Hybrids of two closely related tropical sea urchins (genus Echinometra): evidence against postzygotic isolating mechanisms

A series of cross-fertilization experiments were conducted with two unnamed, sympatric species of sea urchins in the Echinometra mathaei species complex, Echinometra sp. A (Ea) and Echinometra sp. C (Ec). Heterogametic fertilization success was high when eggs of Ec and sperm of Ea were involved, and low with eggs of Ea and sperm of Ec. Hybrids produced from crosses in either direction developed normally to sexually mature adults; Ea x Ea were largest in test size, followed by Ec (ova) x Ea (sperm), Ea (ova) x Ec (sperm), and Ec x Ec, respectively. Color patterns of the hybrids were closer to the maternal coloration, whereas other characters such as relative test dimensions and spine lengths, morphology of tubefoot and gonad spicules, and gamete sizes were intermediate. Fertilization rates in F1 backcrosses were high, minimizing the possibility that hybrid infertility is a postzygotic mechanism of reproductive isolation. On the other hand, intensive surveys failed to find individuals with hybrid characteristics in the field, suggesting that natural hybridization between the two species is rare. Prezygotic isolating mechanisms, such as microhabitat separation and gamete incompatibility, at least between Ea eggs and Ec sperm, most likely maintain the genetic integrity of these two closely related species.     

Comparative studies on the energy metabolism in spermatozoa of four closely related species of sea urchins (genus Echinometra) in Okinawa

Sea urchins of the genus Echinometra are abundant on Okinawa reef flats in southern Japan. The Okinawan Echinometra is designated into four sympatric and closely related species: A, B, C, and D (Ea, Eb, Ec, and Ed). The sperm head size and shape gradually changes to become longer and more slender according to the following order: Ea, Eb, Ec, and Ed. To obtain information regarding speciation in Okinawan Echinometra, this study examined comparatively the energy production system of spermatozoa of Ea, Eb, Ec, and Ed. All spermatozoa contained cholesterol and several kinds of phospholipids. Glycogen, glucose, and triglyceride were present at extremely low levels. After incubation in sea-water, a decrease in the level of phosphatidylcholine (PC) was observed in all spermatozoa concomitantly with activation of motility and respiration. The hydrolysis of PC correlated with the activity of phospholipase A2. Interestingly, the amount of PC consumed, the respiratory rate, and the phospholipase A2 activity in spermatozoa of Ea and Eb were approximately two-fold higher than those of Ec and Ed. Ultrastructural studies showed that lipid bodies within mitochondria were present in the midpieces of all species of spermatozoa. They became small or disappeared after incubation in seawater. Thus, the results obtained strongly suggest that spermatozoa of Ea, Eb, Ec, and Ed all use PC located in the lipid bodies as a substrate for energy metabolism. Also, it seems likely that energy production activities in Ea and Eb spermatozoa are stronger than those in Ec and Ed. The properties of energy metabolism in different species of sea urchin may be related to their habitat.     

Hybridization and F1 backcrosses between two closely related tropical species of sea urchins (genus Echinometra) in Okinawa

Summary

Four species of sea urchins of the genus Echinometra, designated species A, B, C, and D, occur along the coast of Okinawa; they are distinguished by color pattern and other characteristics. The two most distinct species, Echinometra sp. A (Ea) with white-tipped spines and Echinometra sp. D (Ed) with nearly black spines (possibly E. oblonga), were examined for potential hybridization through a series of fertilization experiments and rearing of the resulting hybrids. Fertilization was reciprocally asymmetrical; Ed ova were readily fertilized by sperm of Ea, but less than 20% of the ova of Ea were fertilized by sperm of Ed. Nevertheless, hybrids resulting from crosses in both directions developed normally through larval and juvenile stages to produce sexually mature adults. Larvae of Ed♀×Ea♂ and Ed♂×Ed♀ were larger than those of Ea♂ × Ed♀ and Ea♂ × Ea♀, reflecting the larger eggs of Ed. However, Ea♂×Ea♀ juveniles and later stages, up to maturity, were consistently larger than those of equivalent ages of the other crosses; largest to smallest were Ea♂×Ea♀, Ed♂×Ea♀, Ea♂ × Ed♀, and Ed♂×Ed♀, respectively. Ova and sperm of the hybrids were viable and fertilizable in all combination of crosses, although fertilization of those of Ed♂×Ea♀ hybrids was consistently higher than those of Ea♂ × Ed♀ hybrids. These experiments represent the first reported successful production of hybrid sea urchins with viable gametes. They indicate that reproductive isolation is achieved by prezygotic isolating mechanisms in these two species. Of these mechanisms, gametic incompatibility is probably only partly involved, if at all, and differences in spawning times, habitat segregation, or other factors appear to be important for maintaining reproductive isolation in these closely related species.     

Ingestion and transformation of algal turf by Echinometra mathaei on Tiahura fringing reef (French Polynesia)

The sea urchin Echinometra mathaei is the most abundant herbivore on many tropical reefs. We studied the ingestion and digestion diel rhythms, transformation of algal turf and bioerosion attributable to this species on the Tiahura fringing reef in French Polynesia. Ingestion rates showed a circadian rhythm with most feeding taking place during the night. Absorption of food occurred throughout the day with urchins digesting food outside of the feeding period. A total of 73% of the faecal pellets consisted of CaCO(3) eroded from the reef, 20% consisted of organic matter and 7% the refractory organic matter. Of the organic matter, lipids, carbohydrates and chlorophyll were digested and absorbed and proteins were expelled in the faecal pellets. An average individual bioerosion of 0.32 g day(-1) was estimated for E. mathaei from approximately a 35-mm test diameter on the Tiahura fringing reef. We further estimated that E. mathaei release 70.5 g m(2) y(-1) of carbohydrates, 43.8 g m(2) y(-1) of lipid, 23.3 g m(2) y(-1) of protein and 2.0 g m(2) y(-1) of total chlorophyll pigments.     

Strong reproductive isolation between closely related tropical sea urchins (genus Echinometra)

Morphological, mitochondrial DNA, and single-copy nuclear DNA differences show that the tropical sea urchin Echinometra mathaei is composed of at least four independent gene pools. Evolutionary distance between species measured with restriction-site changes (for mitochondrial DNA) and thermal renaturation (for single-copy nuclear DNA) is 1%-3% nucleotide divergence. Thus these are the most closely related sea urchin species known. Despite this genetic similarity, strong blocks to interspecific fertilization exist in this genus. Between two Hawaiian species, few eggs are fertilized in hybrid crosses, even in the presence of excess sperm. Microscopic examination of such crosses shows that sperm attachment to heterologous eggs is inhibited. Measures of genetic distance between species can help reveal the tempo of speciation and allow comparisons of morphological, biochemical, and ecological characteristics to be made in an evolutionary framework. Our results show that strong reproductive isolation can evolve by changes in egg-sperm recognition without extensive genetic divergence between species. Such mechanisms are most easily studied in free-spawning animals such as sea urchins but as well may represent an important aspect of speciation in species with internal fertilization.     

Absorption efficiencies of Lytechinus variegatus (Lamarck) (Echinodermata: Echinoidea) for selected marine plants

The absorption efficiencies of the sea urchin Lytechinus variegatus (Lamarck) were calculated by an indicator method. Absorption efficiencies for total organic material, protein, lipid, and carbohydrate were highest with Halimeda incrassata Ellis as food (43 %, 65 %, 71 % and 35 %, respectively), and lowest with Eucheuma isiforme C. Agardh as food (?35%, ?42%, ?56%and ?34%, respectively). Moderately positive absorption efficiencies for total organic material were found with Thalassia testudinum König & Sims and Ulva lactuca L. as food, and low positive absorption efficiencies for total organic material are found with Syringodium filiformis Kützing and Sargassum sp. as food. Absorption efficiencies of the sea urchin Strongylocentrotus purpuratus (Stimpson) (calculated from the literature) were much greater than those of Lytechinus for carbohydrate, but less for protein and lipid. Both sea urchins showed negative efficiencies for some nutrient fractions of their foods. The absorption index (mg nutrient fraction absorbed/g dry wt ingested) showed that Thalassia supplied the most protein and total organic material (43 mg/g dry wt ingested and 142 mg/g dry wt ingested, respectively). In general, the absorption indices indicate that Lytechinus obtains more protein and lipid, but less carbohydrate from its food than does Strongylocentrotus. In the field Lytechinus has primarily a detrital diet.     

Isolation of elatol from Laurencia microcladia and its palatability to the sea urchin Echinometra lucunter

Elatol was isolated as the major compound from the red alga Laurencia microcladia Kütz. collected in Southern Brazil. This is the first report of elatol in this species. We also investigated the herbivore behaviour of the black sea urchin Echinometra lucunter (Linnaeus, 1758) towards L. microcladia, Ulva fasciata Delile and Gracilaria domingensis (Kütz.) Sond. ex Dickie through live algal multiple-choice feeding assay, as well as artificial feeding assay. The sea urchins ate the crude algae L. microcladia and pellets containing the powdered algae, extract and all tested concentrations of elatol, suggesting that this seaweed and its main compound are palatable for E. lucunter.     

Concentrations of trace elements in sea urchins and macroalgae commonly present in <Emphasis Type="Italic">Sargassum</Emphasis> beds: implications for trophic transfer

The aim of the study was to evaluate the dynamic of cadmium, lead, copper, zinc, and iron among lower trophic levels, sea urchins and macroalgae. Diets and isotopic values were used in combination to explore trophic positions and potential transference of metals from primary producers to consumers. Concentrations of trace elements were measured in two species of sea urchin (Tripneustes depressus and Eucidaris thouarsii) and nine macroalgae that are usually used as food in four Sargassum beds, one of which is close to a phosphorite mine. Specimens were collected when Sargassum fronds were at their greatest (winter) and lowest (summer) abundance. Highest concentrations of Cd, and Cu in both urchin species were recorded in winter at the site near the phosphorite mine. Concentrations of Pb in T. depressus were below the detectable limit, whereas E. thouarsii, which in addition to a high concentration of Pb, had high amounts of Cu and Zn. Gut content analysis, indicates that the diet of both sea urchins at the four sites and two collection dates is mainly macroalgae. The δ ¹⁵N isotopic values in sea urchins in a typical Sargassum bed were in good agreement with a diet dominated by macroalgae, with T. depressus having herbivorous habits and E. thouarsii having omnivorous habits in this environment. We found macroalgae important in the dynamics of metals in food webs, potentially contributing to transferring Cd, Cu, and Zn to key invertebrate species, such as sea urchins, indicating connectivity of food webs and ecological structuring of marine environments.     

Variation in abundance and spatial distribution of ecomorphs of the sea urchins,Echinometra sp. nov A and E. sp. nov C on a Fijian reef

This paper presents novel spatial distribution and abundance patterns of ecomorphs of Echinometra sp. nov A andE. sp. nov C (hereafter referred to as E. sp. A and E. sp. C, and collectively as Echinometra spp.). Echinometraspp. were surveyed on the Nukubuco Reef, to determine the within-reef variations in size-frequency and density atseveral spatial scales [between positions (east versus west), zones (crest versus flat), sites and quadrats]. The scalesof variation were greatest for both species between positions and zones. Echinometra sp. A was more readily foundon the calmer flats while E. sp. C preferred the high-energy crests. The distribution of ecomorphs of Echinometraspp. varied between habitats (position x zone). Echinometra sp. A ecomorphs were more abundant on all locationsand showed different size-class distributions compared with E. sp. C. Greater urchin numbers were observed onboth zones of the eastern position than the western position. Echinometra sp. A showed marginal aggregationpattern while E. sp. C showed a stronger tendency to cluster.     

Quantitative analysis of gametic incompatibility between closely related species of neotropical sea urchins

Species of the sea urchin genus Echinometra found on the two coasts of Panamá are recently diverged and only partially isolated by incomplete barriers to interspecific fertilization. This study confirms previous work that revealed incompatibility between the eggs of the Atlantic E. lucunter and the sperm of the other two neotropical species, whereas eggs of its sympatric congener E. viridis and allopatric E. vanbrunti are largely compatible with heterospecific sperm. Here we quantify fertilization using a range of sperm dilutions. We demonstrate a much stronger block to cross-species fertilization of E. lucunter eggs than was previously shown at fixed sperm concentrations, and mild incompatibility of the other two species' eggs where previous crosses between species were not distinguishable from within-species controls. Additionally, we present evidence for intraspecific variation in egg receptivity towards heterospecific sperm. Our findings here again discount the "reinforcement model" as a viable explanation for the pattern of prezygotic isolation. Gamete incompatibility in these Echinometra has appeared recently-within the last 1.5 million years-but is weaker in sympatry than in allopatry. Accidents of history may help explain why incompatibility of eggs emerged in one species and not in others. Compensatory sexual selection on sperm in this species could follow, and promote divergence of proteins mediating sperm-egg recognition.     

Survival of seaweeds ingested by three species of tropical sea urchins from Brazil

Three species of sea urchins, Echinometra lucunter, Lytechinus variegatus, and Arbacia lixula, common on the southeastern coast of Brazil, were selected for experiments on seaweed survival after passage through their digestive tract. L. variegatus and E. lucunter have more than 80% of their diet composed of algae. C. 24% of the species of algae present in the digestive tract survived. A. lixula has only 29% of its diet composed by algae, but the survival rate of the algae is c. 50%.Although Rhodophyta represent 51% of the ingested algae, their survival rate in the faeces is only 19%, while the Chlorophyta, with a survival rate of 63%, comprise only about 26% of the diet. Most species of algae that survived ingestion were r-strategists.     

Effect of macrophyte diet and initial size on the survival and somatic growth of sub-adult <Emphasis Type="Italic">Strongylocentrotus purpuratus</Emphasis>: a laboratory experimental approach

Recently recruited urchins from the same brood, but with different initial sizes, may respond differently to similar environmental factors. The aim of this study was to assess and compare the effects of starvation and diet on the survival, growth rates in size and weight, and gonad index among small and large sub-adult purple sea urchins, Strongylocentrotus purpuratus. Small urchins ranged from 7.3 to 7.8 mm and large urchins from 11.8 to 14.1 mm (test diameters). Two independent experiments were performed. In the first experiment, sea urchins were fed during 22 weeks on Egregia menziesii (ad libitum) and for only 1 day month⁻¹ (starved condition). Feeding regime significantly affected survival, somatic growth rate in size and weight, and gonad index, with higher means in the ad libitum treatments than in starving conditions. A recurrent cannibalism event by conspecifics occurred in small sea urchins under starving conditions. In the second experiment, sea urchins were fed during 13 weeks ad libitum with four diets: kelp (E. menziesii), coralline algae (Bossiella orbigniana), eelgrass (Phyllospadix scouleri) and a mixed diet of the three species. Survival was not affected by diet or urchin size, but diet significantly affected somatic growth rate in size and weight and gonad index. Kelp promoted the highest growth rate (2.23 ± 0.21 mm month⁻¹), the mixed diet produced an intermediate growth (1.26 ± 0.21 mm month⁻¹), while the lowest values corresponded to coralline algae and the eelgrass (0.30 ± 0.12 and 0.10 ± 0.03 mm month⁻¹, respectively, means ± SE). The mean growth rate of small urchins (averaging all diets) was higher than in large specimens (1.17 ± 0.37 and 0.77 ± 0.28 mm month⁻¹, respectively).     

Prior residence and body size influence interactions between black sea urchins

Body size and prior residence can modulate agonistic interaction in several animal species, but scientists know little about these relationships in echinoderms. In this study, we tested the effects of these traits on interactions in the black sea urchin (Echinometra lucunter). After a sea urchin was isolated for 24-h in a glass tank to establish prior residence, we introduced an intruder animal adjacent to the resident in the tank and observed interactions for 30 min. The intruder animal was larger, smaller, or size-matched to the resident. We found body size and prior residence concomitantly modulated interactions among black sea urchins, with prior residence as the major determinant. Black sea urchins mainly exhibited opponent inspection and fleeing responses during interaction to avoid fights, especially when a fight could be seriously disadvantageous (small intruder vs. large resident).     

The Endocommensal Ciliates of Venezuelan Sea Urchins

SYNOPSIS. Two hundred and fifty-nine specimens of the sea urchins Echinometra lucunter (201), Diadema antillarum (52), Tripneustes ventricosus (5) and Eucidaris tribuloides (1), collected on the littoral of the Federal District of Venezuela, were examined for intestinal ciliates.
Iron hematoxylin and silver-impregnation staining technics permitted determination of eight of the nine ciliates found: Anophrys aglycus, A. elongata, Cohnilembus caeci, Biggaria bermudensis, B. echinometris, Cyclidium rhabdotectum, Metopus circumlabens and M. rotundus . An unidentified species of Euplotes was also found.     

Conspecific sperm precedence in two species of tropical sea urchins

Conspecific sperm precedence occurs when females are exposed to sperm from males of multiple species, but preferentially use sperm of a conspecific. Conspecific sperm precedence and its mechanisms have been documented widely in terrestrial species, in which complex female behaviors or reproductive tract morphologies can allow many opportunities for female choice and sperm competition, however, the opportunity for conspecific sperm precedence in free spawning marine invertebrates has been largely ignored. Two sea urchin species, Echinometra oblonga and E. sp. C, have high levels of interspecific fertilization in no-choice lab crosses, but no natural hybrids have been found. We performed competitive fertilization assays to test for conspecific sperm precedence and found that eggs of both species showed a marked preference for conspecific sperm when fertilized with heterospecific sperm mixtures. Strong rejection of heterospecific sperm would not have been predicted from no-choice assays and helps explain the lack of natural hybrids. We also found significant variation in hybridization success among crosses. Conspecific sperm precedence in free spawning invertebrates shows that the simple surfaces of eggs and sperm provide ample opportunity for egg choice and sperm competition even in the absence of intricate behavior or complex reproductive morphologies.     

Food niche overlap among neotropical frugivorous bats in Costa Rica

Food habits of 15 species of frugivorous bats were studied at La Selva Biological Station, Costa Rica. Eight hundred and fifty-four (854) fecal samples and 169 samples from fruit parts and seeds discarded by bats beneath feeding roosts were analyzed. During eight months of study, 47 fruit species consumed by bats were identified. Five plant genera (Cecropia, Ficus, Piper Solanum, and Vismia) constituted 85% of all plants found in fecal samples. Feeding niche breadth differed significantly among the six most common species of frugivorous bats (Artibeus jamaicensis, Carollia sowelli, C. castanea, C. perspicillata, Dermanura sp., and Glossophaga commissarisi). All species, except for Dermanura sp., showed a diet dominated by one or two plant species. This suggests a pattern of resource partitioning at a generic level, in which Carollia consumed mainly Piper, Artibeus consumed Ficus and Cecropia, and Glossophaga consumed Vismia. Cluster analysis revealed higher values of food niche overlap in congeneric species than among species of different genera. Results show that if food is a limiting factor, mechanisms other than trophic selection must reduce interspecific interference or competition for food in this frugivorous bat guild.     

Reproductive character displacement and the genetics of gamete recognition in tropical sea urchins

Reproductive character displacement occurs when sympatric and allopatric populations of a species differ in traits crucial to reproduction, and it is commonly thought of as a signal of selection acting to limit hybridization. Most documented cases of reproductive character displacement involve characters that are poorly understood at the genetic level, and rejecting alternative hypotheses for biogeographic shifts in reproductive traits is often very difficult. In sea urchins, the gamete recognition protein bindin evolves under positive selection when species are broadly sympatric, suggesting character displacement may be operating in this system. We sampled sympatric and allopatric populations of two species in the sea urchin genus Echinometra for variation in bindin and for the mitochondrial cytochrome oxidase I to examine patterns of population differentiation and molecular evolution at a reproductive gene. We found a major shift in bindin alleles between central Pacific (allopatric) and western Pacific (sympatric) populations of E. oblonga. Allopatric populations of E. oblonga are polyphyletic with E. sp. C at bindin, whereas sympatric populations of the two species are reciprocally monophyletic. There is a strong signal of positive selection (P(N)/P(S) = 4.5) in the variable region of the first exon of bindin, which is associated with alleles found in sympatric populations of E. oblonga. These results indicate that there is a strong pattern of reproductive character displacement between E. oblonga and E. sp. C and that the divergence is driven by selection. There is much higher population structure in sympatric populations at the bindin locus than at the neutral mitochondrial locus, but this difference is not seen in allopatric populations. These data suggest a pattern of speciation driven by selection for local gamete coevolution as a result of interactions between sympatric species. Although this pattern is highly suggestive of speciation by reinforcement, further research into hybrid fitness and egg-sperm interactions is required to address this potential mechanism for character displacement.     

Fish predation on sea urchins on the Great Barrier Reef

Predators are important for regulating adult sea urchin densities. Here, we employ remote underwater video cameras to record diurnal predation on tethered sea urchins at Lizard Island on the Great Barrier Reef (GBR). We identified four fish predators of adult sea urchins (Balistoides viridescens, Balistapus undulatus, Lethrinus atkinsoni and Choerodon schoenleinii). Predator activity appeared to be site-specific. Balistoides viridescens and B. undulatus (f: Balistidae) were the two most important predators of Echinometra mathaei with the former handling E. mathaei significantly faster (mean 0.7?min) than B. undulatus (5.2?min). Balistoides viridescens also successfully preyed on 70?% of detections, while C. schoenleinii, B. undulatus and L. atkinsoni preyed on just 33, 17 and <1?%, respectively. Additionally, B. viridescens were behaviourally dominant among predator species and were observed as aggressors in 30 encounters with B. undulatus and 8 encounters with L. atkinsoni. In only one encounter was B. viridescens the recipient of any aggression (from B. undulatus). In terms of relative vulnerability, of the three sea urchin species examined, E. mathaei were more vulnerable to predation than Diadema setosum or Echinothrix calamaris, with mean handling times of 1.2, 4.8 and 10.3?min, respectively. Balistoides viridescens and B. undulatus both appear to be able to play an important role as predators of sea urchins on the relatively intact coral reefs of Lizard Island. However, B. viridescens emerge as the most efficient predator in terms of handling speed and the proportion of detections preyed upon. They were also the behaviourally dominant predator. This preliminary study of the predators of sea urchins on the GBR highlights the potential significance of relatively scarce but functionally important species.     

Reproductive periodicity of the echinoids Diadema and Echinometra on the two coasts of Panama

Patterns of reproductive periodicity in the regular echinoid species Diadema mexicanum A. Agassiz and Echinometra vanbrunti A. Agassiz from the Bay of Panama, and Diadema antillarum Phillipi, Echinometra lucunter (Linnaeus) and E. viridis A. Agassiz from the Caribbean coast of the Isthmus of Panama exhibit dissimilarities that reflect the differences of the environments they inhabit. Populations of the two species from the seasonal Bay of Panama display synchronous, well-defined, reproductive cycles. Spawning appears to be timed so that newly metamorphosed sea urchins, rather than larvae, can benefit from the increased food production concomitant with dry season upwelling. On the less seasonal Caribbean shore reproductive periodicity is less defined. Populations of Diadema antillarum and Echinometra lucunter from the vicinity of the Panama Canal show indications of periodicity, while those from a locality 20 km to the east display little tendency for synchrony between individuals. The much rarer E. viridis, on the other hand, maintains well-defined, population-wide cycles in both localities. It is suggested that in a constant environment the intensity of selection for synchrony between individual gametogenic cycles may be inversely proportional to population density.