Estuarine Larval Transport
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Estuarine biological/physical environment
Selective Tidal Stream Transport
Endogenous rhythms vs. exogenous cues
Scalar vs. vector cues
A few examples of crab larval behavior
River
Shallower
Warmer
Fresher
Ocean
Deeper
Colder
Saltier
Water level goes up and down
with tidal cycle.
Range can be a few meters.
Diurnal tide:
~24 hour cycle
Semidiurnal tide:
~12 hour cycle
Low tide
High tide
Estuarine ecosystem includes intertidal zones
Estuaries have terrestrial and aquatic predators
Sea birds
Raccoons
Sharks & rays
Jellyfish
Many fish
Ctenophores
Juvenile fish
Sea turtles
Estuaries are regions of environmental extremes
Depending on balance of tidal forcing and river input:
• Temperature
– Shallow water warms up faster than deep water
– Temperatures up to >30 oC, like bath water
– Can vary by many degrees in a single tidal cycle
• Salinity
– From 0 ppt (fresh) to 32 ppt or more (marine)
– Can vary by many ppt in a single tidal cycle
Low salinity can be a major stressor for marine
and estuarine animals
Osmoregulation: Active regulation of the salt content in
bodily fluids
Adult animals can bury themselves in the mud where
salinity and temperature are relatively constant.
Larvae are in the water column and have no protection
against heat and low salinity.
Currents can be used by larvae to get into or out of estuary
Salt-wedge
Partially
Mixed
Well-Mixed
Flux of larvae (horizontal motion) depends on
velocity and concentration
Flux =
[#/m2/s]
velocity x concentration
[m/s] x [#/m3]
Queiroga & Blanton 2005
Vertical migration patterns lead to
Selective Tidal Stream Transport (STST)
Flood-tide Transport
Move into estuary
Ebb-tide transport
Move out of estuary
Nocturnal flood-tide transport
Move into estuary at night
Forward &
Tankersley 2001
Two crabs from San Diego Bay: different STST
strategies
Lined shore crab
Pebble crab
DiBacco et al. 2001
Shore crab
Pebble crab
(Ebb-tide transport)
Surface
(NO migration)
Surface
Mid-depth
Mid-depth
Bottom
Bottom
Ebb tide
DiBacco et al. 2001
Ebb tide
Virtual larvae with and without vertical migration
have different export rates
Ebb tide transport
Flood: sink to bottom
Ebb: swim to surface
Shore crab
DiBacco et al. 2001
No vertical migration
Pebble crab
Behavior can have internal or external cues
• Endogenous rhythms
– Synchronized with day/night or tidal cycle
• Responses to Exogenous cues
– Physical cues: temperature, salinity, light,
pressure, currents, turbulence
– Chemical cues: from food, predators,
others of same species
Tides affect environmental conditions
(exogenous cues)
Flood tide
Ebb tide
(water comes in from sea)
(water goes out to sea)
– Temperature drops 
– Temperature increases 
– Salinity increases 
– Salinity drops 
– Depth/pressure increase 
– Depth/pressure drop 
Exogenous cues - two types of behaviors
-kinesis
Non-directional movement in response to a stimulus
Temperature
Pressure
Thermokinesis
Barokinesis
Salinity
Halokinesis
-taxis
Directional movement in response to a stimulus
Light
Gravity
Current
Phototaxis
Geotaxis
Rheotaxis
Two types of cues
• Scalar
– Cue has only magnitude, no direction
– Includes most water column properties:
temperature, salinity, density, concentration of
chemicals
• Vector
– Cue has both magnitude and direction
– Velocity is a vector (by definition)
– Gravity, light, pressure (pseudo-vector)
Vector cues increase/decrease vertically
+
Light
Pressure
-
+
Gravity
+
Taxis is positive or negative depending on
direction of movement
Blue crab life cycle
Adult
Zoea (7 stages):
1 to 1.5 months
Megalopa stage:
up to 2 months
Salinity tolerance
Larvae: >20 ppt
Adults: 3 to 15 ppt
Juvenile crab
(20 molts)
Blue crab fishery
• Tastiest crab species in US?
• Chesapeake Bay fishery worth:
– $200 million in 1994
– $55 million in 2000
• Fishery affected by:
– Habitat loss
– Pollution
– prey shortage: oysters, clams
– excess predators: birds, fish
– low recruitment since 1998
2008 - Fishery declared a federal disaster
- New rules on harvest of females
2010 - Recovery to above target
Abundance Target >215 million
Unsustainable
<70 million
http://chesapeakebay.noaa.gov/fish-facts/blue-crab
2014 – New harvest
reductions
Chesapeake Bay is a major blue crab habitat
Adults tolerate this
salinity range
Larvae tolerate this
salinity range
Blue crab life cycle with migration
-Females do the work of getting larvae out to sea
-Larvae use nocturnal ebb migration to escape
HWS = High Water Slack
LWS = Low Water Slack
Queiroga & Blanton 2005
Megalopae return by nocturnal flood tide
transport
HWS = High Water Slack
LWS = Low Water Slack
Queiroga & Blanton 2005
Percent of larvae in top of chamber
Megalopae swim up in response to increasing
pressure, salinity (flood tide indicators)
Tankersley et al. 1995
Even more megalopae swim when both turbulence and
salinity increase (flood tide indicator)
[but not when salinity decreases (ebb tide indicator)]
Welch and Forward 2001
Blue crab megalopae have complex behaviors
Exogenous cues for swimming up on nocturnal flood tide
• Increase in pressure (pseudo-vector)
• Increase in salinity (scalar)
• Increase in turbulence (scalar)
• Dark
But….
• Increase in turbulence
+ decrease in salinity
• Daytime
+ estuarine water
No reaction
Harris mud crab
Native to East coast of North America
-Has invaded inland lakes, Panama Canal
-Alters food webs
-Fouls water intake pipes
-Virus carrier, infects shrimp and blue crabs
Larvae:
Tolerate wide range of salinities >2.5 ppt
Have long spines to deter predators
Better equipped to stay in an estuary
Mud crab map
Early stages stay
at mid-depth
Late stages sink
HWS = High Water Slack
LWS = Low Water Slack
Queiroga & Blanton 2005
Salinity
Current velocity
Mean depth of earlystage mud crab larvae
Cronin 1982
Mud crab larvae have complex behaviors
Swim up in response to:
Increase in salinity (scalar)
Decrease in temperature (scalar)
Increase in pressure (pseudo-vector)
Sink in response to:
Decrease in salinity (scalar)
Increase in temperature (scalar)
Decrease in pressure (pseudo-vector)
Plus a negative feedback model:
in dark, negative geotaxis (vector; swim up)
in light, negative phototaxis (vector; sink)
Crab larvae have many different behavioral
strategies for migration in/out of estuaries
• Pebble crabs
– Larvae do not vertically migrate
– High dispersal within estuary, little export
• Blue crabs
– Early-stage larvae exported to shelf
– Late-stage larvae have complex behaviors
for getting back into the estuary
• Mud crabs
– Early-stage larvae have complex behaviors
for staying in the estuary
– Estuarine throughout life cycle