Comparative genetics of scyphozoan species reveals the geological history and contemporary processes of the Mediterranean Sea

Jellyfish are useful genetic indicators for aquatic ecosystems as they have limited mobility and are highly exposed to the water column. By using comparative genomics and the molecular clock (timetree) of Rhizostoma pulmo, we revealed a divergence point between the East and West Mediterranean Sea (MS) populations that occurred 4.59 million years ago (mya). It is suggested that the two distinct ecological environments we know today were formed at this time. We propose that before this divergence, the highly mixed Atlantic and Mediterranean waters led to the wide dispersal of different species including R. pulmo. At 4.59 mya, the Western and Eastern MS were formed, indicating the possibility of a dramatic environmental event. For the first time, we find that for the jellyfish we examined, the division of the MS in east and west is not at the Straits of Sicily as generally thought, but significantly to the east. Using genomics of the Aurelia species, we examined contemporary anthropogenic impacts with a focus on migration of scyphozoa across the Suez Canal (Lessepsian migration). Aurelia sp. is among the few scyphozoa we find in both the MS and the Red Sea, but our DNA analysis revealed that the Red Sea Aurelia sp. did not migrate or mix with MS species. Phyllorhiza punctata results showed that this species was only recently introduced to the MS as a result of anthropogenic transportation activity, such as ballast water discharge, and revealed a migration vector from Australia to the MS. Our findings demonstrate that jellyfish genomes can be used as a phylogeographic molecular tool to trace past events across large temporal scales and reveal invasive species introduction due to human activity.     

Effects of low dissolved oxygen on survival and asexual reproduction of scyphozoan polyps (Chrysaora quinquecirrha)

Hypoxic conditions are common in many coastal environments such as Chesapeake Bay. While medusae appear to be quite tolerant of low dissolved oxygen (DO) concentrations, the effects of hypoxia on the benthic polyp stages are unknown. Chrysaora quinquecirrha (DeSor) polyps, and were subjected to 5 DO treatments (air-saturated [control], 3.5, 2.5, 1.5 and 0.5 mg l^–1) in the laboratory. Polyp survival and development were documented over 24 d. Virtually no mortality occurred in any treatment during the first 5 d. Total polyp mortality after 24 d was 59.3% at the lowest DO concentration, whereas <3% mortality was observed in the air-saturated treatment. Formation of stolons and strobilae occurred in all treatments, however, the proportions of polyps undergoing stolonation and strobilation were significantly greater in all DO concentrations above 0.5 mg l^–1. Polyp encystment was not observed in any treatment over the course of the 24 d experiment. These results indicate that polyps can survive and asexually propagate even during prolonged exposure to hypoxic conditions.     

秋季降温期水温和饵料量对沙蜇无性繁殖的影响

20世纪末以来,沙蜇在东亚水域频繁暴发,不仅影响正常的渔业生产活动,也给海洋生态系统健康带来极大危害。在室内模拟秋季降温条件,研究了不同水温（4种降温条件）与饵料量（0.42、1.25、和4.2 μg C螅状体^-1d^-1）对沙蜇螅状体足囊繁殖与横裂生殖的影响。结果显示：1）秋季降温速度快慢导致的积温差异对沙蜇螅状体生成足囊数有显著影响（P < 0.001）,积温越大,螅状体生成的足囊数量越多,发现两者具有较好的线性关系（y=142.59x+208.39;R²=0.967）,低积温条件下的足囊萌发也较少;2）饵料量对螅状体无性繁殖的影响是有一定阈值范围的,在一定饵料量范围内,螅状体生成的足囊数量随饵料量的增加而增加,而超过一定阈值范围,过剩的饵料影响水质对沙蜇螅状体反而产生负面影响。3）秋季新繁育出来的当龄沙蜇螅状体可在越冬前发生横裂生殖,但横裂生殖的螅状体比例和放碟数量均较少。试验结果说明,全球变化背景下,积温升高、饵料量小幅增加均可能导致沙蜇秋季有性繁殖的当龄螅状体在越冬前生成更多的足囊,促进诱发翌年水母暴发。     

温度、投饵频次对白色霞水母无性繁殖与螅状体生长的影响

白色霞水母是我国近海主要大型灾害水母种类之一,其暴发性增殖严重破坏了海洋生态系统平衡.在室内控制条件下,研究了温度(7.5、11、14.5、18、21.5和25℃)和投饵频次(1次/2d、1次/8d和1次/16d)对白色霞水母无性繁殖与螅状体生长的影响.结果显示,白色霞水母足囊繁殖的适宜温度为18-25℃,足囊繁殖随温度和投饵频次的增加而增加.温度对白色霞水母横裂率和横裂次数的影响显著,温度越高,白色霞水母发生横裂生殖的时间越早,横裂生殖速度越快,重复横裂次数越多,释放的碟状体数量也越多.横裂率和横裂次数随投饵频次的增加而递增.白色霞水母螅状体在7.5-25℃范围的成活率均为100％,其生长速度随温度和投饵频次的增加而增加.温度和投饵频次对白色霞水母螅状体足囊繁殖、横裂率和螅状体生长具有明显的交互效应.螅状体的横裂次数和初生碟状幼体伞径随螅状体柄径增大而递增,呈线性相关.研究表明,温度、投饵频次即营养条件显著影响着白色霞水母的种群数量,说明海水水温上升、富营养化或渔业资源锐减导致的浮游动物量增加均可能诱发白色霞水母暴发性增殖.结论为进一步探索大型水母暴发的生态环境机理提供重要科学依据.     

Constraints on the evolution of asexual reproduction

Sexual reproduction is almost ubiquitous among multicellular organisms even though it entails severe fitness costs. To resolve this apparent paradox, an extensive body of research has been devoted to identifying the selective advantages of recombination that counteract these costs. Yet, how easy is it to make the transition to asexual reproduction once sexual reproduction has been established for a long time? The present review approaches this question by considering factors that impede the evolution of parthenogenesis in animals. Most importantly, eggs need a diploid chromosome set in most species in order to develop normally. Next, eggs may need to be activated by sperm, and sperm may also contribute centrioles and other paternal factors to the zygote. Depending on how diploidy is achieved mechanistically, further problems may arise in offspring that stem from 'inbreeding depression' or inappropriate sex determination systems. Finally, genomic imprinting is another well-known barrier to the evolution of asexuality in mammals. Studies on species with occasional, deficient parthenogenesis indicate that the relative importance of these constraints may vary widely. The intimate evolutionary relations between haplodiploidy and parthenogenesis as well as implications for the clade selection hypothesis of the maintenance of sexual reproduction are also discussed.     

Use of respiration rates of scyphozoan jellyfish to estimate their effects on the food web

One of the main objectives of research on jellyfish is to determine their effects on the food web. They are voracious consumers that have similar diets to those of zooplanktivorous fish, as well as eating microplankton and ichthyoplankton. Respiration rates (RRs) can be used to estimate the amount of food needed to balance metabolism, and thereby estimate minimum ingestion. We compiled RRs for scyphozoan medusae in three suborders (Semeaostomeae, Rhizostomeae, and Coronatae) to determine if a single regression could relate RRs to mass for diverse scyphomedusan species. Temperature (7–30°C) was not a significant factor. RRs versus wet weight (WW) regressions differed significantly for semeaostome and rhizostome medusae; however, RRs versus carbon mass over five-orders of magnitude did not differ significantly among suborders. RRs were isometric against medusa carbon mass, with data for all species scaling to the power 0.94. The scyphomedusa respiration rate (SRR) regression enables estimation of RR for any scyphomedusa from its carbon mass. The error of the SRR regression was ±72%, which is small in comparison with the 1,000-fold variation in field sampling. This predictive equation (RR in ml O₂ d⁻¹ = 83.37 * g C^0.940) can be used to estimate minimum ingestion by scyphomedusae without exhaustive collection of feeding data. In addition, effects of confinement on RRs during incubation of medusae were tested. Large medusae incubated in small container volumes (CV) relative to their size (ratios of CV:WW < 50) had RRs ~one-tenth those of medusae in relatively larger containers. Depleted oxygen during incubation did not depress RRs of the medusae; however, swimming may have been restricted and respiration reduced in consequence. We briefly review other problems with RR experiments and suggest protocols and limitations for estimating ingestion rates of jellyfish from metabolic rates.     

Grazing by two species of limpets on artificial reefs in the northwest Mediterranean

The extensive presence of artificial reefs in marine coastal habitats demands a better understanding of the extent to which these structures can be considered surrogates of natural rocky shores for populations of plants and animals. The primary aim of this study was to test the hypothesis that removing limpets from the midlittoral of artificial breakwaters in the northwest Mediterranean led to changes in assemblages similar to those observed on rocky shores in the same area. Orthogonal combinations of the presence/absence of two species of limpets, P. aspera and P rustica, were produced using manual removals from June 1997 to February 1998. To test the hypothesis that the effects of limpets were variable at spatial scales comparable to those investigated on rocky shores, we repeated the experiment at two locations tens of kilometres apart, and on two reefs within each location a few kilometres apart. The results revealed strong and relatively consistent negative effects of limpets on filamentous algae, whereas interactions with other members of assemblages were complex and variable. Several taxa (Cyanophyta, encrusting and articulated coralline algae, Ralfsia and Rissoella) were abundant at one location but nearly absent at the other. This large-scale variability in patterns of distribution generated inconsistencies in the effects of limpets between locations. Within locations, several effects of P. aspera and P. rustica were observed, ranging from independent effects on some organisms, to additive or interactive effects on others. Apparently, the removal of filamentous algae by limpets resulted in positive indirect effects on Ralfsia and Rissoella. Collectively, these effects were comparable to those described for rocky shores in the northwest Mediterranean. The processes accounting for large-scale variation in grazing, however, appeared different between the natural and the artificial habitat.     

Food supply effects on the asexual reproduction and respiratory metabolism of Aurelia aurita polyps

Hydrobiologia - Ecological speciation, which relies heavily on selection driving the emergence of new species, has become the primary paradigm through which rapid species radiations are understood....     

Influence of nitrogen nutritions on asexual reproduction of five species Sphaeropsis sacc. in culture

Effect of fourteen different nitrogen sources on five species ofSphaeropsis i. e.S. cactinae, S. cycadis, S. jodhpurensis, S. muehlenbeckiae andS. punicae, obtained respectively from the infected parts ofCitrus medica, Cycas circinalis, Cordia gharaf, Muehlenbeckia platycladoes andPunica granatum was studied, under cultural conditions on Asthana and Hawker's Medium A by substituting its nitrogen with equal quantity of an other nitrogen source. All the present species reproduced well on potassium nitrate, sodium nitrate, ammonium sulphate and ammonium nitrate. None of the present species could grow on sodium nitrite. Out of all amino acids, no one was found to be excellent for the growth and sporulation of the present fungi. Most of the amino acids supported poor growth and sporulation of three of the five species ofSphaeropsis, S. cactinae, S. cycadis andS. punicae on L-asparagine, Glycine, L-cystine and L-alanine. Maximum and minimum pH for growth and sporulation ranged from 6.5 to 7.8 and 5.3 to 4.2 respectively. Decrease in adjusted pH was also observed for all the species on L-asparagine and DL-valine.     

The effects of continuous light and light intensity on the reproduction rates of twenty-two species of marine phytoplankton

The acclimated reproduction rates of 22 species of marine phytoplankton were measured at 0.01, 0.023, 0.1, and 0.23 ly/min in continuous light and in a 14: 10 h light: dark cycle. Three species that reproduced exponentially at all four light intensities in the 14: 10 LD regime did not reproduce at all in continuous light at any of the light intensities. One species, which reproduced at the two lowest light intensities in the 14: 10 LD regime, failed to reproduce at all in continuous light at any light intensity examined. Seven species reproduced more slowly in continuous light than in the 14: 10 LD regime at most or all light intensities. Four species reproduced at roughly the same rate in both light regimes. Five species reproduced more rapidly in continuous light.

No general phylogenetic trend could be discerned from the responses of the species to the different light intensities or to continuous light. In general, species from coastal regions can reproduce as rapidly or more rapidly in continuous light than in a 14: 10 LD cycle, while most species from oceanic regions are harmed by continuous light. A phylogenetic trend in maximum potential reproduction rate is apparent, with diatoms being the fastest, dinoflagellates the slowest, and coccolithophores somewhat intermediate.     

Six new Richtersia species (Nematoda, Selachinematidae) from the Mediterranean Sea

Six new species of the genus Richtersia Steiner, 1916 are described from the Mediterranean Sea: R. bathyalis sp.n., R. spinosa sp.n., R. heipi sp.n., R. mediterranea sp.n., R. staresensis sp.n. and R. coomansi sp.n. Some morphological adaptations of R. bathyalis, R. spinosa and R. heipi are discussed in relation to their small body size. The structure of the stomatal region, labial region and spicules is described in detail and probably holds for the entire genus.     

Existence of two sexual races in the planarian species switching between asexual and sexual reproduction

In certain planarian species that are able to switch between asexual and sexual reproduction, determining whether a sexual has the ability to switch to the asexual state is problematic, which renders the definition of sexuals controversial. We experimentally show the existence of two sexual races, acquired and innate, in the planarian Dugesia ryukyuensis. Acquired sexuals used in this study were experimentally switched from asexuals. Inbreeding of acquired sexuals produced both innate sexuals and asexuals, but inbreeding of innate sexuals produced innate sexuals only and no asexuals. Acquired sexuals, but not innate sexuals, were forced to become asexuals by ablation and regeneration (asexual induction). This suggests that acquired sexuals somehow retain asexual potential, while innate sexuals do not. We also found that acquired sexuals have the potential to develop hyperplastic and supernumerary ovaries, while innate sexuals do not. In this regard, acquired sexuals were more prolific than innate sexuals. The differences between acquired and innate sexuals will provide a structure for examining the mechanism underlying asexual and sexual reproduction in planarians.     

A new species of Caprella (Crustacea: Amphipoda) from the Mediterranean Sea

A new caprellid amphipod, Caprella tavolarensis n. sp., is described based on specimens collected from a Posidonia oceanica seagrass meadow at the Tavolara-Punta Coda Cavallo Marine Protected Area (Sardinia, Mediterranean Sea). The species is close to Caprella liparotensis, but can be clearly distinguished by smaller size, presence of a short rostrum, body elongate and dorsally smooth, absence of serrate carina on the basis of gnathopod 2 and pereopods, mouthparts scarcely setose, absence of fine setae on peduncle of antenna 1 and absence of swimming setae on antenna 2. The number of caprellid species reported from the Mediterranean Sea has increased from 23 (1993) to 41 (2010), consequently, further taxonomical studies should be addressed to properly estimate the total amphipod diversity in the Mediterranean Sea.     

Effects of pH on asexual reproduction and statolith formation of the scyphozoan, <Emphasis Type="Italic">Aurelia labiata</Emphasis>

Although anthropogenic influences such as global warming, overfishing, and eutrophication may contribute to jellyfish blooms, little is known about the effects of ocean acidification on jellyfish. Most medusae form statoliths of calcium sulfate hemihydrate that are components of their balance organs (statocysts). This study was designed to test the effects of pH (7.9, within the average current range, 7.5, expected by 2100, and 7.2, expected by 2300) combined with two temperatures (9 and 15°C) on asexual reproduction and statolith formation of the moon jellyfish, Aurelia labiata. Polyp survival was 100% after 122 d in seawater in all six temperature and pH combinations. Because few polyps at 9°C strobilated, and temperature effects on budding were consistent with published results, we did not analyze data from those three treatments further. At 15°C, there were no significant effects of pH on the numbers of ephyrae or buds produced per polyp or on the numbers of statoliths per statocyst; however, statolith size was significantly smaller in ephyrae released from polyps reared at low pH. Our results indicate that A. labiata polyps are quite tolerant of low pH, surviving and reproducing asexually even at the lowest tested pH; however, the effects of small statoliths on ephyra fitness are unknown. Future research on the behavior of ephyrae with small statoliths would further our understanding of how ocean acidification may affect jellyfish survival in nature.     

Annual cycle and diversity of species and infraspecific taxa of Ceratium (Dinophyceae) in the Ligurian Sea,northwest Mediterranean1

We examined the well‐documented and species‐rich dinoflagellate genus Ceratium Schrank in the northwest Mediterranean Sea as a possible model for marine phytoplankton diversity and as a biological indicator of global climate change. First, we investigated the influence of counting effort; we then documented temporal changes in Ceratium specific and infraspecific taxa over 2 years (2002 and 2003) in the Villefranche Bay based on a monthly net sampling. Finally, we tried to identify factors associated with shifts in biodiversity. The calculation of taxonomic diversity, regularity, and richness were highly dependent on counting effort. We determined that a minimal sample volume of ～70 L was needed to obtain a good estimation of species richness. The annual cycle was characterized by a seasonal trend of high winter species richness followed by low spring biodiversity. Infraspecific variability not only appeared to depend on water temperature but also seemed to be influenced by bottom‐up control and was strongly affected by top‐down control. Thus, the occurrence of high concentrations of salps (Thalia democratica) and copepods larger than 2 mm (Calanus helgolandicus) coincided with a drastic decrease of Ceratium abundance and diversity during spring 2003. Ceratium is sensitive to both abiotic and biotic factors and could prove to be a good candidate as a biological indicator of global change.     

A new species of Diplecogaster (Actinopterygii: Gobiesocidae) from the Mediterranean Sea

Diplecogaster umutturali sp. n. is described from five specimens, ranging from 9.5 to 26.9 mm in standard length. This clingfish is known to occur only at three localities along the northern Levant coasts, eastern Mediterranean Sea and is the third known species of the genus Diplecogaster in the Mediterranean and Black Seas. The species is distinguished from all its congeners in having a small snout, short caudal peduncle, large eyes, short anal fin and upper attachment of gill membrane opposite the 7th-8th pectoral ray. Diplecogaster umutturali sp. n. shows a 5.3% genetic divergence from its close relative D. bimaculata at the cytochrome oxidase I locus (COI).

http://www.zoobank.org/urn:lsid:zoobank.org:pub:376298FE-C8A7-44BB-9B12-721E41D7F355     

The effects of sexual and asexual reproduction on geographic variation in the sea anemone Actinia tenebrosa

Allelic and genotypic frequencies were determined for samples from 35 widely distributed Australasian colonies of Actinia tenebrosa and 2 South African colonies of A. equina. These data provided no evidence of gene flow between Australisian and South African Actinia colonies and indicated that there may be some restriction of gene flow between widely separated Australasian colonies.Both species are viviparous, and brooded A. tenebrosa are known to be produced asexually. The present data indicate that, within both species, almost all genotypic diversity is generated by sexual reproduction with recombination. Sexually produced juveniles appear to be widely dispersed and panmixis may occur over thousands of kilometres. However, successful sexual recruitment must be episodic or rare. Colonies on stable shores displayed relatively low levels of genotypic diversity, as compared with expectations for sexually reproducing populations, indicating strong local effects of asexual recruitment. Clonal genotypes may be spread over hundreds of metres of shore, but are typically restricted to discrete colonies. Asexual recruitment is highly localised and asexual dispersal appears to be limited by lengths of shore (500 m) which are unsuitable for colonization. Colonies on unstable shores are significantly more diverse genotypically and show little evidence of clonal proliferation.     

Growth and asexual reproduction of the sea anemone Metridium: comparative laboratory studies of three species

The rate of growth, individual size attained, and mode and frequency of asexual reproduction were investigated in three species of the sea anemone Metridium. The species differ in ecological distribution, characteristic habitat, and morphology. M. exilis Hand is characteristically intertidal and occurs in aggregations of numerous small (0.2 cm² pedal disc area, pda) individuals. Growth in laboratory culture ceased at 1.2 cm² pda despite copious feeding. Feeding accelerated the rate of asexual reproduction by binary fission but did not result in larger individuals. Individuals of M. senile (L.) are larger and reproduce both sexually and asexually by fragmentation. The species occurs from the mid-intertidal to the shallow subtidal in central California. Individuals from intertidal populations grew rapidly in laboratory culture when fed daily (mean pda after 5 months in culture was 45 cm²). The regenerative ability of M. senile is excellent: 100% of excised fragments (including column and pedal disc tissue) had completely regenerated, including tentacle formation, after 3 wk with no mortality. An undescribed species of Metridium is much larger (individuals grow >/ l m in height) and apparently is exclusively subtidal in central California. Asexual reproduction is rare or absent and no pedal lacerates were produced by individuals during the period of study. The regenerative ability of Metridium sp. is poor: 53% of the excised fragments had regenerated after 19 wk with 25% mortality. The three species thus show distinct patterns of growth and asexual reproduction, paralleling their morphological, ecological, and genetic distinctness.