Resemblances in the early development of two sea urchins: Toxopneustes roseus (Euechinoidea: Echinacea) and Rhyncholampas pacificus (Euechinoidea: Irregularia) from different habitats in the southern Mexican Pacific.

Sea urchins play a key role in the marine environment, contributing to maintain a balance in benthic ecosystems. Toxopneustes roseus acts as a regulator of rhodolith beds and is a key species as a bioturbation promoter, while Rhyncholampas pacificus moderates the detritus content of sediment through various mechanisms and contributes to accelerating the circulation of organic matter. However, nothing is known about their early development, so the objective of this research is to characterize the embryonic and larval development of specimens of the two species from the southern Mexican Pacific and identify the causes that produce their differences. The embryonic development of T. roseus lasted approximately 20 â€‹h; the echinopluteus larva appeared at 23 â€‹h and culminated in around 12 days with an eight-armed larva. Metamorphosis was reached at 18 days. The embryonic development of R. pacificus lasted about 15 â€‹h; the larva emerged at 20 â€‹h and culminated in about two days with an eight-armed larva. The metamorphosis was completed in 9 days. The reproductive output of both species determines their development time and the structure of their larvae; therefore, the energy of R. pacificus is invested in reaching metamorphosis earlier to ensure that its larvae, which are not very abundant, settle and recruit to the population. Regarding T. roseus, the longest permanence in the plankton is efficiently maintained by its numerous larvae through the presence of mobile arms and the shift of the swimming function to the epauletes. This is the first work that characterizes the early development of a species of the genus Toxopneustes and of the only living species of the genus Ryncholampas. The information generated in this work is essential to gaining knowledge about these groups of echinoids, especially the effect of the environment on their early development.

Trioecy is maintained as a time-stable mating system in the pink sea urchin Toxopneustes roseus from the Mexican Pacific.

Trioecy is a sexual system that consists of the co-occurrence of females, males and hermaphrodites in a population and is common in plants; however, in animals it is uncommon and poorly understood. In echinoderms, trioecy had never been recorded until now. Frequencies of females, males, and hermaphrodites were evaluated and gametogenic development was histologically characterized in a population of Toxopneustes roseus inhabiting the Mexican Pacific. Trioecy in this population is functional and temporally stable, since the three sexes coexisted in each sampling month. The hermaphrodites presented similar gametogenic development as the females and males and participated during the spawning season, contributing to the population's reproductive process. Trioecy is considered an evolutionarily transitory state, and it is extremely difficult to explain its presence in a species. We hypothesize that continuous ocean warming represents a threat to the survival of this population of T. roseus, since its early developmental stages, which represent a population bottleneck, are more vulnerable to high temperatures than other sea urchins inhabiting the area, while its population density is significantly lower. These conditions generate a strongly stressed environment, which is the determining factor that maintains the stability of trioecy in the species in which it has been studied.

Effect of temperature increase on fertilization, embryonic development and larval survival of the sea urchin Toxopneustes roseus in the Mexican south Pacific.

Toxopneustes roseus performs a key role in the eastern tropical Pacific as a strategic herbivore and bioturbation promoter. We evaluated the effect of temperature on the fertilization success, embryonic development and larval survival of T. roseus under laboratory conditions, to understand how the increase in ocean temperature could affect it in a global warming. The highest percentage of fertilization occurred in gametes that were exposed to 30 °C, and a significant negative effect of 32 °C was evidenced by the lowest percentage. There was also a deleterious effect in embryos exposed to 32 °C, resulting in an abnormal development at all the time points. The highest percentage of larval survival occurred at 30 °C, while the lowest percentage occurred at 32 °C. The results suggest that T. roseus probably lives near its upper thermal limit, and future ocean warming could threaten the permanence of the species in the eastern tropical Pacific, or at least lead to contraction or fragmentation of its range limits. Therefore if sea temperature rises globally, it could cause the disappearance of these populations that are living at the edge of their thermal tolerance, but for other populations located in more temperate latitudes, it could propitiate favorable conditions for fertilization and survival of embryos and larvae.

Density, spatial distribution and mortality rate of the sea urchin Diadema mexicanum (Diadematoida: Diadematidae) at two reefs of Bahías de Huatulco, Oaxaca, Mexico / Densidad, distribución espacial y tasa de mortalidad de Diadema mexicanum (Diadematoida: Diadematidae) en dos arrecifes de Bahías de Huatulco, Oaxaca, México

Abstract Diadema mexicanum, a conspicuous inhabitant along the Mexican Pacific coast, is a key species for the dynamics of coral reefs; nevertheless, studies on population dynamics for this species are scarce. Monthly sampling was carried out between April 2008 and March 2009 at Isla Montosa and La Entrega, Oaxaca, Mexico using belt transects. Population density was estimated as well as abundance using Zippin's model. The relationship of density with sea-bottom temperature, salinity, pH, and pluvial precipitation was analyzed using a step by step multiple regression analysis. Spatial distribution was analyzed using Morisita's, Poisson and negative binomial models. Natural mortality rate was calculated using modified Berry's model. Mean density was 3.4 ± 0.66 ind·m-2 in La Entrega and 1.2 ± 0.4 ind·m-2 in Isla Montosa. Abundance of D. mexicanum in La Entrega was 12166 ± 25 individuals and 2675 ± 33 individuals in Isla Montosa. In Isla Montosa there was a positive relationship of density with salinity and negative with sea-bottom temperature, whereas in La Entrega there was not a significant relationship of density with any recorded environmental variable. Monthly mortality rate was 0.10 in La Entrega and 0.15 in Isla Montosa. Spatial distribution pattern was aggregated for both localities during the sampling period with oscillations in the intensity of aggregation. Higher density and abundance, and a low mortality rate, indicate better conditions for the population of D. mexicanum in La Entrega, contrasting with those of Isla Montosa where apparently there are more stressful conditions associated to stronger oceanographic conditions and a higher sedimentation rate resulting from the proximity of the Copalita River. Rev. Biol. Trop. 63 (Suppl. 2): 173-182. Epub 2015 June 01.

Resumen . Diadema mexicanum es un habitante conspicuo de las costas del Pacífico Mexicano y una especie clave en los arrecifes de coral, sin embargo son escasos los estudios acerca de la dinámica poblacional de esta especie. En el presente trabajo se muestreó mensualmente de abril 2008 a marzo 2009 en Isla Montosa y La Entrega usando transectos de banda. Se estimó la densidad poblacional, así como la abundancia mediante el modelo de Zippin. Se evaluó la relación entre la variación mensual de la densidad y la temperatura, salinidad, pH y precipitación pluvial usando un análisis de regresión lineal múltiple paso a paso. La distribución espacial se analizó con los modelos de Morisita, Poisson y Binomial Negativa. Se calculó la tasa de mortalidad natural usando el modelo de Berry modificado. La densidad media fue 3.4 ± 0.66 ind m-2 en La Entrega y 1.2 ± 0.4 ind m-2 en Isla Montosa. La abundancia en La Entrega fue 12 166 ± 25 individuos y 2 675 ± 33 individuos en Isla Montosa. Se registró una relación positiva de la densidad con la salinidad y negativa con la temperatura en Isla Montosa, mientras que en La Entrega ninguna variable ambiental se relacionó significativamente con la densidad. La distribución espacial fue agregada en ambas localidades con oscilaciones en la intensidad de agregación. La mortalidad mensual fue 0.1 en La Entrega y 0.15 en Isla Montosa. Los resultados indican mejores condiciones para la población de D. mexicanum en La Entrega en comparación con Isla Montosa, donde las condiciones oceanográficas son más severas y la tasa de sedimentación es más alta, asociada a la cercanía del Río Copalita.