14.5 Threats
14.5.1 Mass Mortalities of Echinoderms
Mass mortalities have been reported as far back as 1965, when Glynn (1968)
described the mass mortalities of echinoids and other reef flat organisms concurrent with midday and low water exposures in La Parguera, Puerto Rico. All dead
urchins and some other echinoderms were counted and removed from the sampling
plot at Caracoles Reef in La Parguera over a two day period. Population sizes of
living organisms were determined from m
2 quadrat counts. Tolerance of desiccation was also investigated by exposing 25 individuals of each of the five echinoid species (T. ventricosus, L. variegatus, E. lucunter, Brissus unicolor (Leske),
and D. antillarum on coral rubble in open sunlight (unprotected) over a period of
six hours. The same desiccation experiment was performed, but this time the sea
urchins were provided with some protection. In addition, Glynn (1968) investigated the effects of solar radiation on the sea urchin survival. Fifteen individuals of
three species of sea urchins were exposed to sunlight in water baths just deep
enough to cover the animal. The most severe mortality of echinoids was during
April 14, 1965. Tripneustes ventricosus experienced the highest mortality
(86.4 %), L. variegatus 64 %, B. unicolor 25.9 % and E. lucunter 1.2 %. Lytechinus variegatus and T. ventricosus were found dead in shallow water zones and
had not been exposed to drying. Before the mass mortality L. variegatus had
an aggregated mean density of 57.6 ind m
-2 , T. ventricosus 13.6 ind m
-2 .
Echinometra lucunter was very abundant, numbering [1,000 individuals.
Diadema antillarum and B. unicolor were scarce but showed a significant increase
in densities by spring 1966. There was also a noticeable recovery of T. ventricosus
(35.6 ind m
-2 ) and L. variegatus (539.6 ind m
-2 ) the following fall.
There was two other mass die offs on May 16 and June 28, 1965 reported by
Glynn (1968). On those episodes, there were two species of ophiuroids that were
affected seriously by extreme temperatures (39 °C), Ophioderma appressa (Say)
and Ophiocoma echinata. During the summer months (May–August) there were a
series of minus tides during midday when heat and desiccation approached
extreme levels. Glynn (1968) frequently observed temperatures exceeding 32 °C
and, during May and June 1965, extreme temperatures as high as 39 °C.
Other echinoderm species involved in these three mass mortalities were:
D. antillarum, Ophiomyxa flaccida (Say), Ophiothrix angulata, Ophiocoma riisei
(Lütken), Ophiactis savignyi, Holothuria (Halodeima) mexicana, and H. (Thymiosycia) arenicola (Semper). For the desiccation experiment, the loss in weight as a
function of exsiccation showed a linear relationship for both protected and
unprotected individuals. The loss in weight by B. unicolor increased to 70 % from
the initial wet weight after 6 h. Echinometra lucunter had the highest loss of
weight (50 % of initial wet weight after 6 h). All the unprotected urchins died after
3 h of exposure except E. lucunter. For the protected urchins, T. ventricosus had
the highest resistance to desiccation (80 % of the initial wet weight). For the solar
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S. M. Williams et al.
14.5.1 Mass Mortalities of Echinoderms
Mass mortalities have been reported as far back as 1965, when Glynn (1968)
described the mass mortalities of echinoids and other reef flat organisms concurrent with midday and low water exposures in La Parguera, Puerto Rico. All dead
urchins and some other echinoderms were counted and removed from the sampling
plot at Caracoles Reef in La Parguera over a two day period. Population sizes of
living organisms were determined from m
2 quadrat counts. Tolerance of desiccation was also investigated by exposing 25 individuals of each of the five echinoid species (T. ventricosus, L. variegatus, E. lucunter, Brissus unicolor (Leske),
and D. antillarum on coral rubble in open sunlight (unprotected) over a period of
six hours. The same desiccation experiment was performed, but this time the sea
urchins were provided with some protection. In addition, Glynn (1968) investigated the effects of solar radiation on the sea urchin survival. Fifteen individuals of
three species of sea urchins were exposed to sunlight in water baths just deep
enough to cover the animal. The most severe mortality of echinoids was during
April 14, 1965. Tripneustes ventricosus experienced the highest mortality
(86.4 %), L. variegatus 64 %, B. unicolor 25.9 % and E. lucunter 1.2 %. Lytechinus variegatus and T. ventricosus were found dead in shallow water zones and
had not been exposed to drying. Before the mass mortality L. variegatus had
an aggregated mean density of 57.6 ind m
-2 , T. ventricosus 13.6 ind m
-2 .
Echinometra lucunter was very abundant, numbering [1,000 individuals.
Diadema antillarum and B. unicolor were scarce but showed a significant increase
in densities by spring 1966. There was also a noticeable recovery of T. ventricosus
(35.6 ind m
-2 ) and L. variegatus (539.6 ind m
-2 ) the following fall.
There was two other mass die offs on May 16 and June 28, 1965 reported by
Glynn (1968). On those episodes, there were two species of ophiuroids that were
affected seriously by extreme temperatures (39 °C), Ophioderma appressa (Say)
and Ophiocoma echinata. During the summer months (May–August) there were a
series of minus tides during midday when heat and desiccation approached
extreme levels. Glynn (1968) frequently observed temperatures exceeding 32 °C
and, during May and June 1965, extreme temperatures as high as 39 °C.
Other echinoderm species involved in these three mass mortalities were:
D. antillarum, Ophiomyxa flaccida (Say), Ophiothrix angulata, Ophiocoma riisei
(Lütken), Ophiactis savignyi, Holothuria (Halodeima) mexicana, and H. (Thymiosycia) arenicola (Semper). For the desiccation experiment, the loss in weight as a
function of exsiccation showed a linear relationship for both protected and
unprotected individuals. The loss in weight by B. unicolor increased to 70 % from
the initial wet weight after 6 h. Echinometra lucunter had the highest loss of
weight (50 % of initial wet weight after 6 h). All the unprotected urchins died after
3 h of exposure except E. lucunter. For the protected urchins, T. ventricosus had
the highest resistance to desiccation (80 % of the initial wet weight). For the solar
462
S. M. Williams et al.