Life at the edge local adaptation and range limits for two estuarine sea slugs
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Life at the edge: local adaptation and range limits for two estuarine sea slugs. Hanna Koch Dr. Patrick Krug California State University, Los Angeles. Local Adaptation. Selection for low salinity tolerance. Selection for high thermal tolerance. Migration (gene flow). Env. 1 - Los Angeles.

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Life at the edge local adaptation and range limits for two estuarine sea slugs

Life at the edge:local adaptation and range limits for two estuarine sea slugs

Hanna Koch

Dr. Patrick Krug

California State University, Los Angeles

Local adaptation
Local Adaptation

Selection for low salinity tolerance

Selection for high thermal tolerance


(gene flow)

Env. 1 - Los Angeles

Env. 2 – San Fran

  • Natural selection produces adaptation, but:

    • Slxn doesn’t always favor same traits in every habitat

    • Animals don’t always stay in one place

    • Adaptation results from selection on a heritable trait

    • Gene flow opposes adaptation

Range limits
Range Limits

  • Represent species’ limits to adaptability

  • Useful for ID’ing & testing limiting factors that shape distributions/abundances of organisms

    • Geographic barriers

    • Abiotic (temperature, salinity)

    • Genetic Constraints (what inhibits adaptation)

    • Biotic Interactions (competitive exclusion)

Intertidal ecosystem
Intertidal ecosystem

  • Model system:

    • Ideal for range limit studies

    • Easily tracked along 1-D, narrow shoreline

      • North & south endpoints

    • Distribution spatially & temporally restricted by tides

      • Exposed to extreme temps & salinities

      • No subtidal refuge

      • Climate change can lead to range shifts

Alderia modesta alderia willowi
Alderia modesta - Alderia willowi

Vaucheria longicaulis

  • Estuarine sea slugs

  • Live on exposed mudflats of salt marshes along North America’s Pacific coast

  • Live & feed on alga that grows on the mudflats

willowi, modesta

Dynamic Range Boundary

Bodega Bay

Alderia modesta

Northern sp.

Tomales Bay (HI)

Tomales Bay (OF)

Alderia willowi

Southern sp.

SF Bay

Range edge vs range center
range edge vs. range center

A. willowi

  • Tomales Bay (stressful):

    • range edge

    • least freshwater input compared

      to surrounding bays (Bodega & SF)

      • Tectonically formed

  • Los Angeles (optimal):

    • range center

    • More stable environment than TB

      • Warmer temps

      • 4-fold less precipitation

      • Preferred higher salinities

Tomales Bay

Range edge


Range center


1. The range-edge population is more locally adapted to low salinity stress than range center

2. Local adaptation over the rainy season is more pronounced at the range edge

3. Low salinity tolerance is a genetic, heritable trait & is favored at the range edge

Methods measuring low salinity tolerance time to death
Methods- measuring low salinity tolerance (time to death)

Vital Staining

2 ‰ SW

Results 1 summer local adaptation across range
Results 1 (summer): local adaptation across range

ANOVA: F2,57 = 10.85, p < 0.001

A. willowi

B > A,

p < 0.001




Mean Time to Death (min)

Bolinas Tomales Bay

Los Angeles

(Range center)

(Range edge)

Range edge exhibits greater local adaptation to low salinity stress, even during summer before the winter rains

Results 2 local adaptation over growing season
Results 2: local adaptation over growing season

Range Center

Range Edge

ANOVA: F1,38 = 244.12,

p < 0.0001

A. willowi

ANOVA: F1,38 = 6.0, p = 0.019

A. willowi

2 days

Mean Time to Death (min)

5.4 hrs

3.7 hrs

3 hrs

September ’10 December ‘10

September ’10 December ‘10

Populations become more adapted over rainy season & even more so @ range edge

Results 3 low salinity tolerance is a heritable trait
Results 3: low salinity tolerance is a heritable trait

A. willowi

A. willowi

Mean Time to Death (min)

Range Center

Range Edge

Generation 0 Generation 1 Generation 2

Low salinity tolerance is (A) genetically based

(B) favored at the range edge

Results 4 comparing low salinity tolerance between sister species
Results 4:comparing low salinity tolerance between sister species

ANOVA: F1,38 = 10.4, p < 0.005

~3 days

Mean Time to Death (min)

~2 days

A. modesta A. willowi

modesta’sability to withstand lower salinities for longer provides greater local adaptation & competitive advantage over willowi in SF


  • Gradients in salinity drive variations in local adaptation for A. willowi across its range

    • @ range center, LA: nat’lslxn on low S tolerance is relaxed

      • Slight local adaptation, only seen over rainy season

    • @ range edge, Tomales Bay: strong slxn on low S tolerance

      • Strong local adaptation, Low S tolerance conferred to offspring

  • A. willowi northern range limit currently fixed @ TB

    • Genetic constraints

  • In SF, the most adapted willowi still never reach same level of tolerance to very low salinity as modesta does

Overall study significance
Overall Study Significance

  • If we can better understand how tolerance for the physical environment limits the geographical range of species, then we can make better predictions of potential ecosystem responses to climate change, which is critical to effective management and conservation

Thank you
Thank You

  • The Krug Lab:

    Dr. Patrick Krug, Dr. Jann Vendetti, Betsy Shimer, Dominique Gordon, Matthew Garchow, Angela Llaban, Julia Vo, Diane Rico, John Martin

Climate change
Climate Change

  • As global temperatures rise:

    • Changes in salinity & rainfall patterns

    • Worldwide range limit shifts

  • Previous studies suggest poleward (upper) bounds set by physiological limits; equatorial (lower) bounds set by interspecific competition

Daily exposure to 2‰ water Daily exposure to heat stress

…correlated with seasonal shifts in temperature & salinity

Seasonal shifts in presence / abundance…

Population Dynamics in Mill Valley - SF

Abiotic stress
Abiotic Stress

  • Salinity gradients can determine distributions w/in estuaries

  • Salinity has not yet been linked to N-S range endpoints in a intertidal animal, yet is critical for estuarine taxa dist. Along coastlines w/ strong latitudinal gradient in precipitation like NE Pacific