Didemnid Ascidians Research Articles

An unusual cyanophyte, containing phycourobilin and symbiotic with ascidians and sponges

A study was made of the pigment composition and ultrastructure of a unicellular cyanophyte living in symbiosis with colonial didemnid ascidians and encrusting sponges collected from the southern end of the Great Barrier Reef, Australia, in 1981–1984. The ascidians were Trididemnum tegulum Kott and T. clinides. Kott; the sponges were Prianos aff. melanos de Laubenfels, Spirastrella aff. decumbens Ridley and an unidentified brown fleshy sponge (BFS). This cyanophyte seems to be identical with Synechocystis trididemni Lafargue et Duclaux. A phycoerythrin containing both phycourobilin and phycoerythrobilin chromophores was shown to be present; the urobilin was carried on α and β subunits, no γ subunit was found. A second phycoerythrin possessing only erythrobilin chromophores was also present. In thin-sections the cells showed no central DNA-containing nucleoid, and an unusual thylakoid arrangement with some thylakoids having greatly expanded lumens forming pseudo-vacuoles in the centre of the cell. Freeze-fracture showed 11 to 12 nm particles on both PF (protoplasmic face) and EF (exoplasmic face) faces of thylakoids. In many ways, the ultrastructure resembled that of the chlorophyll-b containing prokaryote Prochloron spp.

Prochloron--a status report.

Prochloron is a genus of prokaryotic algae with photosynthetic pigments like those of chlorophytes. Prochlorophytes are almost invariably found associated as symbionts with marine protochordates (didemnid ascidians), and so far none has been successfully grown in sustained culture away from in host. Based on materials collected from nature, information of various sorts (biochemical, physiological, cytological and fine-structural) has been obtained, indicating many resemblances (and probably close phylogenetic affinities) between prochlorophytes and cyanophytes. Nevertheless they are distinguished by certain unique combinations of characters. Some of the data support the symbiogenesis theory for the origin of green-plant chloroplasts. Other possibilities are briefly discussed.

Dynamics of the Epifauna of the Bivalve Pinna Bicolor: Interactions Among Recruitment, Predation, and Competition

The epifauna on the shells of a subtidal population of the bivalve Pinna bicolor is composed mainly of encrusting bryozoans and serpulid polychaetes, with tunicates and sponges relatively uncommon. Only a small subset of the observed interactions between sessile organisms were capable of changing the patterns of abundance on individual Pinna shells. Didemnid tunicates were capable of excluding most other species, but the most common competitive interactions, those between encrusting bryozoans, had variable outcomes and frequently did not result in the exclusion of the loser from a patch (shell). Sponges were also capable of excluding bryozoans and serpulids, although at slower rates than tunicates. Serpulids were overgrown by all other species, although overgrowth usually did not result in their exclusion from patches. Predators were, in general, not capable of influencing the abundance of any species greatly. The exception involved two species of monacanthid fish, juveniles of which ate newly metamorphosed didemnid ascidians. These fish thus could prevent monopolization of some patches by tunicates. The effects of predation by juvenile fish and overgrowth by tunicates were variable in space and time. The interaction between variable tunicate recruitment and predation was important. Didemnids recruited in low numbers and, even in a year of high recruitment, did not colonize a large number of patches; in addition, most recruits were eaten by fish. Predation by fish played no role in another year, because few tunicates recruited. It is likely that the abundance of fish was also variable, since their recruitment is also likely to be variable in time. The combination of these factors has resulted in few patches being colonized by tunicates. In the 2 yr of the study, none of the fifty patches under observation was colonized successfully by didemnids, and observations on other Pinna shells suggest that no substantial establishment occurred in the 2—3 yr preceding this study. Thus, in most patches, predation and competition did not strongly influence the abundance of sessile organisms. The patterns of occupation of individual patches, and the overall abundance of sessile species in the community, could be explained adequately by the observed patterns of recruitment. This situation contrasts with the occupation of patches on the pilings of a nearby pier, where competitive interactions between occupants of a patch are the most important factor. The results of these experiments provide a mechanism to explain the previously noted difference in the abundance of sessile invertebrates on the shells of Pinna bicolor and on the pilings of the pier.

Ecology and colonial dynamics of some Pacific reef flat Didemnidae (Ascidiacea)

Non-cryptic didemnid ascidians containing the prokaryote Prochloron are abundant on the intertidal flat of many Pacific reefs. Species with sheet-like colonies (e.g. Diplosoma similis, Trididemnum paracyclops) are characteristic of high wave energy situations and clean hard surfaces; didemnids in which the colony divides repeatedly into small cormomeres (e.g. Diplosoma virens, D. multipapillata, Lissoclinum voeltzkowi) are in less exposed situations, often on algae or unstable substrata. The unusual back-reef cascade habitat of D. multipapillata on a Fijian fringing-reef is described. In Fiji, replicative division leads to population build-up between June and October, with a hot season decline in November-December. The effects of torrential rain in April 1980 are described. Studying L. voeltzkowi in permanent quadrats has shown that the populations of cormomeres are dynamic, with much movement occurring. The process of colony division is described in Diplosoma virms from the Great Barrier Reef. Possible advantages of didemnid dynamism discussed include maximization of periphery to area (perhaps increasing the efficiency of feeding and/or growth), the exclusion of competitors, and the mingling of clones to facilitate cross-fertilization.

Two new genera of didemnid ascidians from tropical Australian waters

Atriolum has characters that suggest affinities with Leptoclinides and Didemnum respectively. Its large zooids and functional rather than vestigial oviduct are primitive characters previously not known in the Didemnidae. A thoracic brood pouch is probably convergent with similar structure in the Holozoidae and does no t reflect phylogeny. Type material of this monotypic genus is from Murray I. and Raine I., northern Queensland . Polysoma resembles certain Diplosoma spp. in the absence of spicules, and the form o f the colony and zooids. It is distinguished by its arc of numerous scattered testis follicles and straight vas deferens. Type material of this mono typic genus is from Port Essington, Northern Territory.

Occurrence, structure and synthesis of 3-(N-Methylamino)glutaric acid, an amino acid from Prochloron didemnii

3-(N-Methylamino)glutaric acid has been identified as a new free amino acid in extracts from Prochloron didemnii (Lewin), a unique prokaryotic algal symbiont associated with certain didemnid ascidians. Its structure was established by elucidation of the mass spectra of its TMSi and other derivatives and confirmed by synthesis.

Photosynthetic unit size, carotenoids, and chlorophyll—protein composition of Prochloron sp., a prokaryotic green alga

Six samples of the prokaryotic, unicellular algae Prochloron sp., which occur in association with didemnid ascidians, were collected from various localities in the tropical Pacific Ocean, and their pigments and chlorophyll-protein complexes were identified and characterized. No phycobilin pigments were detected in any of the species. Chlorophylls a and b were present in ratios of a/b = 4.4-6.9. The major carotenoids were beta-carotene (70%) and zeaxanthin (20%). Minor carotenoids of one isolate were identified as echinenone, cryptoxanthin, isocryptoxanthin, mutachrome, and trihydroxy-beta-carotene; no epsilon-ring carotenoids were found in any sample. Except for the absence of glycosidic carotenoids, the overall pigment composition is typical of cyanobacteria. A chlorophyll a/b-protein complex was present in Prochloron; it was electrophoretically and spectrally indistinguishable from the light-harvesting chlorophyll a/b-protein of higher plants and green algae. It accounted for 26% (compared to approximately 50% in green plants) of the total chlorophyll; 17% was associated with a P700-chlorophyll a-protein. The photosynthetic unit size of 240 +/- 10 chlorophylls per P700 in Prochloron was about half that of eukaryotic green plants. A model is proposed for the in vivo organization of chlorophyll in Prochloron.

Anoplodelphys G. Nov., Copépode Notodelphyidae Parasite De Didemnidae (Ascidies Aplousobranches) En Méditerranée 1)

Based upon a large collection from the western Mediterranean, a description of a new genus of notodelphyid parasitic copepod from didemnid ascidians is given. The new genus Anoplodelphys is erected for three new species found in different host species. Anoplodelphys clearly belongs to the ophioseidimorph group of notodelphyids; within this group, the genus is mainly distinguished by the complete lack of true buccal appendages, feature in common with only four previously described species. The discovery of this new genus increases the polyphyletic trend within the ophioseidimorph notodelphyids. The host specificity is extremely narrow: all three new species of Anoplodelphys live in several species of the large genus Didemnum. Some remarks on the host parasite relationships conclude the paper.

Identification of chlorophyll b in extracts of prokaryotic algae by fluorescence spectroscopy.

Solvent extracts of three different prokaryotic algae from three species of didemnid ascidians contained pigments identified, on the basis of their fluorescence excitation (E)and fluorescence emission (F)spectral maxima (measured in nm) at 77K, as chlorophyll a (E 449, F 678) and chlorophyll b (E 478, F 658). The release of algae on cutting or freezing Diplosoma virens was accompanied by a strong unidentified acid that converted these pigments to pheophytins. This unexpected finding provided further confirmation of the identity of the chlorophylls on the basis of the fluorescence spectra at 77K of pheophytin a (E 415, F 669) and pheophytin b (E 439, F 655). There was no evidence for the presence of the fluorescent bilin pigments found in other prokaryotic blue-green algae. Chlorophyll a/b ratios ranged from 2.6 to 12.0 in algae from different ascidians. The photosynthetic membranes were not organized into appressed thylakoids or grana in the algae from any of the three species of ascidians. The relationship between these observations and those in higher eukaryotic organisms is discussed.

Studies of the relationships between the ascidian Diplosoma virens and its associated microscopic algae. I. Photosynthetic characteristics of the algae.

The microscopic algae contained within the cloacal cavity of the ascidian Diplosoma virens have a high photosynthetic capacity [up to 5.3 �gC (�g chl a)-1 h-1], giving thecolony a high photosynthesis: respiration ratio (maximum 8.0-8 5). Photosynthesis is saturated at a quantum flux density of 120 × 1015 quanta cm-2 s-1 (400-700 nm) and occurs at its maximum at temperatures between 30 and 35� C. The photosynthetic capacity is inhibited by chloramphenicol but not by cycloheximide. This is interpreted as indicating an affinity of these algae with the prokaryotes rather than the Chlorophyta, in spite of their possession of chlorophyll b (chlorophyll a : b ratio = 6.0) as an accessory pigment. In this and other respects the algae resemble those (Prochloron) described from other didemnid ascidians. Use of a buffered extraction medium has allowed separation from the animal tissue of photosynthetically active algal cells.

Associations of microscopic algae with didemnid ascidians

Abstract Microscopic epizoic algae were found, forming coloured patches on the surfaces of calcified colonial ascidians (Didemnum spp.) along shores of the Gulf of California. The commonest of these is an apparently undescribed species of Synechocystis. Other algae more rarely associated with these ascidians include pink unicellular and filamentous cyanophytes and diatoms.