Advanced Cambrian hydroid fossils (Cnidaria: Hydrozoa) extend the medusozoan evolutionary history

Primitive cnidarians are crucial for elucidating the early evolution of metazoan body plans and life histories in the late Neoproterozoic and Palaeozoic. The highest complexity of both evolutionary aspects within cnidarians is found in extant hydrozoans. Many colonial hydrozoans coated with chitinous exoskeletons have the potential to form fossils; however, only a few fossils possibly representing hydroids have been reported, which still require scrutiny. Here, we present an exceptionally well-preserved hydroid found in the Upper Cambrian Fengshan Formation in northern China. It was originally interpreted as a problematic graptolite with an uncertain systematic position. Based on three characteristic morphological traits shared with extant hydroids (with paired hydrothecae, regular hydrocaulus internodes and special intrathecal origin pattern of hydrocladium), we propose this fossil hydroid as a new genus, Palaeodiphasia gen. nov., affiliated with the advanced monophyletic hydrozoan clade Macrocolonia typically showing loss of the medusa stage. More Macrocolonia fossils reviewed here indicate that this life strategy of medusa loss has been achieved already as early as the Middle Devonian. The early stratigraphical appearance of such advanced hydroid contrasts with previous molecular hypotheses regarding the timing of medusozoan evolution, and may be indicative for understanding the Ediacaran cnidarian radiation.     

Fossilization processes of graptolites: insights from the experimental decay of Rhabdopleura sp. (Pterobranchia)

Laboratory experiments documenting the decomposition pattern of extant organisms are used to reconstruct the anatomy and taphonomy of fossil taxa. The subclass Graptolithina (Hemichordata: Pterobranchia) is a significant fossil taxon of the Palaeozoic era, represented by just one modern genus, Rhabdopleura. The rich graptolite fossil record is characterized by an almost total absence of fossil zooids. Here we investigated the temporal decay pattern of Rhabdopleura sp. tubes, stolons and single zooids removed from the tubarium. Tubes showed decay after four days, when fuselli began to separate from the tube walls. This rapid loss may explain the absence of fuselli from some graptolite fossils. The black stolon did not show decay until day 155. One day after their removal, zooids quickly decomposed in the following temporal sequence: (1) tentacles; (2) ectoderm; (3) arms; (4) gut; (5) cephalic shield, leading to complete disappearance of recognizable body parts in the majority of experimental zooids within 64–104 h. The most resistant zooid features to decay (61 days) were black‐pigmented granules. These results indicate that tubes and the black stolon would persist for weeks across death, transport and burial, whereas a complete decay of zooid features occurs in few days, providing an explanation for the overall poor record of fossil graptolite zooids and suggesting that recorded silhouettes of fossil zooids may be attributed to fossil decay‐resistant pigments.     

Two new species of Electra (Bryozoa, Cheilostomata) from the Miocene of the Aquitaine Basin, France

Post-Cretaceous examples of Electridae, a primitive family of cheilostome bryozoans, are poorly represented in the fossil record, probably because of their thinly calcified zooids and preference for nearshore environments. Two new electrid species are here described from the Lower Miocene (Burdigalian) of Pontpourquey, Aquitaine, France: Electra triaurata nov. sp. and Electra aquitanica nov. sp. Both species belong to extant species groups, the E. indica and E. biscuta groups, respectively, that presently occur in the Indo-Pacific; both are the only fossil examples of these species groups. Whereas E. triaurata nov. sp. has uniserial colonies, zooids with porous gymnocysts, three flattened spines and basal windows allowing etching of the substrate to produce the trace fossil Leptichnus, E. aquitanica nov. sp. has multiserial colonies and zooids with a proximal gymnocyst bearing 2 to 5 spines.     

Feeding ecology and trophic impact of the hydroid Obelia dichotoma in the Kongsfjorden (Spitsbergen, Arctic)

Obelia dichotoma is a thecate hydroid with a worldwide distribution, occurring mainly on shallow water hard substrates. Since the trophic ecology of hydroids in polar waters is badly understood, the aim of the present work was to study qualitatively and quantitatively the diet of these organisms in an Arctic environment and to determine their trophic significance. For this purpose, the density of the hydroid population was documented, and simultaneously, zooplankton was sampled in two different years (1997 and 1998). Prey capture rates were estimated by analysing the gastrovascular content of the polyps in a diurnal cycle. Additionally, the digestion time of O. dichotoma was measured by laboratory feeding experiments using diatoms as food items. The analyses of the gastrovascular cavities of the polyps sampled during the diurnal cycles showed that O. dichotoma fed mainly on faecal pellets, organic matter and microalgae. Zooplankton prey was also observed, but gastrovascular contents and zooplankton abundance did not show any correlation in both years. The consumption rates of the hydroid populations differed between the 2 years. It was almost double (8.9 mg Carbon m^?2) in 1998 compared to 1997 (5.5 mg Carbon m^?2). The significance of the environmental variability in the feeding ecology and population dynamics of hydroids under Arctic conditions is discussed.     

A new hydroid from the Upper Cretaceous of Mississippi

A new fossil hydroid is reported as an organic impression on a calcareous gastropod steinkern from the Prairie Bluff Chalk (Maastrichtian), Oktibbeha County, Mississippi. This is the first such hydroid reported from the Upper Cretaceous of the Atlantic Coastal Plain. The fossil organism consists of anastomosing hydrorhizae forming a holdfast, a fascicled hydrocaulus, and elongated, crenulated and ribbed hydrothecae. The fossil is unlike other Mesozoic hydroids that have been reported from Europe and North America; it is described as Mesodendrium oktibbehaensis gen. et sp. nov. and tentiatively referred to an extant family, the Campanulariidae (Calyptoblastina). The complete preservation of the holdfast, hydrocaulus and hydrothecae suggests that this hydroid lived inside gastropod shells. In analogy with Recent symbiotic hydroids inhabiting mollusc shells, the new specimen described here possibly represents the oldest known example of a symbiotic relationship between hydroids and hermit crabs.     

Effects of the 1997–1998 El Niño on the cnidarian community of a high turbidity coral reef system (northern Bahia,Brazil)

We are reporting on the results of a 4-year study that allowed for the analysis of the effects of the 1997–1998 El Niño Southern Oscillation (ENSO) event on the cnidarian community of the naturally high-turbidity reefs of northern Bahia, Brazil. Cnidarian densities were significantly different between pre- and post-ENSO years, with significant consequent changes in community composition. BIOENV analysis identified variations in turbidity, mean temperature variance, and cloud cover as the factors best explaining changes in the community. We suggest that the 1997–1998 ENSO event, the most intense on record, had a differential effect on the cnidarian community. Highest mortalities were suffered by minute hydroid colonies, partial mortality by octocorals and actiniarians, while large hydroid colonies and scleractinians were the least affected. Such an ability to survive under the stressful environmental regime imposed by this ENSO event is attributed to the presence of morphological pre-adaptations (e.g., size of colony polyps, abundance of certain nematocyst types) and the inherent phenotypic plasticity that results from long-term exposure to naturally stressful conditions.     

Hydroid assemblages from the Bellingshausen Sea (Antarctica): environmental factors behind their spatial distribution

Although scientific knowledge about the biodiversity of Antarctic benthic hydrozoans has considerably increased in recent years, little is known about their spatial distribution and underpinning factors. Trying to contribute to filling this gap, benthic hydroid spatial distribution in the Bellingshausen Sea (Southern Ocean) was studied. Samples were collected at 32 stations at depths between 86 and 3,304 m during Spanish Antarctic expeditions in 2003 and 2006. Sediments and bottom water properties were analyzed using an USNEL-type box corer and a Neil Brown Instrument System Mark III CTD, respectively. Forty species were reported (Acryptolaria sp., Stegopoma plicatile, Staurotheca dichotoma having the highest percentages of occurrence), representing ca. 19 % of the species richness of the known benthic hydroid fauna of the Southern Ocean. Three well-defined assemblages (shallow, deep and transitional) were established based on significant differences in species occurrence. Benthic hydroid spatial distribution in the Bellingshausen Sea seems to be controlled mainly by depth and substrate (most hydrozoan species are epibiotic), by species dispersal abilities and by species resilience to changing hydrodynamic conditions. The level of species richness found in the present study, compared with other Antarctic areas, gives support to arguments stated by authors against the idea that the Bellingshausen Sea is “a benthos desert” controlled by oligotrophic conditions and intense iceberg traffic.     

Colony integration and the expression of the Hox gene, Cnox-2, in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa).

The stolonal mat is an anatomical feature correlated with increased colonial integration in several lineages of the cnidarian class Hydrozoa. Cnox-2 is a Hox gene known to be expressed in the body column of the cnidarian polyp. We report the pattern of Cnox-2 expression in both the stolonal mat and free stolons of the hydroid Hydractinia symbiolongicarpus. The gene is found to have high levels of expression in the mat similar to that found in the basal portion of the polyp, but it is not detectably expressed in those regions of free stolons where polyps are budded. These findings suggest that the stolonal mat arose via an expansion of the basal ectoderm of the polyp.     

埃迪卡拉纪至寒武纪苗岭世刺细胞动物化石研究现状与展望

刺细胞动物是一类具有刺细胞的水生无脊椎动物,分布在世界各地的海洋和淡水中。作为后生动物最早分化出的一支,刺细胞动物对研究后生动物的起源和早期演化具有极其重要的意义,也为研究后生动物系统发育、地层对比和古地理恢复等方面提供了重要的科研线索。本文简要介绍了刺细胞动物早期(埃迪卡拉纪至寒武纪苗岭世)的化石记录和研究现状,将刺细胞动物的早期演化分为四个时期,分别为前寒武纪、寒武纪纽芬兰世、寒武纪第二世和寒武纪苗岭世,并对上述四个时期的刺细胞动物进行论述。目前的研究表明:1)前寒武纪的刺细胞动物化石记录虽然比较丰富,但充满争议,目前尚未有比较确切的刺细胞动物记录;2)寒武纪纽芬兰世如宽川铺生物群中保存了较多的刺细胞动物胚胎及发育序列化石,是研究刺细胞动物演化的重要窗口;3)寒武纪第二世,以澄江生物群为代表的布尔吉斯页岩型化石保存了软躯体的刺细胞动物。与此同时,在第二世中已经出现了最早的珊瑚化石记录;4)寒武纪苗岭世刺细胞动物主要以布尔吉斯页岩型化石保存(如布尔吉斯页岩生物群和凯里生物群),同时期在美国犹他州发现的刺细胞动物印痕化石表明了这个时期几乎所有的刺细胞动物大类已经出现。本文根据上述时期的研究,对今后刺细胞动物化石的研究提供一些建议和讨论。     

Establishment of primary cell culture from the temperate symbiotic cnidarian, Anemonia viridis

The temperate symbiotic sea anemone Anemonia viridis, a member of the Cnidaria phylum, is a relevant experimental model to investigate the molecular and cellular events involved in the preservation or in the rupture of the symbiosis between the animal cells and their symbiotic microalgae, commonly named zooxanthellae. In order to increase research tools for this model, we developed a primary culture from A. viridis animal cells. By adapting enzymatic dissociation protocols, we isolated animal host cells from a whole tentacle in regeneration state. Each plating resulted in a heterogeneous primary culture consisted of free zooxanthellae and many regular, small rounded and adherent cells (of 3–5 μm diameter). Molecular analyses conducted on primary cultures, maintained for 2 weeks, confirmed a specific signature of A. viridis cells. Further serial dilutions and micromanipulation allowed us to obtain homogenous primary cultures of the small rounded cells, corresponding to A. viridis “epithelial-like cells”. The maintenance and the propagation over a 4 weeks period of primary cells provide, for in vitro cnidarian studies, a preliminary step for further investigations on cnidarian cellular pathways notably in regard to symbiosis interactions.     

A new species of the cheilostome bryozoan Chiastosella in the Southern Ocean,past and present

Understanding whether marine calcifying organisms may acclimatise to climate change is important with regard to their survival over the coming century. Due to cold waters having a naturally higher CO₂ uptake, the Southern Ocean provides an especially good opportunity to study the potential impact of climate change. In 2011, a new cheilostome bryozoan species—Chiastosella ettorina sp. nov.—was dredged from Burdwood Bank, Southern Ocean, at 324–219-m depth during the Nathaniel B Palmer Cruise. This species had previously been collected in 1902 from the same area at 100-m depth, but was incorrectly identified as Chiastosella watersi, an encrusting species from New Zealand. The availability of samples of the same species, from the same general location, but collected 109 years apart allowed us to investigate morphological modifications potentially arising from environmental changes. We found a significant difference in zooid size, with the oldest and shallowest specimens having smaller zooids than the recently collected deeper specimens. This difference in zooid size appears to be unrelated to known sources of environmental variation such as temperature and salinity, and it could represent the extremes of the zooid size range of C. ettorina. An alternative explanation is that acidifying waters may have caused zooids to grow more slowly, resulting in a final larger size.     

Tectonic stability since the last interglacial offsets the Glorieuses Islands from the nearby Comoros archipelago

The four fossil terraces of the Glorieuses Islands are described, and new dates are provided to resolve their stratigraphy, depositional setting, and tectonic behavior. Most outcrops consist of a single sedimentary unit that represents the remains of an extensive reef flat dominated by Isopora palifera corals. At Lys Island, this unit is locally overlain by dipping layered beds composed of Halimeda segments, tentatively interpreted as storm overwash. Reliable U/Th dates obtained from corals sampled from the fossil outcrops mostly fall between 127 and 123 kyr, suggesting that these reefs formed exclusively during the first MIS-5e sea-level highstand, when sea level reached +3 m. The mean elevation of these terraces being +4.5 m, an uplift of 0.012 ± 0.002 mm year^?1 is inferred. This relative tectonic stability contrasts with the subsidence reported from Mayotte Island, suggesting a different geologic setting for the nearby Comoros and Glorieuses archipelagoes.     

The dormant buds of Rhabdopleura compacta (Hemichordata)

Rhabdopleura has an overwintering stage that consists of two layers of cells surrounding a central yolk mass. This cellular part is surrounded by a thick electron dense capsule which is secreted by the bud itself. The capsule is probably impervious and protective to its contents. Blood vessels join the buds to the zooids of the colony. They form the probable route of transfer of yolk from the zooids to the dormant bud. The capsule of the dormant bud has some structural features in common with the black stolon of the adult zooids. The black stolon is probably formed in a manner similar to that which made the fusellar fabric of the periderm of fossil graptolities.     

New material of Diacodexis (Mammalia,Artiodactyla) from the early Eocene of Southern Europe

Diacodexeidae are the first representatives of Artiodactyla in the fossil record. Their first occurrence is at the very base of the Ypresian (earliest Eocene, 56.0 Ma) with Diacodexis, a genus well diversified during the early Eocene in Europe, especially during the MP7–MP8 + 9 interval. However, most of European species are documented by scarce material, retrieved from single localities. In this work, we describe new Diacodexis material from ～MP7 and ～MP8 + 9 localities of Southern Europe, including material of D. antunesi from Silveirinha, considered as the most primitive European Diacodexis species, and material from three localities from Southern France (Fordones, Palette, and La Borie). The new material documents Diacodexis premolar morphology and deciduous dentition which bear potentially important phylogenetic information, as well as astragali, including a specimen from Silveirinha that constitutes the earliest occurrence of an astragalus of the genus Diacodexis in the European fossil record. Investigation of the enamel microstructure reveals that early European species had a simple enamel pattern with one-layered Schmelzmuster composed of ‘basic’ radial enamel only, instead of the two-layered Schmelzmuster (thin radial enamel + thick layer of Hunter-Schreger bands) observed on North American species and so far considered to represent the primitive condition within Artiodactyla. In accordance with previous studies, our observations highlight that Diacodexis gigasei from Belgium is morphologically closer to the North American species D. ilicis than to D. antunesi from Portugal. The latter species, together with D. aff. antunesi from Fordones, appears to be morphologically closer to the Asiatic taxa D. indicus and D. pakistanensis. Finally, we found numerous similarities between D. cf. gigasei from Palette and D. gigasei, a result that challenges the intra-European provincialism that characterizes the earliest Ypresian. Diacodexis gigasei could be one of the rare species shared by the northwestern and southwestern European bioprovinces.     

Looking for long-term changes in hydroid assemblages (Cnidaria,Hydrozoa) in Alboran Sea (South-Western Mediterranean): a proposal of a monitoring point for the global warming

In the last 20–30 years, the temperature of the Mediterranean Sea has increased and global warming is allowing the establishment of tropical-affinity species into more temperate zones. Sessile communities are particularly useful as a baseline for ecological monitoring; however, a lack of historical data series exists for sessile marine organisms without commercial interest. Hydroids are ubiquitous components of the benthic sessile fauna on rocky shores and have been used as bio-indicators of environmental conditions. In this study on the benthic hydroid assemblages of the Chafarinas Islands (Alboran Sea, South-Western Mediterranean), we characterized the hydroid assemblages, identified the bathymetric gradients, and compared them with a previous study carried out in 1991. Hydroid assemblages showed a significant difference both between year and among depths. Furthermore, eight species not present in 1991 were found, including two possible new species and the tropical and subtropical species Sertularia marginata. Due to its strategic position at the entrance of the Mediterranean and the existence of previous data on hydroid assemblages, the Chafarinas Islands are proposed as a possible monitoring point for entrance of Atlantic tropical species into the Mediterranean Sea.     

Water transparency affects the survival of the medusa stage of the invasive freshwater jellyfish <Emphasis Type="Italic">Craspedacusta sowerbii</Emphasis>

Craspedacusta sowerbii is a global invader of inland waters, but little is known on what environmental factors control its colonization success. We hypothesized that water transparency and thus, exposure to UV radiation are relevant for the colonization success of this relatively transparent invasive hydroid. Here we used field and laboratory experiments to assess the effect of natural solar radiation and artificial UV radiation on the medusa stage of C. sowerbii across natural and simulated water transparency gradients in three Chilean Patagonian lakes. Short-term exposure of the jellyfish to artificial UV radiation under low Dissolved Organic Carbon (DOC) treatments induced mortality and caused sublethal effects such as swimming anomalies and production of reactive oxygen species. Outdoor exposure of jellyfish to full solar radiation in the most transparent water (Lake Ranco; DOC = 0.6 mg l^?1) resulted in almost complete mortality. However, higher DOC contents provided partial or almost complete protection against the adverse impact of UV radiation. Overall, our findings provide evidence of the role of underwater light conditions in the underlying mechanisms that may favor the invasion of this gelatinous alien species in inland waters. The results imply that under the current “brownification” of lakes, an increase in water color might provide more favorable conditions for the invasion process of this hydroid.     

Intracellular pH of symbiotic dinoflagellates

Intracellular pH (pH_i) is likely to play a key role in maintaining the functional success of cnidarian–dinoflagellate symbiosis, yet until now the pH_i of the symbiotic dinoflagellates (genus Symbiodinium) has never been quantified. Flow cytometry was used in conjunction with the ratiometric fluorescent dye BCECF to monitor changes in pH_i over a daily light/dark cycle. The pH_i of Symbiodinium type B1 freshly isolated from the model sea anemone Aiptasia pulchella was 7.25 ± 0.01 (mean ± SE) in the light and 7.10 ± 0.02 in the dark. A comparable effect of irradiance was seen across a variety of cultured Symbiodinium genotypes (types A1, B1, E1, E2, F1, and F5) which varied between pH_i 7.21–7.39 in the light and 7.06–7.14 in the dark. Of note, there was a significant genotypic difference in pH_i, irrespective of irradiance.     

A new deep-sea hydroid (Cnidaria: Hydrozoa) from the Bering Sea Basin reveals high genetic relevance to Arctic and adjacent shallow-water species

The marine benthic fauna in Arctic shallow-water is reported to be a relatively young assemblage by species of either Pacific or Atlantic affinity. Whether current deep-sea Pacific species are included in the affinity or not is unknown. Combining morphological comparisons and genetic analyses, a new deep-sea hydroid to science, Sertularia xuelongi sp. nov. (Cnidaria: Hydrozoa: Sertulariidae), is described from the northern margin of the Bering Sea Basin at depths of 800–1570 m collected in 2010. It is characterized by slender and zigzag-shaped hydrocauli, alternately arranged hydrothecae and the absence of distal-lateral horns in fully matured female gonothecae. Its distribution, currently known only from Bering Sea Basin, suggests that it could not be an Arctic species with Pacific affinity. However, phylogenetic analyses based on the mitochondrial 16S rRNA gene show that it is clustered into a distinctive clade with four closely related species recorded from shallow-water of Northwest France, Iceland, Chukchi Sea and/or Bering Sea. In addition, its sequence similarity is highly relevant to these four species: Sertularia argentea (98.6 %), S. cupressina (98.8 %), S. plumosa (98.8 %) and S. robusta (99.4 %). All these provide a new insight into the relevance of North Pacific deep-sea species to the benthic fauna in Arctic and adjacent shallow-water. The taxonomic restriction of the genus Sertularia and the re-validation of the genus Polyserias are discussed. Future researches on more deep-sea species from Pacific and/or Atlantic are required to understand the evolution and speciation pattern involved in polar relevance.     

Mid-Pliocene bald ibis (<Emphasis Type="Italic">Geronticus</Emphasis> cf. <Emphasis Type="Italic">calvus</Emphasis>; Aves: Threskiornithidae) from the Cradle of Humankind,Gauteng, South Africa and its environmental and evolutionary implications

We document the presence of the bald ibis genus Geronticus Wagler, 1832 (Aves: Threskiornithidae) from the mid-Pliocene (ca. 3–3.5 Ma) of South Africa based on an incomplete skull from the Bolt’s Farm Cave System (Cradle of Humankind, Gauteng, South Africa). The fossil cranium is distinct on morphometric and structural grounds from Geronticus apelex, the only other Pliocene Geronticus described from Southern Africa, but is very close in dimensions and general morphology to the extant G. calvus of South Africa, and the Bolt’s Farm fossil ibis is therefore attributed to G. cf. calvus. Modern Geronticus ibises are localised to temperate, open grasslands and semi-arid steppe, and nest exclusively on cliffs and similar rocky eminences. Given its attribution as G. cf. calvus, the Bolt’s Farm ibis was likely similar in ecology to the extant G. calvus, suggesting that the habitat surrounding the Bolt’s Farm fossil site during the mid-Pliocene featured open grassland and presence of cliffs. This record constrains the divergence between G. calvus and its putative ancestor G. apelex to the mid-Pliocene and implies that G. calvus has possibly been subject to “evolutionary/morphological stasis” for more than 3 million years. This postulated stasis would be consistent with the notion that extant genera with few species (i.e. high genus-to-species ratios) show low rates of phenotypic diversification and change through the Neogene.