Population structures and levels of connectivity for Scyphozoan and Cubozoan jellyfish

Understanding the hierarchy of populations from the scale of metapopulations to mesopopulations and member local populations is fundamental to understanding the population dynamics of any species. Jellyfish by definition are planktonic and it would be assumed that connectivity would be high among local populations, and that populations would minimally vary in both ecological and genetic clade-level differences over broad spatial scales (i.e., hundreds to thousands of km). Although data exists on the connectivity of scyphozoan jellyfish, there are few data on cubozoans. Cubozoans are capable swimmers and have more complex and sophisticated visual abilities than scyphozoans. We predict, therefore, that cubozoans have the potential to have finer spatial scale differences in population structure than their relatives, the scyphozoans. Here we review the data available on the population structures of scyphozoans and what is known about cubozoans. The evidence from realized connectivity and estimates of potential connectivity for scyphozoans indicates the following. Some jellyfish taxa have a large metapopulation and very large stocks (>1000 s of km), while others have clade-level differences on the scale of tens of km. Data on distributions, genetics of medusa and polyps, statolith shape, elemental chemistry of statoliths and biophysical modelling of connectivity suggest that some of the ~50 species of cubozoans have populations of surprisingly small spatial scales and low levels of connectivity. Despite their classification as plankton, therefore, some scyphozoans and cubozoans have stocks of small spatial scales. Causal factors that influence the population structure in many taxa include the distribution of polyps, behavior of medusa, local geomorphology and hydrodynamics. Finally, the resolution of patterns of connectivity and population structures will be greatest when multiple methods are used

Behavioural and oceanographic isolation of an island‑based jellyfish (Copula sivickisi, Class Cubozoa) population

Cubozoan jellyfish are classified as plankton despite the strong swimming and orientation abilities of cubomedusae. How these capabilities could affect cubozoan population structures is poorly understood. Medusae of the cubozoan Copula sivickisi can uniquely attach to surfaces with the sticky pads on their bells. Biophysical modelling was used to investigate the spatial scales of connectivity in a C. sivickisi population. When the medusae were active at night they could maintain their observed distribution on fringing reef if they attached to the reef when the current speed exceeded a moderate threshold. This behaviour facilitated the isolation of a C. sivickisi population on reefs fringing Magnetic Island, Queensland, Australia. Within this distribution, there was considerable within bay retention and medusae rarely travelled > 3 km. The few (< 0.1%) medusae lost from the island habitat were largely advected into open water and away from the mainland coast which lies 8 km from the island. Given that successful emigration is unlikely, the island population probably represents a stock that is ecologically distinct from any mainland populations. The cosmopolitan distribution of C. sivickisi could contain incipient or cryptic species given the small scales of connectivity demonstrated here

Bio-physical determinants of sediment accumulation on an offshore coral reef: A snapshot study

Sediments are found on all coral reefs around the globe. However, the amount of sediment in different reservoirs, and the rates at which sediments move between reservoirs, can shape the biological functioning of coral reefs. Unfortunately, relatively few studies have examined reef sediment dynamics, and associated bio-physical drivers, simultaneously over matching spatial and temporal scales. This has led to a partial understanding of how sediments and living reef systems are connected, especially on clear-water offshore reefs. To address this problem, four sediment reservoirs/sedimentary processes and three bio-physical drivers were quantified across seven different reef habitats/depths at Lizard Island, an exposed mid-shelf reef on the Great Barrier Reef. Even in this clear-water reef location a substantial load of suspended sediment passed over the reef; a load theoretically capable of replacing the entire standing stock of on-reef turf sediments in just 8 h. However, quantification of actual sediment deposition suggested that just 2 % of this passing sediment settled on the reef. The data also revealed marked spatial incongruence in sediment deposition (sediment trap data) and accumulation (TurfPod data) across the reef profile, with the flat and back reef emerging as key areas of both deposition and accumulation. By contrast, the shallow windward reef crest was an area of deposition but had a limited capacity for sediment accumulation. These cross-reef patterns related to wave energy and reef geomorphology, with low sediment accumulation on the ecologically important reef crest aligning with substantial wave energy. These findings reveal a disconnect between patterns of sediment deposition and accumulation on the benthos, with the ‘post-settlement’ fate of sediments dependent on local hydrodynamic conditions. From an ecological perspective, the data suggests key contextual constraints (wave energy and reef geomorphology) may predispose some reefs or reef areas to high-load turf sediment regimes

Wind Conditions on the Great Barrier Reef Influenced the Recruitment of Snapper (Lutjanus carponotatus)

Most coral reef fishes have a pelagic larval stage before recruiting to reefs. The survival of larvae and their subsequent recruitment can drive the dynamics of reef populations. Here we show that the recruitment of the snapper Lutjanus carponotatus to One Tree Island in the Capricorn Bunker Group, in the southern Great Barrier Reef, was highly variable over 23 years. We predicted that the currents in the Capricorn Bunker Group, including their wind driven components and the Capricorn Eddy (a nearby transient oceanic eddy), would affect patterns of recruitment. A biophysical model was used to investigate this prediction. L. carponotatus were collected from One Tree Island and the dates when they were in the plankton as larvae were determined from their otoliths. The winds present during the pelagic phases of the fish were examined; they were found to have survived either longshore (SSE) winds that induced little cross shelf movement in the larval plume or cross shelf (ENE) winds that induced little longshore movement. The unidirectional transportation of the larval plume in these conditions was favorable for recruitment as it kept the plume concentrated in the Capricorn Bunker Group. These winds were more prevalent in the periods of peak L. carponotatus production that preceded high recruitment. Dispersal under average winds (6.2 m s−1 from the prevailing ESE) and strong winds (velocity 1.5 times average), with and without the Capricorn Eddy, was also modeled. Each of these combinations were less favorable for recruitment than the longshore and cross shelf winds larval L. carponotatus survived before reaching OTI. The larval plume was comparatively less concentrated in the Capricorn Bunker Group under average winds. Strong winds transported the larval plume far longshore, to the NW, away from the Capricorn Bunker Group, while the Capricorn Eddy transported larvae seaward into oceanic waters. Larval swimming could counteract these dispersive forces; however, significant dispersion had occurred before larvae developed strong swimming and orientation abilities. This study provides a physical proxy for the recruitment of snapper. Further, we have demonstrated that great insights into recruitment variability can be gained through determining the specific conditions experienced by survivors

Swimming behaviour can maintain localised jellyfish (Chironex fleckeri: Cubozoa) populations

The potentially lethal Chironex fleckeri (Class Cubozoa) inhabits estuarine and nearshore coastal waters in the western Pacific. The spatial scales of connectivity between C. fleckeri populations are poorly understood. Biophysical modelling of Port Musgrave, a 17 × 21 km shallow bay in tropical Australia, was used to investigate the potential for connections between populations separated by medium (10s of km) to small (100s of m) spatial scales. We measured the swimming speeds and orientations of medusae ranging in size from 4 to 12 cm interpedalial distance (the distance between two adjacent corners on the bell of the medusae). Medusae swam longshore at average speeds (5.3 ± 3.5 cm s-1 SD) that exceeded the local average current speeds (2.7 ± 2.4 cm s-1). These and other ecological data were used to parameterise the biophysical model. No medusae modelled as passive were advected from the bay in 14 d; <2.5% of swimming medusae were lost. When medusae swam directionally, a high percentage aggregated in shallow waters within 10s to 100s of m of the seeding locations. Newly metamorphosed medusae are likely to be retained in the bay through a combination of ‘sticky water’ (i.e. water with reduced current speeds, reduced through diversion around obstacles) in shallow complex habitats and favourable currents. C. fleckeri are vulnerable to low salinities; however, modelling a strong flood revealed higher salinity refugia in shallow water. As there was high retention within the system, we conclude that populations of C. fleckeri inhabiting shallow, semi-enclosed estuarine bays probably represent stocks. Within these stocks, swimming and favourable currents may minimise connectivity and maintain populations at multiple spatial scales

Experimental validation of the relationships between cubozoan statolith elemental chemistry and salinity and temperature

Knowledge surrounding the movements of jellyfish is limited, but elemental chemistry has the potential to elucidate these movements. The objective of this study was to experimentally validate elemental chemistry as a technique which may provide insight into the movements of cubozoans. The approach used a laboratory experiment on the well-known cubozoan species Chironex fleckeri, examining the relationship between statolith elemental chemistry and temperature and salinity. Strong evidence was found that statolith Sr:Ca varied with temperature and that this was independent of variation in salinity. Sr:Ca ratios in saltwater varied little with variation in temperature or salinity. Accordingly, a physiological mechanism within C. fleckeri must have affected statolith Sr:Ca, causing it to vary with temperature. Based on the experimental data from this study and correlative evidence that Sr:Ca varied with temperature in another cubozoan, Copula sivickisi, we provide robust evidence that statolith Sr:Ca can be utilised as a proxy for temperature and may be applicable to other species of cubozoans. Ba:Ca in statoliths was found to vary with both temperature and salinity. As a result, it was determined that Ba:Ca profiles in statoliths have the potential to help resolve jellyfish movements in some circumstances. The use of elemental chemistry to elucidate horizontal or vertical movements of cubozoan species has significant potential and application

The study of sediments on coral reefs: a hydrodynamic perspective

There is a rich literature on coral reef sediments. However, this knowledge is spread among research fields, and the extent to which major sediment reservoirs and reservoir connecting processes have been quantified is unclear. We examined the literature to quantify where and how sediments have been measured on coral reefs and, thereby, identified critical knowledge gaps. In most studies, sediments in one reservoir or one sedimentary process were quantified. The measurement of water column sediments (55% of reservoir measurements) and sediment trapping rates (42% of process measurements) were over-represented. In contrast, sediments on reef substrata, and the transition of sediments from the water column to the benthos, were rarely quantified. Furthermore, only ~20% of sediment measurements were accompanied by the quantification of hydrodynamic drivers. Multidisciplinary collaborative approaches offer great promise for advancing our understanding of the connections between sediment reservoirs, and the sedimentary and hydrodynamic processes that mediate these connections

Wind Conditions on the Great Barrier Reef Influenced the Recruitment of Snapper (Lutjanus carponotatus)

Most coral reef fishes have a pelagic larval stage before recruiting to reefs. The survival of larvae and their subsequent recruitment can drive the dynamics of reef populations. Here we show that the recruitment of the snapper Lutjanus carponotatus to One Tree Island in the Capricorn Bunker Group, in the southern Great Barrier Reef, was highly variable over 23 years. We predicted that the currents in the Capricorn Bunker Group, including their wind driven components and the Capricorn Eddy (a nearby transient oceanic eddy), would affect patterns of recruitment. A biophysical model was used to investigate this prediction. L. carponotatus were collected from One Tree Island and the dates when they were in the plankton as larvae were determined from their otoliths. The winds present during the pelagic phases of the fish were examined; they were found to have survived either longshore (SSE) winds that induced little cross shelf movement in the larval plume or cross shelf (ENE) winds that induced little longshore movement. The unidirectional transportation of the larval plume in these conditions was favorable for recruitment as it kept the plume concentrated in the Capricorn Bunker Group. These winds were more prevalent in the periods of peak L. carponotatus production that preceded high recruitment. Dispersal under average winds (6.2 m s −1 from the prevailing ESE) and strong winds (velocity 1.5 times average), with and without the Capricorn Eddy, was also modeled. Each of these combinations were less favorable for recruitment than the longshore and cross shelf winds larval L. carponotatus survived before reaching OTI. The larval plume was comparatively less concentrated in the Capricorn Bunker Group under average winds. Strong winds transported the larval plume far longshore, to the NW, away from the Capricorn Bunker Group, while the Capricorn Eddy transported larvae seaward into oceanic waters. Larval swimming could counteract these dispersive forces; however, significant dispersion had occurred before larvae developed strong swimming and orientation abilities. This study provides a physical proxy for the recruitment of snapper. Further, we have demonstrated that great insights into recruitment variability can be gained through determining the specific conditions experienced by survivors

Behavioural maintenance of highly localised jellyfish (Copula sivickisi, class Cubozoa) populations

The medusae of cubozoan jellyfishes have sophisticated behaviours and are strong swimmers. Therefore, they have the potential to influence their distribution and connectivity among populations. We used ecological and behavioural data in combination with local oceanography to estimate the potential of medusae in the cubozoan Copula sivickisi to disperse from local populations at scales of hundreds of meters to kilometres. The distribution of C. sivickisi was mapped on a fringing reef at Magnetic Island, Queensland, Australia, with underwater jellyfish camera units (JCams). The availability of reef habitat, dominated by Sargassum sp. algae and coral, had a significant effect on the abundance of medusae. Medusae were 11–7 times more abundant at shallow (≤ 4.1 m) and mid-depth (4.2–7 m) sites with high to moderate habitat availability, compared to deep sites (≥ 7.1 m) where habitat availability was low. Further, medusae were absent at sites far from suitable habitat, both alongshore and in deeper waters. Medusae displayed preferential habitat selection. They were found low in the water column near reefs in depth stratified plankton tows and they preferentially attached to Sargassum in a habitat choice experiment. The swimming speeds of C. sivickisi medusae were determined experimentally and were equivalent to or faster than most of the current speeds measured where the populations occured. The results suggested that medusae can attach to habitat and swim against currents to maintain positions on reefs, thereby restricting dispersal. Incipient speciation is highly likely within the species' cosmopolitan distribution

Benthic cyanobacterial mat formation during severe coral bleaching at Lizard Island: The mediating role of water currents

Cyanobacterial mats are increasingly recognised as a symptom of coral reef change. However, the spatial distribution of cyanobacterial mats during coral bleaching has received limited attention. We explored cyanobacterial mat distribution during a bleaching event at Lizard Island and considered hydrodynamics as a potential modifier. During bleaching cyanobacterial mats covered up to 34% of the benthos at a transect scale, while some quadrats (1 m2) were covered almost entirely (97.5%). The spatial distribution of cyanobacterial mats was limited to areas with slower water currents. Coral cover declined by 44% overall, although cyanobacterial mats were not spatially coupled to the magnitude of coral loss. Overall, the marked increase in cyanobacterial mat cover was an ephemeral spike, not a sustained change, with cover returning to 0.4% within 6 months. Cyanobacterial mats clearly represent dynamic space holders on coral reefs, with a marked capacity to rapidly exploit change, if conditions are right