Larval dispersal and connectivity Flashcards
For marine species, the larval phase is the dominant dispersal stage and a driver of population connectivity.
Why is understanding the drivers of larval dispersal is a biophysical problem?
- Biological – (processes we can study) processes influencing offspring production, growth - linked to larval duration, development, and survival - types and abundances od predation
- Physical – advection and diffusion properties of water circulation. Where will they be pushed or pulled to?
- Biophysical – interactions between biological and physical i.e. vertical swimming behaviour and water column stratification (do we see big seasonal differences in where they are going to end up?).
What are some of the processes that are going to drive dispersal?
Time and space scales relevant to different approaches to the study of larval dispersal. A challenge for the future is to integrate these methods.
- Physical oceanography
- Larval behavior
- Tracking fingerprint
- Population genetics
- Historical Processes (Geology/Climate)
Break these processes down into thinking about what’s going on with our larvae, on their journey and how that maps onto what we see and where we find them.
- Spawning output
- Larval dispersal (Currents)
- Larval dispersal + behaviour
- predator/prey mediated survival
- Available settlement habitat
- Post - settlement (larval condition)
What are some of the physical processes affecting the larvae?
- Retention mechanisms
- Coastal topography creates regions of reduced flow and/or retention
- Seabed topography can aid retention as near-shore flows are reduced by bed friction
- Frontal convergences and submesoscale eddies may enhance retention of larvae
- Deepwater currents
- Hydrographic features
- Disruption of current flow along oceanic ridge axes
- ‘Stepping stone’ model
Retention mechanisms
Various different oceanographic and hydrographic retention mechanisms (get spawned but never released from the local environment - often coastal or estuarine) (Epifanio & Garvine, 2001, Est. Coast. Shelf. Sci. 52: 51-77)
Shallow water systems were once considered open with a free exchange of larvae and lack of physical barriers, yet now understand that more physical barriers occur - such as the Humber estuary.
Topography
- Coastal topography creates regions of reduced flow and/or retention (Mace & Morgan, 2006).
- is it flat sandy beaches/bays and coves - changing the flow
- Seabed topography can aid retention as near-shore flows are reduced by bed friction
- The way in which water behaves when it gets shallower interacts with the friction generated forces on the seabed - having an effect on that flow.
- A rough seabed topography can act to increase retention.
- These systems may change over a tidal cycle as the water ebbs and flows
Processes driving dispersal and connectivity in the deep ocean
Many of these processes have not been fully resolved for deep-water systems
Yet abiotic conditions are relatively stable over large distances
- Cosmopolitan distributions of species are common yet many taxa show significant population structure
- Youll find deepwater species of echinoderms in the high north Atlantic as you do in the Wedell sea - absolutely the same species.
- Biology shows a long dispersal but not capable of across the whole ocean - stepping stone process.
- Deepwater currents, direction and speeds can influence the direction dispersal will take place.
- Hydrographic features may trap larvae over seamounts (Brewin et al., 2009, Mar. Ecol. Prog. Ser. 383: 225-237) or pose barriers to dispersal between basins (i.e. Antarctic polar front).
- Seamounts - can entrain plankton in what’s called a tailor column. Can also create a barrier to dispersal for passing organisms.
Mid-ocean ridges.
We may see limited genetic dispersal around these features
- Disruption of current flow along oceanic ridge axes may limit dispersal and generate genetic diversity among hydrothermal vent systems - producing species with smaller ranges (i.e. Bathymodiolus sp. Won et al., 2003, Mol. Ecol. 12: 169-184)
- The sheer distance can form a barrier - even on homogeneous abyssal plains distance effects on dispersal may lead to population differentiation and turnover on regional scales
- Distance effects frequently invoked for chemosynthetic habitats (Shank & Halanych, 2007, Mar. Ecol. Evol. Perspect. 28: 25-35)
- ‘Stepping stone’ model – differences between vent sites at fast and slow spreading centres. The way in which habitat is created and moved is very different in these areas.
- Pacific ocean - fast-spreading centres
- Mid Atlantic ridge - slow-spreading
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Dispersal at vents
- Data looking at the relationship between what we know about current speeds and the larval densities recorded in these areas.
- The density of larvae varies alot for different species and for different ‘set-ups’. 9 degrees north, higher larval density than the sedimented Guaymas Basin.
- The density of larvae may be higher at sedimented sites, higher in places like 9 degrees north - slower current speeds - more retention of larvae.
- Flow is very variable, generally faster around the mid-Atlantic ridge and slow in areas around the east pacific rise
- Can look at how flow varies at different ridge features.
- Fast spreading - East Pacific Rise
- Slow spreading - Mid Atlantic Ridge
- Flow is often low but maybe variable and turbulent
- The position of the larvae in the water column is significant, as flow direction and speed varies with height above the bottom
- Available data suggest that density of larvae may be higher over sedimented sites, such as the Guaymas basin, than at rocky sites (JdF)
Dispersal ability may be inferred from:
Dispersal ability may be inferred from:
- length of larval life (PLD) and the speed and direction of ambient currents;
- direct collections of planktonic larvae;
- studies of respiration
- studies of genetic similarity between disjunct populations
The depths at which larvae disperse may be inferred from biochemical markers, and from studies of physiological tolerances
What evidence has been created from hydrothermal systems that have done direct larval sampling?
- Net sampling - larvae of vent gastropods and Calyptogena were associated with the plume whilst non-vent larvae found only in non-plume seawater
- EPR - Modelled larval dispersal in both plume and near-bottom currents - mean vertical flux of 100 larvae h-1 in a single black smoker plume - plumes are a significant mechanism for dispersal (larvae rise with the plume).
- Demonstrate that the main transport of larvae is by cross-axis flows with tidal excursions of up to 2 km (retention of larvae, trapped by cross-axis tidal flows). Flows near the seabed carried more larvae than those 15m above the bed.
- Suggest that plume-level transport dominates <26% of time,
- even in vigorous black smokers, and that <3% of larvae are entrained in plumes
Vent biogeographic provinces
Vents occurring in separated biogeographic provinces was first proposed by Bachraty in 2009 and supplemented by when Ben went to Antarctica and found new hydrothermal vents and a separate community.
The pacific - along the east pacific rise there are several distinct communities quite close to each other. Segments of ridge can be active whilst others can be dead.
Mid Atlantic - slow-spreading ridge, often contains good connectivity between areas of activity, only separated by large fracture zones.
Faster spreading ridges - much more influence of non-transform default features. Which are little breaks in the ridge structure, which can create a barrier to dispersal.
Examples of potential biogeographic filters or conduits for dispersion of larvae
- NTD offset is short, allowing effective dispersal,
- constricted and/or irregular NTD path hinders dispersal, despite the favourable bottom current regional flow
- dispersal between adjacent segments is aided by prevailing flow direction along interconnecting NTD
- FZ links active segments, and exchange of propagules is relatively unrestricted
- barren segments and adverse flow direction constrain dispersal
- isolated community is prevented from wider dispersal by FZ and inactive adjacent segments
What biological processes have an effect on larval movement in the deep ocean?
Interaction of biological responses and behaviours with the environment can moderate dispersal and connectivity
- Adult nutrition
- Seasonality of spawning behaviour - when they are spawning, whether they are spawning in synchronous nature?
- Maternal condition – egg and larval quality
- Larval behaviour - are they actively orientating or associating themselves with particular features?
Majority of invertebrate larvae are largely passive
Crustacea and fishes – large vertical migrations and directed horizontal swimming
Some larvae can actively orient and navigate to suitable settlement habitats using auditory, olfactory and other cues (Montgomery et al., 2006, Adv. Mar. Biol. 51: 143-96)
Developmental modes - review
Split larvae into groups based on nutrition, as this directly influences what we see in the water column.
Planktotrophic development is planktonic and feeds whilst a planktonic larva – has a long larval life
Pelagic lecithotrophic development is also planktonic but does not feed during its pelagic existence – has a relatively short pelagic life
Non-pelagic lecithotrophic development remains closely associated with the benthos during its larval life
Direct Development Has no free-living larva and emerges from the egg capsule as a fully formed juvenile
Planktonic Larval Duration (PLD)
- From minutes to years
- Species with non-feeding (i.e. lecithotrophic larvae) have a finite suite of resources and, therefore, a limited larval life span
- Feeding larvae have the possibility of delaying metamorphosis to the benthic adult until a suitable settlement site is located
- PLD is one of the fundamental components examined in the study of population connectivity.
- Mytilus edulis - 32 days
- Red crabs 68 - 150 days.
- The longer you can stay in the plankton - the further you can travel
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Larval behaviour - Chesapeake bay
North et al., (2008) Mar. Ecol. Prog. Ser. 359: 99-115
Looked at different current speeds and different behaviours in our larvae.
Four different scenarios
- Passive drifting vs active swimming
- 4 behavioural scenarios
- 3 swimming speeds
The vertically migrating larvae - as you increase their swimming speeds they are able to retain themselves more in the bay - not be dispersed out into the ocean.
Case study – Young et al. (2012). Dispersal of deep-sea larvae from the Intra-American Seas: Simulations of Trajectories using Ocean Models. Integrative and Comparative Biology, 52 (4): 483-496
can read more about, just what ben said about the tubeworm
Looked at oceanic currents taken from models, looks at dispersal of 7 species when you have the biological data available.
Used velocities of oceanic currents (from hindcast models) to assess larval dispersal of 7 species at 2 depths (100 & 500m) – passive particle releases of 200-250 ‘larvae’ per simulation (200)
The study aimed to address the following questions;
- What is the relationship between PLD and dispersal potential for seep and non-seep slope animals?
- How much does dispersal potential vary from year to year?
- Do dispersal models demonstrate potential connectivity among distant deep-sea populations?
- Do larvae migrating to upper water column disperse significantly greater distances than those below the permanent thermocline?
- How do dispersal trajectories relate to known distributions about where we find the adults?
Example - Lamellibranchia luymesi – methane seep tubeworm.
- Only species occurring at all 3 sites
- Modelled predictive releases of a spawning population in the Carribean, at the Bahamas and on the Louisiana slope.
- Predicted with a 390-day post-larval duration, dispersing at 100 and 500m from the surface. Shows the Louisiana slope population has high retention and does not mix into the Atlantic much.metapopulation.
- The population from the Bahamas is being taken north and distributed out, with a large number of losses over the deep north Atlantic.
- 13-month duration - long-lived in the plankton.
- Transport from all sites followed overall pattern of North Atlantic Gyre
- Sufficient retention in all areas – GoM is a ‘sink’ metapopulation
- Larvae entering GoM are entrained without reaching Louisiana slope seep habitats
