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P  H O T I C (A  M P H I ) I  N D E X

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What Is It?

BENCHMARK

Amphistegina is a genus of relatively large (1-3 mm)

Density > 103 #/m2. Bleaching < 25%.

foraminifera (microscopic one-celled organisms with a

Juveniles 30-80% (varies seasonally).

shell) with symbiotic zooxanthellae (algae that produce

Damaged

Damaged tests 5-15%.

oxygen and remove wastes for the host). The Photic

(Amphi) Index is based upon densities and visual

TA R G E T

characterization of populations of Amphistegina. This

Density > 104 #/m2. Bleaching 0%.

is an indicator of general environmental suitability

Juveniles 30-80% (varies seasonally).

for calcifying organisms with algal symbionts and

Damaged tests < 5%.

an indicator of light stress (especially UV radiation).

Unlike the FORAM Index (D6), which measures

RED FL AG

general environmental water quality particularly with

Density < 103 #/m2 or bleaching > 40%

Density

respect to nutrient availability, this indicator looks at a

or % Juveniles < 40% in early summer

stress response primarily associated with light levels

or

or damaged tests > 20%.

affecting the host-symbiont relationship (similarly to

some coral bleaching responses).

Why Do We Measure It?

oxidative stress resulting from intense UV radiation.

Amphistegina are abundant and widely distributed

While most coral bleaching events correlate more

foraminifera (shelled protists), whose shelled bodies

closely with temperature stress, light (particularly UV)

(both alive and dead) are commonly found in the

also plays a contributing role. Comparing the values

sediments on "healthy" reefs. Since the early 1990s,

of the Photic (Amphi) Index (D13), the Coral Bleaching

population densities have decreased and bleaching

Index (D14) and extent of Coral Bleaching (F7) during

(loss or reduction of symbiotic zooxanthellae) has

a bleaching event enables an estimation of the

become prevalent in populations of Amphistegina

relative contributions between ultraviolet radiation

worldwide. Bleaching is often associated with

and temperature stress in the reef environment for

increases in microbial infestations, shell breakage

any given bleaching event.

due to predation, reproductive failure and shell

deformities.

How Do We Measure It?

Amphistegina are known to be sensitive to water

Amphistegina populations are analyzed by collecting

quality deterioration and to bleach in response

reef rubble in areas of concern. The reef rubble is

to excess radiant energy, but not in response to

scrubbed in seawater using a soft brush. The resultant

temperature stress. Thus, this index provides a quick

sediment slurry is rinsed free of most of the loose

and sensitive way to discriminate the extent of photo-

organic matter by decanting, and the sediment is

placed in a dish (e.g., large petri dish) overnight. Live

Amphistegina will crawl to the surface and can be

picked from the sediment. Using a stereomicroscope

at 10-20x magnification, Amphistegina are counted,

measured or "sized" (juveniles are <0.5 mm), and

characterized by their appearance according to

symbiont color and degree of breakage53.

The "health" of the population is based on an evaluation

of densities, size distributions, and prevalence and

severity of bleaching and shell damage.

Pamela Hallock

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