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

•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

Migration(gene flow)

Selection for high thermal

tolerance

Selection for low salinity

tolerance

Env. 1 - Los Angeles Env. 2 – San Fran

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

• 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

•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

Vaucheria longicaulis

Northern sp.

Dynamic Range Boundary

Alderia modesta

Alderia willowiSouthern sp.

willowi, modesta

Bodega Bay

Tomales Bay (HI)

Tomales Bay (OF)

SF Bay

range edge vs. range center• 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 BayRange edge

LARange center

A. willowi

Hypotheses

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

2 ‰ SW

Vital Staining

Methods- measuring low salinity tolerance (time to death)

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

Mea

n T

ime

to

Dea

th (

min

)

A. willowi

(Range center) (Range edge)

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

ANOVA: F2,57 = 10.85, p < 0.001

B > A,p < 0.001

Los Angeles Bolinas Tomales Bay

A B B

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

Range Center

Mea

n T

ime

to D

eath

(m

in)

Range EdgeANOVA: F1,38 = 6.0, p = 0.019

A. willowiANOVA: F1,38 = 244.12, p < 0.0001A. willowi

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

3.7 hrs

5.4 hrs

3 hrs

2 days

September ’10 December ‘10 September ’10 December ‘10

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

Mea

n T

ime

to D

eath

(m

in) A. willowi

Range CenterRange Edge

Generation 0 Generation 1 Generation 2

Low salinity tolerance is (A) genetically based (B) favored at the range edge

A. willowi

Results 4:Results 4:comparing low salinity comparing low salinity

tolerance between sister tolerance between sister species species

Mea

n T

ime

to D

eath

(m

in)

A. modesta A. willowi

ANOVA: F1,38 = 10.4, p < 0.005

~2 days

~3 days

modesta’s ability to withstand lower

salinities for longer provides greater local

adaptation & competitive advantage

over willowi in SF

Conclusions

1. Gradients in salinity drive variations in local adaptation for A. willowi across its range– @ range center, LA: nat’l slxn 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

2. A. willowi northern range limit currently fixed @ TB– Genetic constraints

3.3. In SF, the most adapted In SF, the most adapted willowiwillowi still never reach same level of still never reach same level of tolerance to very low salinity as modesta tolerance to very low salinity as modesta doesdoes

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

• 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

• 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

Population Dynamics in Mill Valley - SF

Seasonal shifts in

presence / abundance…

…correlated with

seasonal shifts in

temperature & salinity

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