<<  <  >  >>

 

DIADEMA ABUNDANCE

ORITY ON

F1 2

RI

RE

TH

EF

S FOR HEAL

What

What Is It?

BENCHMARK

Diadema antillarum, known as the long-spined or

Regional average of ~2 urchins/m2,

Regional

black sea urchin, is easily recognized by its long,

with more than 50% of surveyed sites

needle-like spines, which radiate up to 30 cm from a

showing

showing increasing abundances.

relatively small (7.5 cm) test. The spines are coated

TA R G E T

with a mild toxin.

Working target of 2-6 urchins/m2. More

Diadema urchins are found throughout the Caribbean,

information is needed to determine

as well as in tropical waters of the eastern and

the optimum number of Diadema

western Atlantic Ocean. They live in a wide variety of

habitats, but usually inhabit shallow coral reef and

antillarum

antillarum to adequately remove algae

seagrass areas. During the day, these urchins tend

but not erode the reef framework.

to hide in crevices in the reef or aggregate in groups.

RED FL AG

At night, they are more active on the reef and nearby

seagrasses.

< 1 urchin/m2.

Black sea urchins are very important herbivores on

the reef, as they help to maintain the balance between

coral and algal growth. Algal turfs and macroalgae

reefs. They are key grazers and bioeroders, and their

are their preferred foods, but the urchins can also

long spines provide shelter and protection for small

eat other materials, including live coral. Diadema

fish and invertebrates (e.g., crabs, fishes).

antillarum was once the most abundant and important

Diadema abundance is an indicator of their level of

herbivore on Caribbean reefs42.

herbivory. Diadema reduce macroalgae and increase

In 1983, a lethal disease outbreak rapidly killed

open settlement space for coral recruits. Reefs with

almost 98% of Diadema urchins throughout the

more Diadema tend to have less macroalgae and

Caribbean, in what is considered to be the most

often are healthier (or have the potential to be).

severe and significant mass mortality for a marine

Healthy reefs should have a D. antillarum density

organism in modern times43. This significant loss of

that can provide a balance between survival of coral

Diadema has contributed to a shift in many coral reef

recruits, algal coverage, and sustainable bioerosion

communities from coral dominance to macroalgal

rates. An average-sized Diadema urchin weighs about

dominance. This shift was even more severe in areas

100 g and eats approximately 1-2% of its body weight

where overfishing had also reduced the numbers and

in algae per day. To keep algae cropped on a typical

sizes of herbivorous fishes.

shallow patch reef, then, 400 to 800 g/m2 of urchins

Very little recovery on the regional scale has been

are needed -- about 4 to 8 urchins per m2 44. This

observed, although some sites now host abundant

calculation assumes grazing by Diadema only. Ideally,

Diadema populations, and anecdotal information

herbivorous fish would also be present, and fewer

indicates some recovery may be underway. Several

urchins would be required.

factors may be contributing to the lack of large-scale

How

How Do We Measure It?

recovery. Despite their protective spines, Diadema are

a common food for numerous reef consumers (e.g.,

Divers normally count the number of juvenile and adult

queen triggerfish and other fin fish, lobster, fighting

Diadema antillarum along belt transects also used to

conch). Larval supply may be limited by the very low

count fish or measure coral cover. AGRRA and MBRS

densities of reproducing adults. Recruitment may

methodologies recommend six transects (each 1 m

also be limited by inadequate substrate, predation

wide and 10 m long) per site3,8.

pressures, contaminants (e.g., metals) or poor water

quality.

Measurements need to be standardized for the time

of day to the extent possible (usually made between

Why Do We Measure It?

Why

10:00 a.m. and 2:00 p.m.).

Diadema urchins play several important roles on

74