Ultrastructure of the coelom in the brachiopod Lingula anatina

The organization of the coelomic system and the ultrastructure of the coelomic lining are used in phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians. In the current study, the organization of the coelom of the lophophore in the brachiopod Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (= canals). The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium, alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and podocyte‐like cells. Connections between cells of the coelomic lining are provided by adherens junctions, tight‐like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges. The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated myoepithelium can be considered as the ancestral type of coelomic lining. The many different kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial cells and muscle cells.     

Regional specification during embryogenesis in the inarticulate brachiopod Discinisca.

The process of embryogenesis is described for the inarticulate brachiopod Discinisca strigata of the family Discinidae. A fate map has been constructed for the early embryo. The animal half of the egg forms the dorsal ectoderm of the apical and mantle lobes. The vegetal half forms mesoderm and endoderm and is the site of gastrulation; it also forms the ectoderm of the ventral regions of the apical and mantle lobes of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg along the future plane of bilateral symmetry of the larva. The timing of regional specification in these embryos was examined by isolating animal, vegetal, or lateral regions at different times from the 2-cell stage through gastrulation. Animal halves isolated at the 8-cell and blastula stages formed an epithelial vesicle and did not gastrulate. When these halves were isolated from blastulae they formed the cell types typical of apical and mantle lobes. Vegetal halves isolated at all stages gastrulated and formed a more or less normal larva; the only defect these larvae had was the lack of an apical tuft, which normally forms from cells at the animal pole of the embryo. When lateral isolates were created at all developmental stages, these halves gastrulated. Cuts which separated presumptive anterior and posterior regions generated isolates at the 4-cell and blastula stages that formed essentially normal larvae; however, at the midgastrula stage these halves formed primarily anterior or posterior structures indicating that regional specification had taken place along the anterior-posterior axis. The plane of the first cleavage, which predicts the plane of bilateral symmetry, can be shifted by either changing the cleavage pattern that generates the bilateral 16-cell blastomere configuration or by isolating embryo halves prior to, or during, the 16-cell stage. These results indicate that while the plane of the first cleavage predicts the axis of bilateral symmetry, the axis is not established until the fourth cleavage. The development of Discinisca is compared to development in the inarticulate brachiopod Glottidia of the family Lingulidae and to Phoronis in the phylum Phoronida.     

Predatory borings in the inarticulate brachiopod Artiotreta from the Silurian of Oklahoma

A sample containing inarticulate acrotretid brachiopods, from the Silurian Clarita Formation of Oklahoma, was studied to determine the occurrence, nature and frequency of boring in the shells, and the possible identity of the boring organism. Specimens of Artiotreta parva Ireland, 1961 are abundant and specimens of Opsiconidion n. sp. and Acrotretella siluriana Ireland, 1961 are rare in the samples studied. All of the borings discovered are in A. parva except for one, which is in a specimen of O. n. sp. The boreholes are considered to be of predatory origin, and are similar in morphology to borings made by Recent gastropods. The small countersunk boreholes are equally frequent on pedicle and brachial valves, but preferentially located in particular regions of each valve. Boring frequency is high. The identity of the boring organism has not been determined, but is considered to be a soft bodied predator or an archaeo-gastropod.     

Population structure, growth and distribution of Lingula anatina (Brachiopoda) in Queensland, Australia

Size-frequency data collected in large samples from five populations of Lingula anatina from North Queensland intertidal sandflats conformed to previously known brachiopod size-frequency distributions. Probability paper analysis of the samples and constituent sub-samples confirmed that all the distributions had a hidden poly-modality resulting from a long but defined breeding season. Two patterns of annual recruitment were revealed; the first is the periodic failure of the populations to gain recruits and the second is one of patchy distribution of settlement cohorts. These patterns introduce sampling bias into the derivation of lingulid size frequency curves unless compensated by more extensive sampling methods than have previously been considered adequate. The interpretation of year classes has enabled a growth curve to be derived. The populations inhabited predominantly sandy sub-strates and displayed aggregated distributions. Littoral distribution of the species is regarded as optimal rather than marginal.     

Genetic structure of three populations of the 'living fossil' brachiopod Lingula from Queensland, Australia

An electrophoretic survey of enzyme variation demonstrates that the genomes of three populations of the inarticulate brachiopod Lingula contain a higher level of variability than the average for marine, tropical invertebrates, and for other brachiopods. Genetic distances between populations from localities up to 1200 km apart were very small, indicating that the populations belong to a single species, L. anatina Lamarck. These data suggest that larval dispersal is capable of maintaining a homogeneous gene pool over this range. It is predicted that the dispersal mechanism is sufficiently efficacious to ensure gene-flow throughout most of the Indo-West Pacific distribution of Lingula. Further investigations of genetic distance should support the hypothesis, based on studies of adult and larval morphology, that many presently recognized species should be reassigned to L. anatina.     

Modern Data on the Innervation of the Lophophore in Lingula anatina (Brachiopoda) Support the Monophyly of the Lophophorates

Evolutionary relationships among members of the Lophophorata remain unclear. Traditionally, the Lophophorata included three phyla: Brachiopoda, Bryozoa or Ectoprocta, and Phoronida. All species in these phyla have a lophophore, which is regarded as a homologous structure of the lophophorates. Because the organization of the nervous system has been traditionally used to establish relationships among groups of animals, information on the organization of the nervous system in the lophophore of phoronids, brachiopods, and bryozoans may help clarify relationships among the lophophorates. In the current study, the innervation of the lophophore of the inarticulate brachiopod Lingula anatina is investigated by modern methods. The lophophore of L. anatina contains three brachial nerves: the main, accessory, and lower brachial nerves. The main brachial nerve is located at the base of the dorsal side of the brachial fold and gives rise to the cross neurite bundles, which pass through the connective tissue and connect the main and accessory brachial nerves. Nerves emanating from the accessory brachial nerve account for most of the tentacle innervation and comprise the frontal, latero-frontal, and latero-abfrontal neurite bundles. The lower brachial nerve gives rise to the abfrontal neurite bundles of the outer tentacles. Comparative analysis revealed the presence of many similar features in the organization of the lophophore nervous system in phoronids, brachiopods, and bryozoans. The main brachial nerve of L. anatina is similar to the dorsal ganglion of phoronids and the cerebral ganglion of bryozoans. The accessory brachial nerve of L. anatina is similar to the minor nerve ring of phoronids and the circumoral nerve ring of bryozoans. All lophophorates have intertentacular neurite bundles, which innervate adjacent tentacles. The presence of similar nerve elements in the lophophore of phoronids, brachiopods, and bryozoans supports the homology of the lophophore and the monophyly of the lophophorates.     

Breedng season, larval development and diseprsal of Lingula anatina (Brachiopoda, Inarticulata) from Townsville, Australia

A discrete but protracted breeding season, extending from October to April, was defined for Lingula anatina from Townsville, Australia, using gonad indices, sizes of developing ova, occurrence of planktonic larvae and analysis of size frequency distribution of an adult population. Comparison of temperate and tropical records suggests that the duration of the breeding season is regulated by temperature. The minimum age at which larvae achieve competence to settle is approximately the same in temperate and tropical waters, but in both regions development and growth occur at a very variable rate. A number of morphological features which can be considered precursors to settlement are closely associated in their development, which may proceed in response to settlement cues provided by proximity to the substratum. The varying intensity of such cues in different environments may be responsible for delays before some larvae realize competence to settle, accounting for much of the variability in the development process. Accordingly, differences between collections of Indo-West Pacific Lingula larvae can be rationalized; all records are now referred to L. anatina . Extension of pelagic larval life in lingulid larvae appears to involve a strategy different from that of most other marine invertebrates.     

Two distinct subunits of hemerythrin from the brachiopod Lingula reevii: an apparent requirement for cooperativity in O2 binding

Reported are results on the subunit composition of octameric hemerythrin (Hr) from the brachiopod Lingula reevii. Unlike most other Hrs, L. reevii Hr shows cooperativity in O2 binding. Purified L. reevii Hr was found to consist of two different subunits in approximately equimolar proportions. These two subunits differ in molecular weight by approximately 1000. Amino acid sequence data for the first 24 residues of the two subunits, labeled alpha and beta, show 70% identity with each other. Comparisons to amino acid sequences of other Hrs show approximately 50% identity in the first 24 residues and that both the alpha and beta subunits of L. reevii Hr have one residue deleted at their amino termini. Very recently, one other Hr, that from the brachiopod Lingula unguis, was also shown to contain equimolar proportions of two different subunits [Satake, K., Yugi, M., Kamo, M., Kihara, H., & Tsugita, A. (1990) Protein Seq. Data Anal. 3, 1-5], and this Hr also shows cooperativity in O2 binding. An alpha 4 beta 4 octamer is, therefore, proposed to be a common feature of those Hrs that show such cooperativity. Likely arrangements of alpha and beta subunits within an alpha 4 beta 4 octamer having the same configuration of subunits as that in other octameric Hrs are proposed. The most probable arrangements can be readily derived from physically reasonable restrictions on the types of intersubunit interactions and on transmission of allosteric effects.     

Nucleotide sequence of 5S ribosomal RNA from Lingula anatina. A study on the molecular evolution of 5S ribosomal RNA from a living fossil

By chromatography on columns of DEAE-Sephadex A-50 and Sephadex G-100, and electrophoresis on polyacrylamide gel, 5S rRNA was purified from a low-molecular-weight RNA fraction extracted from the total tissues of Lingula anatina. Complete digests of the 5S rRNA with RNase T1 [EC 3.1.4.8] and pancreatic RNase [EC 3.1.4.22] were sequenced by conventional column chromatography procedures. The nucleotide sequence of this RNA was determined mainly by a chemical method for sequencing the RNA 3' end-labeled with 32P (1), with the complement of the oligonucleotide catalog obtained by the complete RNase digestions of the RNA. By comparing the sequences of several invertebrate, vertebrate, and Chlorella 5S rRNAs, a phylogenic tree of the rRNAs was constructed and the time of divergence of Lingula was estimated.     

仿刺参体腔细胞和血细胞类型及体腔细胞数量研究

本文应用光镜、电镜技术研究了仿刺参(Apostichopus japonicus)体腔细胞和血窦中血细胞的形态、结构、周年体腔细胞总数及主要类型的细胞数量。研究结果表明,仿刺参体腔细胞和血细胞形态结构相同,作者将体腔细胞分为淋巴样细胞、球形细胞、变形细胞、透明细胞、纺锤细胞和结晶细胞,其中球形细胞又分为Ⅰ型、Ⅱ型、Ⅲ型。血窦中血细胞除未见结晶细胞外,其余与体腔细胞相同。体腔细胞和血细胞均以淋巴样细胞、变形细胞、球形细胞为主,透明细胞、纺锤细胞次之,结晶细胞数量最少。从形态和结构分析,作者认为仿刺参体腔细胞和血细胞来源可能相同。周年平均的体腔细胞总数为(3.85±0.21)×106个/mL,淋巴样细胞为(1.58±0.11)×106个/mL,占细胞总数的(41±1.47)%;球形细胞为(1.16±0.13)×106个/mL, 占细胞总数的(30±0.89)%;变形细胞为(0.99±0.07)×106个/mL,占总细胞数的(25±0.98)%,三种主要类型细胞占细胞总数的(96±1.53)%。     

On the origin systematic position of the calcareous-shelled inarticulate brachiopods

Available data on the anatomy, ontogeny embryology of Brachiopoda ( sensu lato ) suggest that this Phylum in the traditional view is in fact a clade of organization includes two stocks of lophophorate organisms of quite different origins. Their rank does not correspond to existing Class divisions. The phosphatic-shelled in articulates are regarded as a separate Class Lingulata. The Phylum Brachiopoda ( sensu stricto ) is restricted to the calcareous-shelled inarticulate articulate lineages. Ancestors of the calcareous-shelled brachiopods probably differentiated from the protolophophorates before the radiation of the other lophophorate stocks took place. The articulation of the valves appeared independently in several lineages during the early stages of brachiopod evolution.     

Structure of the brachiopod lophophore

Data on the development, structure, and functional morphology of the brachiopod lophophore are analyzed. The common origin of the tentacle apparatus in Lophophorata from the postoral ciliary band of the larva is shown. The brachiopod lophophore is based on the brachial axis consisting of the brachial fold running along the row of tentacles. The brachial axis may be attached to the brachial (dorsal) mantle lobe (trocholophe, schizolophe, and ptycholophe lophophores) or extend freely into the mantle cavity to form coiling brachia (spirolophe, zygolophe, and plectolophe lophophores). The circulation of water flows through the mantle cavity in the brachiopods with attached and free lophophores is described. A new hypothesis on the sorting of particles suspended in water during filtration is proposed.     

Respiration and the function of brachiopod punctae

When tied shut, three genera of punctate terebratulids from the Northeast Pacific [Terebratulina unguicula (Terebratulidina), Terebratalia transversa and Laqueus californianus (Terebratellidina)] take up O₂ from the external water (at one-third the rate when gaping). The impunctate rhynchonellid Hemithiris psittacea does not, indicating that punctae admit O₂ when the valves are closed. The shells of T. transversa vary from smooth and globose to Spirife r-like alate and costate morphs. The latter have more punctae and greater uptake of O₂ through the shell. The alate/costate morphs are most abundant where turbidity is likely to prompt closure. Punctael respiration is faster through the thin shells of Laqueus than the thick ones of Terebratalia. O₂deprived articulates gape widely and close slowly when disturbed, but there is no evidence of O₂debt. They regulate O₂consumption at the same rate from saturation to 0.5 ml·l¹, and thus appear adapted to low O₂levels. These results have implications for the energetic efficience of articulates and the evolutionary patterns of bivalve molluscs v. articulates.     

可口革囊星虫体液细胞的显微和亚显微结构

本文利用光学显微镜和透射电镜研究了可口革囊星虫体液细胞的显微和亚显微结构。研究发现该星虫体液中含有红细胞、颗粒细胞、桑椹胚样细胞和卵母细胞。可口革囊星虫的红细胞数量较多,红细胞呈盘状,从一边或两边略凹陷,最典型的是从三边发育的二裂片内陷,直径为24μm-32μm。圆形红细胞,核位于中央,细胞内很少有细胞器,细胞质内充满嗜锇的血红蛋白。体腔液内含有两种类型的颗粒细胞。Ⅰ型细胞含有大量的小颗粒,是典型的变形细胞,细胞质里含有大量的线粒体、最典型的高尔基体、内质网和各种形状不一的小池。Ⅱ型细胞含有大量的嗜锇细胞质。在可口革囊星虫体液中含有不同发育时期的卵母细胞。     

Chemico-structural phylogeny of the discinoid brachiopod shell

Stratiform shells of living discinids are composed of membranous laminae and variously aggregated, protein-coated granules of apatitic francolite supported by proteinaceous and chitinous nets in glycosaminoglycans (GAGs) to form laminae in rhythmic sets. The succession is like that of living lingulids but differs significantly in the structure of the periostracum, the nature of baculate sets and in its organic composition. In particular, discinids have a higher level of amino acids although with relatively lower acidic and higher basic concentrations; and their overall lower organic content is owing to lower levels of hydrophilic components, like GAGs and chitin. The organic constituents are not completely degraded during fossilization; but data are presently too meagre to distinguish between linguloid and discinoid ancestries. Many differences among three of the four described extant genera emanate from transformations with a long geological history. Pelagodiscus is characterized by regular, concentric rheomorphic folding (fila) of the flexible periostracum and the plastic primary layer and by sporadically developed hemispherical imprints of periostracal vesicles. Both features are more strikingly developed in Palaeozoic discinids. In the oldest discinid, the Ordovician Schizotreta, and the younger Orbiculoidea and related genera, vesicles were persistent, hexagonal close-packed arrays fading out over fila. They must have differed in composition, however, as the larger vesicles of Schizotreta were simple (possibly mucinous), whereas the smaller vesicles of Orbiculoidea and younger genera were composites of thickly coated spheroids, possibly of lipoproteins, which survive as disaggregated relicts in Pelagodiscus. Baculate sets within the secondary layer are also less well developed in living discinids, being incipient in Pelagodiscus and restricted to the dorsal valve of Discinisca. The trellised rods (baculi) with proteinaceous cores are composed of pinacoids or prisms of apatite, depending on whether they are supported by chitinous nets or proteinaceous strands in GAGs. This differentiation occurred in Schizotreta but in that stock (and Trematis) the baculate set is symmetrical with baculi subtended between compact laminae, whereas in younger and post-Palaeozoic species the outer bounding lamina(e) of the set is normally membranous and/or stratified. The most striking synapomorphy of living discinids is the intravesicular secretion of organsiliceous tablets with a crystalline habit within the larval outer epithelium and their exocytosis as a close- or open-packed, transient, biomineral cover for larvae. Canals, on the other hand, are homologous with those pervading lingulid shells. Both systems interconnect with chitinous and proteinaceous sets and have probably always served as vertical struts in an organic scaffolding supporting the stratiform successions. A phylogenetic analysis based mainly on shell structure confirms the discinoids as the sister group of the linguloids but, contrary to current taxonomic practice, also supports the inclusion of acrotretoids within a ''discinoid'' clade as a sister group to the discinids.     

Mesozoic brachiopod migrations and the opening of the North Atlantic

The distribution of a number of Mesozoic brachiopods indicates a migration up the eastern side of the North Atlantic starting in the early Jurassic and reaching as far north as Greenland by the beginning of Cretaceous times. The brachiopods evidently reached the new Atlantic via the Rif mountains of northern Morocco and migrated both north and south from there. Some of them came from the central High Atlas which represents a gulf open to the east but closed to the west. The western High Atlas, on the other hand, was populated by successive waves of forms from the west. Brachiopods and other distinctive forms of “Tethyan” origin clearly extended considerably farther north on the eastern side of the Atlantic than they did on the west. This probably implies an early establishment of the Gulf Stream. Some Tethyan brachiopods which did not extend to the western end of the Mediterranean are found instead to have spread via northeast Siberia to cordilleran North America.     

The mitochondrial genome of the brachiopod Laqueus rubellus

The complete nucleotide sequence of the 14,017-bp mitochondrial (mt) genome of the articulate brachiopod Laqueus rubellus is presented. Being one of the smallest of known mt genomes, it has an extremely compact gene organization. While the same 13 polypeptides, two rRNAs, and 22 tRNAs are encoded as in most other animal mtDNAs, lengthy noncoding regions are absent, with the longest apparent intergenic sequence being 54 bp in length. Gene-end sequence overlaps are prevalent, and several stop codons are abbreviated. The genes are generally shorter, and three of the protein-coding genes are the shortest among known homologues. All of the tRNA genes indicate size reduction in either or both of the putative TPsiC and DHU arms compared with standard tRNAs. Possession of a TV (TPsiC arm-variable loop) replacement loop is inferred for tRNA(R) and tRNA(L-tag). The DHU arm appears to be unpaired not only in tRNA(S-tct) and tRNA(S-tga), but also in tRNA(C), tRNA(I), and tRNA(T), a novel condition. All the genes are encoded in the same DNA strand, which has a base composition rich in thymine and guanine. The genome has an overall gene arrangement drastically different from that of any other organisms so far reported, but contains several short segments, composed of 2-3 genes, which are found in other mt genomes. Combined cooccurrence of such gene assortments indicates that the Laqueus mt genome is similar to the annelid Lumbricus, the mollusc Katharina, and the octocoral Sarcophyton mt genomes, each with statistical significance. Widely accepted schemes of metazoan phylogeny suggest that the similarity with the octocoral could have arisen through a process of convergent evolution, while it appears likely that the similarities with the annelid and the mollusc reflect phylogenetic relationships.