidona coriacea (P. c), P. australis (P. a) and P. sinuosa (P. s) showing size of wing in relation to the seed (scale = 5000 µm), (B) differences in the width of the wing taken at the widest point for seeds (n = 30) for Posidonia species, (C) surface Area, (D) volume and (E) surface Area to Volume ratio determined from micro-CT are shown for 3 paired replicate seeds for each species with and without the wing.

Figure 2

Scanning electron micrographs of surface (A,C,E) and cross-section (B,D,F) of a keel of P. coriacea (A,B), P. australis (C,D) and P. sinuosa (E,F). Bars = 200 µm (A,C,E) or 20 µm (B,D,F). The arrowhead in (A,C,E) indicates base of the membranous wing; W, wing; S, seed. The arrows in (B,D,F) indicate ‘flattened’ cells.

Figure 3

Variation in rotational (A), drag (B) and lift (C) coefficients (normalized by the seed plan area) for P. coriacea, P. australis and P. sinuosa with and without a wing for different positions on the seabed.