Biotic interaction and synecology in a Late Cretaceous coral-rudist biostrome of southeastern Spain

A coral-rudist biostrome exposed in Campanian limestones near the village of Tabernas de Valldigna in south-east Spain was analysed with respect to its palaeontology, sedimentology and palaeoecology. Special attention was given to possible evidence for synecological interactions between corals and rudists. Changes in the rudist shell accretion process are evident in some polished slabs and thin sections and resulted from in vivo contact with coral-colonies. These unusual balcony-like shell protuberances exist where the rudist’s commissure was in contact with corals. They likely represent defence-reactions of rudists against the coral cnidia. Nevertheless, the fossil record of these biotic interactions is rare. This may be due to different growth-rates of rudists and corals, differing shape and size of interacting areas, or different life-spans. In consequence, the discrete ‘window’ of intergroup biotic interaction was small. Sedimentation and resuspension rates were high in the biostrome and corals only established pioneer associations under these unfavourable conditions. A higher diversity of corals is reached, however, when rudists are present. This increase in diversity resulted from the availability of additional ecological niches such as rudist-shell hard substrates and elevation above mobile sediment surface. Rudists on the other hand, received support from stabilisation of their shells through coral encrustation and framework building. In consequence, both groups benefited from their co-existence.     

The structure of the outer shell layer in radiolitid rudists, a morphoconstructional approach

Radiolitids (Bivalvia, Radiolitidae) built the calcite outer layer of their shell with compact structure as many other bivalves, or with repetitively layered non-compact structure following different patterns: continuous radial ridges, discontinuous radial ridges, normal cellular, cellular with radially elongate cells, discontinuous cellular, and discontinuous cellular with elongate cells. Some special structures may develop in localized parts: radial structures and inner-marginal or radial grooves. All these structures are currently used in the diagnosis of the family and for the characterization of subfamilies. Constructional morphology analyses and re-evaluation of all these structures reveal that they were misunderstood in some important aspects in the bivalve Treatise and that, consequently, the diagnosis of the family has to be emended and their subfamilies reconsidered. All outer shell layer structure related characters are described, illustrated, and interpreted under a constructional approach, some characters are proposed to emend the diagnosis of the family, and two possible clades are preliminarily distinguished.     

Algal genotype and photoacclimatory responses of the symbiotic alga Symbiodinium in natural populations of the sea anemone Anemonia viridis

As an approach to investigate the impact of solar radiation on an alga–invertebrate symbiosis, the genetic variation and photosynthetic responses of the dinoflagellate algal symbiosis in an intertidal and a subtidal population of the sea anemone Anemonia viridis were explored. Allozyme analysis of the anemones indicated that the two populations were genetically very similar, with a Nei''s index value of genetic identity (I) of 0.998. The algae in all animals examined were identified as Symbiodinium of clade a by PCR-RFLP analysis of the small subunit ribosomal RNA gene. The symbiosis in the two populations did not differ significantly in algal population density, chlorophyll a content per algal cell or any photosynthetic parameter obtained from studies of the relationship between photosynthesis and irradiance. We conclude that there is not necessarily genetic variation or photosynthetic plasticity of the symbiotic algae in Anemonia viridis inhabiting environments characterized by the different solar irradiances of the subtidal and intertidal habitats.     

Highly infectious symbiont dominates initial uptake in coral juveniles

The majority of reef-building corals acquire their obligate algal symbionts ( Symbiodinium ) from the environment. However, factors shaping the initial establishment of coral–algal symbioses, including parental effects, local environmental conditions and local availability of symbionts, are not well understood. This study monitored the uptake and maintenance of Symbiodinium in juveniles of two common corals, Acropora tenuis and Acropora millepora , that were reciprocally explanted between sites where adult colonies host different types of Symbiodinium . We found that coral juveniles were rapidly dominated by type D Symbiodinium , even though this type is not found in adult colonies (including the parental colonies) in four out of the five study populations. Furthermore, type D Symbiodinium was found in less than one-third of a wide range of coral species ( n  > 50) sampled at the two main study sites, suggesting that its dominance in the acroporid juveniles is not because it is the most abundant local endosymbiotic type. Moreover, dominance by type D was observed irrespective of the light intensity to which juveniles were exposed in a field study. In summary, despite its relatively low abundance in coral assemblages at the study sites and irrespective of the surrounding light environment, type D Symbiodinium is the main symbiont type initially acquired by juveniles of A. millepora and A. tenuis . We conclude that during early ontogeny in these corals, there are few barriers to the uptake of Symbiodinium types which differ from those found in parental colonies, resulting in dominance by a highly infectious and potentially opportunistic symbiont.     

Onset of algal endosymbiont specificity varies among closely related species of Acropora corals during early ontogeny

Juveniles of a number of corals with horizontal transmission of dinoflagellate endosymbionts naturally acquire and maintain Symbiodinium types that differ from those found in adult populations. However, the duration of this early period of symbiont flexibility and successional changes leading to dominance by the characteristic adult (homologous) type are unknown. To document natural succession of Symbiodinium types within juvenile corals, we monitored Symbiodinium communities in juveniles of Acropora tenuis and Acropora millepora for 3.5 years. Juveniles originating from one of three reef populations, characterized by differing adult coral- Symbiodinium associations, were raised in a common environment. In four out of five cases, juveniles became dominated initially by a nonhomologous adult type. Changes in Symbiodinium communities associated with A. tenuis juveniles led to the establishment of the adult homologous association at ∼3.5 years of age. These changes were not linked to the onset of reproductive maturity, but may be linked to micro-environmental changes associated with vertical growth of juvenile corals. We hypothesize that fine-tuning of specificity mechanisms takes place during ontogeny in A. tenuis , leading to the eventual establishment of the adult homologous association. However, Symbiodinium communities in A. millepora juveniles did not change significantly over the 3.5 years, potentially reflecting (i) lack of specificity, (ii) more than a 3.5-year delay in the onset of specificity, or (iii) lack of availability of the adult Symbiodinium type. This study demonstrates that juvenile corals may survive for extended periods of time with nonhomologous Symbiodinium types and that closely related species of Acropora differ in the timing of the onset of specificity for algal symbionts.     

Immunological evidence for a ribulose bisphosphate carboxylase-like protein in the symbiotic dinoflagellate Symbiodinium sp.

Whereas previously there has been no convincing evidence for ribulose bisphosphate carboxylase in dinoflagellates, a strong and highly specific reaction was observed when antibodies to the denatured large subunit of the (silver beet) protein were used to probe Western blots of whole soluble fractions of various Symbiodinium isolates. No reaction was observed using extracts from Symbiodinium isolated from a host which had been maintained under low light intensity. The results imply extensive sequence homology between the large subunit of ribulose bisphosphate carboxylase and a dinoflagellate protein of M , approximately 35 000.     

CHROMOSOMES AND DNA IN SYMBIODINIUM FROM AUSTRALIAN HOSTS1

Chromosome counts are reported for five species of Symbiodinium Freudenthal isolated from the host species Plesiastrea versipora, Aiptasia pulchella, Parerythropodium sp., Zoanthus robustus, and Capnella gaboensis based on light and electron microscope studies. Both techniques resulted in large variations in the apparent numbers of chromosomes per nucleus in Symbiodinium isolated from the same host colony. Adjacent regions of condensed DNA were occasionally linked by thin fibrils of DNA, The results suggest that the regions of condensed nuclear DNA in dinoflagellates may not represent whole chromosomes, and so chromosome counts are considered inappropriate for defining Symbiodinium taxa.     

The assessment of DNA from marine organisms via a modified salting-out protocol

We developed a rapid, practical and non-toxic salting-out method for the extraction of DNA from marine organisms, and tested it on two representative species of Porifera and Cnidaria, both living in association with symbiotic zooxanthellae. We tested the efficiency of the protocol by comparing the output of the method for fresh tissue, frozen tissue and tissue stored in ethanol. It proved to be effective for extracting DNA in the case of the methods of preservation considered here, and for obtaining quantities of DNA comparable to those obtained via the traditional approach. The DNA from both species was of good quality. The DNA obtained was amplified by PCR using specific primers for the large ribosomal subunit, allowing the identification of the presence of both the host and symbiont genomes.     

A Review of: “New Interpretations of Complex Trace Fossils”

Dinosaur Tracks and Traces, edited by David D. Gillette and Martin G. Lockley, 1989. Cambridge University Press, 110 Midland Avenue, Port Chester, New York 10573, USA. lvii + 454 pages. US$54.50.