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

Peter A Todd,Tricia S M Poh,Tjeerd J Bouma,Samantha Lai,Siti Maryam Yaakub

doi:10.3389/fpls.2018.00160

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.

Highlights

The study of the movement ecology determining how seagrasses disperse is critical to understanding the exchange of genetic material, and their persistence in changing environments (McMahon et al, 2014; Weatherall et al, 2016)
The three species settled at significantly different rates (p < 0.01) (Figure 2A), with the majority of Halophila ovalis fragments (77.7%) settling by the end of the experimental period, compared to 55.5% for H. uninervis and only 7.7% for Cymodocea rotundata
None of the H. uninervis or C. rotundata fragments established by the end of the 14 days, whereas 78.2% of the surviving H. ovalis fragments did (Figure 2B)

Summary

Introduction

The study of the movement ecology determining how seagrasses disperse is critical to understanding the exchange of genetic material, and their persistence in changing environments (McMahon et al, 2014; Weatherall et al, 2016). Seeds and pollen for many species tend to be neutrally or negatively buoyant (Harwell and Orth, 2002; Orth et al, 2006), leading to them being transported over short distances and remaining within the parent meadow (Kendrick et al, 2012). Seeds can be dispersed by secondary biotic vectors such as dugongs, fish or waterfowl, for distances ranging from meters to thousands of kilometers (Sumoski and Orth, 2012; McMahon et al, 2014). Long-distance dispersal has generally been attributed to buoyant fruits or reproductive shoots bearing seeds which, depending on hydrodynamic conditions, can travel up to hundreds of kilometers from the source meadow (Harwell and Orth, 2002; Erftemeijer et al, 2008; van Dijk et al, 2009; Ruiz-Montoya et al, 2012)

Methods

Results

Conclusion