4.3.2 Reproduction
Reproductive cycles of echinoids on both sides of the Panama Isthmus have been
studied by Lessios (1981a, 1984, 1985a, 1991). Populations of D. mexicanum and
E. vanbrunti from Punta Paitilla, Isla Urabá and Culebra Island in the seasonal
Gulf of Panama have well-defined, synchronous, reproductive cycles (Lessios
1981a). The majority of individuals reached a peak gonad size in September, with
spawning predicted to occur from September to October. This would allow the
newly metamorphosed sea urchins, rather than the larvae, to benefit from the
increased food production as a result of the dry season upwelling (Lessios 1981a).
Populations of D. antillarum and E. lucunter from Maria Chiquita and Fort
Randolph in the less seasonal Caribbean demonstrated only slight, if any synchrony between individuals. However, E. viridis which is rarer at these locations
had a well defined, population wide cycle. Based on these data Lessios (1981a)
proposed that in a constant environment the intensity of selection for synchrony
between individual gametogenic cycles may be inversely proportional to population density. Later work by Lessios (1985a) on Caribbean echinoids did not
directly support this hypothesis. However, Lessios (1985a) suggested that population density may be one of the factors determining the degree of synchronicity.
This study (1985a) revealed that E. viridis, L. williamsi, Clypeaster rosaceus
(Linnaeus, 1758) and Leodia sexiesperforata (Leske, 1778) are reproductively
active during Panama’s rainy season from May to November, but quiescent during
the dry season (Lessios 1985a), while L. variegatus, T. ventricosus, and Clypeaster
subdepressus (Gray, 1825) have ripe gonads throughout the year. Only E. lucunter
showed non-periodic fluctuations in readiness to spawn.
Lessios (1984, 1991) also worked on lunar spawning periodicity in Panamanian
echinoids. Diadema mexicanum was found to spawn on the third quarter of the
lunar cycle, with a peak three to four days after full moon, while its allopatric
congener D. antillarum, spawned during the first lunar quarter, one to two days
after new moon. Lessios (1984) wrote that it is therefore possible that geminate
species of Diadema have acquired a prezygotic isolating mechanism while in
allopatry and therefore reproductive isolation need not be the product of natural
selection against hybridization, thus casting doubt on the speciation by reinforcement theory.
Lunar spawning rhythms were not found in T. ventricosus, E. viridis,
L. sexiesperforata, C. rosaceus or L. williamsi (Lessios 1991). Eucidaris tribuloides and D. antillarum were found to have a distinct lunar rhythm spawning at
different lunar phases, while L. variegatus has a semilunar rhythm spawning
every new and full moon (Lessios 1991). Such spawning rhythms are unlikely to
be cued by tides as the Atlantic coast experiences only small tidal ranges
(\0.5 m) where water level is determined as much by meteorological conditions
as by tides. Lunar periodicity was therefore shown to not be a lineage specific
trait inherited from a common ancestor, but has evolved independently in different echinoid taxa (Lessios 1991).
4 Echinoderm Diversity in Panama
121
Reproductive cycles of echinoids on both sides of the Panama Isthmus have been
studied by Lessios (1981a, 1984, 1985a, 1991). Populations of D. mexicanum and
E. vanbrunti from Punta Paitilla, Isla Urabá and Culebra Island in the seasonal
Gulf of Panama have well-defined, synchronous, reproductive cycles (Lessios
1981a). The majority of individuals reached a peak gonad size in September, with
spawning predicted to occur from September to October. This would allow the
newly metamorphosed sea urchins, rather than the larvae, to benefit from the
increased food production as a result of the dry season upwelling (Lessios 1981a).
Populations of D. antillarum and E. lucunter from Maria Chiquita and Fort
Randolph in the less seasonal Caribbean demonstrated only slight, if any synchrony between individuals. However, E. viridis which is rarer at these locations
had a well defined, population wide cycle. Based on these data Lessios (1981a)
proposed that in a constant environment the intensity of selection for synchrony
between individual gametogenic cycles may be inversely proportional to population density. Later work by Lessios (1985a) on Caribbean echinoids did not
directly support this hypothesis. However, Lessios (1985a) suggested that population density may be one of the factors determining the degree of synchronicity.
This study (1985a) revealed that E. viridis, L. williamsi, Clypeaster rosaceus
(Linnaeus, 1758) and Leodia sexiesperforata (Leske, 1778) are reproductively
active during Panama’s rainy season from May to November, but quiescent during
the dry season (Lessios 1985a), while L. variegatus, T. ventricosus, and Clypeaster
subdepressus (Gray, 1825) have ripe gonads throughout the year. Only E. lucunter
showed non-periodic fluctuations in readiness to spawn.
Lessios (1984, 1991) also worked on lunar spawning periodicity in Panamanian
echinoids. Diadema mexicanum was found to spawn on the third quarter of the
lunar cycle, with a peak three to four days after full moon, while its allopatric
congener D. antillarum, spawned during the first lunar quarter, one to two days
after new moon. Lessios (1984) wrote that it is therefore possible that geminate
species of Diadema have acquired a prezygotic isolating mechanism while in
allopatry and therefore reproductive isolation need not be the product of natural
selection against hybridization, thus casting doubt on the speciation by reinforcement theory.
Lunar spawning rhythms were not found in T. ventricosus, E. viridis,
L. sexiesperforata, C. rosaceus or L. williamsi (Lessios 1991). Eucidaris tribuloides and D. antillarum were found to have a distinct lunar rhythm spawning at
different lunar phases, while L. variegatus has a semilunar rhythm spawning
every new and full moon (Lessios 1991). Such spawning rhythms are unlikely to
be cued by tides as the Atlantic coast experiences only small tidal ranges
(\0.5 m) where water level is determined as much by meteorological conditions
as by tides. Lunar periodicity was therefore shown to not be a lineage specific
trait inherited from a common ancestor, but has evolved independently in different echinoid taxa (Lessios 1991).
4 Echinoderm Diversity in Panama
121