Unlikely Nomads: Settlement, Establishment, and Dislodgement Processes of Vegetative Seagrass Fragments

Abstract

The dispersal of seagrasses is important to promoting the resilience and long-term survival of populations. Most of the research on long-distance dispersal to date has focused on sexual propagules while the dispersal of vegetative fragments has been largely overlooked, despite the important role this mechanism might play. In this study, we proposed a conceptual model that categorizes vegetative fragment dispersal into seven fundamental steps: i.e., (i) fragment formation, (ii) transport, (iii) decay, (iv) substrate contact, (v) settlement, (vi) establishment, and (vii) dislodgement. We present two experiments focusing on the final steps of the model from substrate contact to dislodgement in four tropical seagrass species (Cymodocea rotundata, Halophila ovalis, Halodule uninervis, and Thalassia hemprichii), which are critical for dispersed vegetative fragments to colonize new areas. We first conducted a mesocosm experiment to investigate the effect of fragment age and species on settlement (i.e., remains on the substrate in a rising tide) and subsequently establishment (i.e., rooting in substrate) rates. To determine dislodgement resistance of settled fragments, we also subjected fragments under different burial treatments to wave and currents in a flume. We found that both initial settlement and subsequent establishment rates increased with fragment age. H. ovalis was the only species that successfully established within the study period. After settlement, dislodgement resistance depended primarily on burial conditions. Smaller species H. ovalis and H. uninervis were also able to settle more successfully, and withstand higher bed shear stress before being dislodged, compared to the larger species T. hemprichii and C. rotundata. However, the ordinal logistic regressions did not reveal relationships between the tested plant morphometrics and the energy needed for dislodgement (with the exception of C. rotundata), indicating that there are potentially some untested species-specific traits that enable certain species to withstand dislodgement better. We discuss the implication our findings have on the dispersal potential for different species and the conservation of seagrasses. This study represents the first effort toward generating parameters for a bio-physical model to predict vegetative fragment dispersal.

Persistent Clones and Local Seed Recruitment Contribute to the Resilience of Enhalus acoroides Populations Under Disturbance

“…No shared multilocus genotypes were found among populations, even within the same lagoon, hence genetic connectivity between populations of E. acoroides is only realized by sexual propagules and confirms the assumption that successful dispersal of vegetative fragments of large-sized seagrass species is low to non-existent ( Lai et al, 2018 ). The genetic structure among lagoons is not explained by geographic distance with distant lagoons Thieu Thrieu and Van Phong Bay (~70 km distance) being less differentiated than neighboring lagoons Xuan Day Bay and Cu Mong (~14 km distance).…”

Global biogeography of chemosynthetic symbionts reveals both localized and globally distributed symbiont groups

“…Thiodiazatropha 16S rRNA gene sequences associated with the roots of various seagrass species around the globe, a discovery the authors subsequently verified with microscopic imaging (37). Vegetative seagrass fragments (shoots and rhizomes) can reestablish in distant locations after detaching from their parents and have great potential for long-distance dispersal (39,40). It is interesting to speculate that lucinid clams and seagrasses from across the globe could form a network of source habitats facilitating the dissemination and dispersal of Ca.…”

The relative importance of light and hydrodynamics in the decay and transport of vegetative seagrass fragments

“…Key parameters, such as mortality rates, buoyancy, and settling velocities, tend to be more readily available for sexual propagules such as fruits and seeds (Ruiz-Montoya et al, 2012;McMahon et al, 2014;Wu et al, 2016), compared to asexual propagules such as vegetative fragments (Weatherall et al, 2016;Lai et al, 2018). Long-distance dispersal in seagrasses is often attributed to fruits, which are buoyant in many species and are known to travel vast distances (McMahon et al, 2014).…”

“…Vegetative fragment dispersal can be divided into seven core processes (Lai et al, 2018) Dislodgement; and (vii) Decay. Together, these processes determine how different species traits and environmental factors affect dispersal via vegetative fragments.…”