Estuarine Larval Transport

• Estuarine biological/physical environment • Selective Tidal Stream Transport• Endogenous rhythms vs. exogenous cues• Scalar vs. vector cues• A few examples of crab larval behavior

RiverShallowerWarmerFresher

OceanDeeperColderSaltier

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

Sea birds Raccoons Sea turtles

Sharks & rays Many fish

Jellyfish Ctenophores Juvenile fish

Estuaries have terrestrial and aquatic predators

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.

Salt-wedge

PartiallyMixed

Well-Mixed

Currents can be used by larvae to get into or out of estuary

Flux of larvae (horizontal motion) depends on velocity and concentration

Flux = velocity x concentration

[#/m2/s] [m/s] x [#/m3]Queiroga & Blanton 2005

Vertical migration patterns lead to Selective Tidal Stream Transport (STST)

Forward & Tankersley 2001

Flood-tide TransportMove into estuary

Ebb-tide transportMove out of estuary

Nocturnal flood-tide transportMove into estuary at night

Two crabs from San Diego Bay: different STST strategies

Lined shore crab

Pebble crab

DiBacco et al. 2001

Shore crab Pebble crab

Surface

Mid-depth

Bottom Bottom

Mid-depth

Surface

Ebb tideEbb tide

DiBacco et al. 2001

(Ebb-tide transport) (NO migration)

DiBacco et al. 2001

Ebb tide transport No vertical migrationFlood: sink to bottomEbb: swim to surface

Virtual larvae with and without vertical migration have different export rates

Shore crab 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

(water comes in from sea)– Temperature drops – Salinity increases – Depth/pressure increase

Ebb tide

(water goes out to sea)– Temperature increases – Salinity drops – Depth/pressure drop

Exogenous cues - two types of behaviors

-kinesis

Non-directional movement in response to a stimulus

Temperature Thermokinesis

Pressure Barokinesis

Salinity Halokinesis

-taxis

Directional movement in response to a stimulus

Light Phototaxis

Gravity Geotaxis

Current 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

Salinity toleranceLarvae: >20 pptAdults: 3 to 15 ppt

Zoea (7 stages):

1 to 1.5 months

Megalopa stage:

up to 2 months

Juvenile crab

(20 molts)

Adult

Blue crab life cycle

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

Unsustainable <70 million

Abundance Target >215 million

http://chesapeakebay.noaa.gov/fish-facts/blue-crab

2008 - Fishery declared a federal disaster - New rules on harvest of females

2010 - Recovery to above target

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

Queiroga & Blanton 2005

HWS = High Water SlackLWS = Low Water Slack

-Females do the work of getting larvae out to sea-Larvae use nocturnal ebb migration to escape

Megalopae return by nocturnal flood tide transport

Queiroga & Blanton 2005

HWS = High Water SlackLWS = Low Water Slack

Megalopae swim up in response to increasing pressure, salinity (flood tide indicators)

Tankersley et al. 1995

Per

cent

of

larv

ae in

top

of

cham

ber

Welch and Forward 2001

Even more megalopae swim when both turbulence and salinity increase (flood tide indicator)[but not when salinity decreases (ebb tide indicator)]

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

Blue crab megalopae have complex behaviors

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 pptHave long spines to deter predatorsBetter equipped to stay in an estuary

Mud crab map

Queiroga & Blanton 2005

HWS = High Water SlackLWS = Low Water Slack

Early stages stay at mid-depth

Late stages sink

Salinity

Current velocity

Mean depth of early-stage mud crab larvae

Cronin 1982

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)

Mud crab larvae have complex behaviors

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