INVERTEBRATES AND FLUID DYNAMICS:

HOLDING ON

BOUNDARY LAYER

Mainstream Velocity (U)

Boundary layer thickness (99%)

Boundary layer thickness (90%)

Dis

tanc

e fr

om s

ubst

rate

Velocity

BOUNDARY LAYER

Using the boundary layer – water pennies (Psephenidae)

BOUNDARY LAYER

Mainstream Velocity (U)

Dis

tanc

e fr

om s

ubst

rate

Velocity

BOUNDARY LAYER

Using the boundary layer – water pennies (Psephenidae)

Other stream insects

Plecoptera

Trichoptera

Ephemeroptera

Stream animals– Strategies for holding on

Baetis

Rhithrogena

Ancylus

Psephenus

Neothremma

Bibliocephala

Stream animals– Strategies for holding on

Extreme Gradients – Swash Zone

Donax

Swash Surfing

Donax

Density

Shape

Weight distribution

Density

2.0

1.0

0

Density(103 kg/m3)

DonaxChione

Mercenaria

Spisula

Macrocallis

ta

Divaric

ella

Tellina

Tagellus

Weight Distribution

Pivot point

AnteriorPosterior

Ventral

Dorsal

Behaviour in a Swash Zone

WAVE STRESS

a. Limitation of size

Water flow100%

90%

Boundary layer

WAVE STRESS

a. Limitation of size

Water flow

WAVE STRESS

b. Holding on – flow tolerance

Flow rate(m/s)

3

0

Time to dislodge

WAVE STRESS

c. Holding on - orientation

Keyhole limpet

WAVE STRESS

c. Holding on - orientation

Water flow

WAVE STRESS

c. Holding on - orientation

<.5 m/s

-90 0 90 -90 0 90

>.5 m/s

Freq

Orientation (º to flow)

WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

1. Testing holding power

WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

1. Testing holding power

Kg required to dislodge

100

0

Foot area (cm2)15

WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

1. Suction?

Atmospheric pressure ≈ 1 kg/cm2

Patella ≈ 3 - 7 kg/cm2

-can’t generate a force > atmospheric pressure

- No negative pressure under foot

WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

Patella

Mucous layer

WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

2. Adhesion area

surface tension

Thickness of fluid

Theoretical adhesion = 600 kg/cm2

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

2. Adhesion

Tenacity(kg/cm2 to detach)

Weight of mucous

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”?

Foot rigidity

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”? Foot rigidity

l

d

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”? Foot rigidity

Tenacity

Flexibility

High

HighLow

Low

In field -

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”? Drag

FLOW

Resistance to water movement depends on:

1) Size

2) Shape

3) Texture

3. WAVE STRESS

d. Holding on - tenacity

What is “tenacity”? Drag

Drag

Side Front Rear

Drag – not well correlated with density

Types of Limpets

Non-Migratory Migratory

-don’t move far

-often fixed and territorial

-low r

-low growth

-move up shore

-high r

-high growth

(need high food intake)

-react to predators by clamping -flee from predators

LESS FLEXIBLE MORE FLEXIBLE

A COMPROMISE OF SEVERAL FACTORS

Sea Urchins - Echinoidea

lunules

WHAT IS THE FUNCTION OF LUNULES?

1) Aid in burrowing

2) Removal of feces

3) Maintain a “communication” with the surface if buried

4) Maintain inclined posture

5) Feeding

Log lunule length

Log test diameter

p < .05

If lunules have a hydrodynamic function-they should grow with the animal

isometric

observed

Time to burrow

Burrowing speed

Plugged Not plugged

230 sec231 sec

Flow through a sand dollar

Lift

Weight

Weight

Lift

Weight

Burrowing to add weight

Sand Dollar reorientation - Dendraster

Adding weight – incorporate magnetite

Reducing lift and drag

Skin drag – important?

Pressure drag – depends on shape

Reducing lift and drag

Skin drag – important?

Pressure drag – depends on shape

Very flat

Reducing lift and drag

Skin drag – important?

Pressure drag – depends on shape

Rounded

Area of lower pressure

How do you reduce lift

Reduce pressure differential between

upper and lower surfaces

Can sand dollars tell the direction of orientation?

Anterior

Posterior

Can sand dollars tell the direction of orientation?

-inverted sand dollars – can flip over more easily with posterior edge facing upstream(i.e critical velocity to re-orient is lower)

Hardy & Merz. 2013. Invert. Bio 132:52

Initial orientation

Final orientation(1 hr)