The prosicular stage of Rhabdopleura (Pterobranchia: Hemichordata)

After settling, the larva of Rhabdopleura surrounds itself with a collagenous dome. Later, the zooid breaks through the wall of the dome and builds the horizontal tube part of the coenecium on to the dome.
The dome is a layered structure, unknown in other parts of the coenecium. whereas the horizontal tube is made up of rings in the classical manner of the adult coenecium. The construction of these two parts is different. The techniques used to reinforce the horizontal tube show a marked similarity to the cortical bandages recently described in the fossil graptolites, and give support to the claim that they are ancestral to Rhabdopleura. There are two sorts of early horizontal tube, one is a straight tube, and the other is longer and coiled. The hole in the dome through which the zooid emerges to build the horizontal tube is probably produced by a chemical boring of the zooid, and supports the hypothesis that the zooids can bore holes in shells and corals.     

Hemichordate skeletal growth: shared patterns in Rhabdopleura and graptoloids

Rhabdopleura shows several features of skeleton growth that are also seen in graptoloids. The similarities between the growth patterns, in terms of the plan towards which the zooids aimed and of their response to environmental disturbance, are profound. Both demonstrate a high degree of genetic control, not only on the gross morphology of the tube or theca but also on the pattern of increments by which this must be achieved. The execution of this 'blueprint' is facilitated by spatial awareness in the zooids of both groups. The main variable left to the graptoloid zooid was the number of increments used in building a theca. Variations in the number of increments probably reflect differences in the productivity of the environment and hence the amount of spare energy in the colony budget. An important new observation is that mortality is common amongst zooids in both Rhabdopleura and graptoloids, with new animals taking over tube or thecal building from where the previous zooid left off. This is identifiable in the increment patterns of tubes and thecae. Several generations of zooids can inhabit a rhabdopleuran tube and can be demonstrated to have inhabited a graptolite theca. This means that innate senescence was not a major cause of death for graptolite colonies. It also means that all thecae might have been continuously occupied and that the colony could have survived significantly bleak environmental conditions by large-scale zooid mortality followed by regeneration. □ Graptolite, ecology, hemichordate, pterobranch, RHABDOPLEURA, growth.     

Does polymorphism predict physiological connectedness? A test using two encrusting bryozoans

A colonial lifestyle necessitates communication between colony members to coordinate functions and enable resource sharing through physiological integration. Colonial integration is predicted to increase with both the size of the colony and the level of specialization (polymorphism). In modular colonies, although integration might be reflected in structural characteristics such as module spacing or branching patterns, physiological integration is fundamentally dependent on the level of connectedness between modules. In cheilostome bryozoans, funicular tissue links adjacent zooids through pores within zooid walls and is the most likely means of nutrient transport within colonies. We sought to determine whether the relative numbers of pores (septulae) and pore plates (septal chambers) per zooid differed across colony regions in a monomorphic species, Watersipora subtorquata, and one showing some polymorphism, Mucropetraliella ellerii. Within each species, the morphology of pore plates corresponded to functional predictions based on their position within the zooid, and connection numbers per zooid increased with colony size. Contrary to expectations, however, the more complex species, M. ellerii, had significantly fewer porous connections per zooid than W. subtorquata. Physiological connectedness was therefore not predicted by simple assessment of polymorphism in these species and may not be sufficient to infer colonial integration in related taxa.     

A study of regression and budding in Perophora viridis

1. A method has been devised for studying the regression of the zooid of Perophora into a stolon and the subsequent differentiation of a new zooid from this stolon. 2. Circulatory cells of the stolon resulting from regression will aggregate into masses larger than the minimal size necessary for differentiation of a zooid, but fail to differentiate into a zooid. 3. The cells of a zooid after staining with neutral red appear in the stolon during regression and finally come to lie in the newly formed zooid. 4. During the cycle of adult zooid to stolon to newly formed zooid, there is no evidence for cell division from studies with tritiated thymidine. 5. It is concluded that under conditions of starvation, an adult zooid furnishes all the cells for the formation of a stolon and the subsequent zooids without cell division.     

Studies on the asexual reproduction in the polystyelid ascidian,Polyzoa vesiculiphora Tokioka

New blastozooids of Polyzoa vesiculiphora, the polysytelid ascidian are produced by pallial budding of three types depending on the method of “isolated bud” formation; stolonic, planktonic and intermediate types. Differences among each type of bud are attributed to behavior of test-vessels composing a part of the bud. Isolated buds produced by each type are essentially equal in terms of their internal structures and their subsequent fate, and develop independently of their parent zooids. New test-vessels originate directly from the epidermis of a “prefunctional zooid,” while the test-vessels derived from the parent zooid finally disintegrate. The new test-vessels extended with branching under the ventral side of a “functional zooid,” ascend to the lateral side of it and participate in bud formation. Budding regions exist in three dimensions on the lateral wall of the mantle of the functional zooid, especially the right posterior part. During the life cycle of one functional zooid, the stolonic type buds appear at early and/or aged stages. Appearances of the stolonic type buds in early stages tend to repress those of the planktonic types. The number of planktonic type buds formed on a functional zooid at the same time is many more than that of the stolonic type. Such budding features are discussed from the viewpoint of behavior of the test-vessel system.     

An early Cambrian hemichordate zooid

Hemichordates are known as fossils from at least the earliest mid-Cambrian Period (ca. 510 Ma) and are well represented in the fossil record by the graptolithinid pterobranchs ("graptolites"), which include the most abundantly preserved component of Paleozoic macroplankton. However, records of the soft tissues of fossil hemichordates are exceedingly rare and lack clear anatomical details. Galeaplumosus abilus gen. et sp. nov. from the lower Cambrian of China, an exceptionally preserved fossil with soft parts, represents by far the best-preserved, the earliest, and the largest hemichordate zooid from the fossil record; it provides new insight into the evolution of the group. The fossil is assigned to the pterobranch hemichordates on the basis of its morphological similarity to extant representatives. It has a zooidal tube (coenecium) with banding throughout comparable to that in the extant pterobranchs and a zooid with paired annulated arms bearing paired rows of annulated tentacles; it also displays a putative contractile stalk. G. abilus demonstrates stasis in pterobranch morphology, mode of coenecium construction, and probable feeding mechanism over 525 million years.     

Multizooidal Budding in Parasmittina trispinosa (Johnston) (Bryozoa,Cheilostomata)

Two principally different wall types occur in the bryozoan colony: Exterior walls delimiting the super-individual, the colony, against its surroundings and interior walls dividing the body cavity of the colony thus defined into units which develop into sub-individuals, the zooids. In the gymnolaemate bryozoans generally, whether uniserial or multiserial, the longitudinal zooid walls are exterior, the transverse (proximal and distal) zooid walls interior ones. The radiating zooid rows grow apically to form “tubes” each surrounded by exterior walls but subdivided by interior (transverse) walls. The stenolaemate bryozoans show a contrasting mode of growth in which the colony swells in the distal direction to form one confluent cavity surrounded by an exterior wall but internally subdivided into zooids by interior walls. In the otherwise typical gymnolaemate Parasmittina trispinosa the growing edge is composed of a series of “giant buds” each surrounded by exterior walls on its lateral, frontal, basal and distal sides and forming an undifferentiated chamber usually 2–3 times as broad and 3 or more times as long as the final zooid. Its lumen is subdivided by interior walls into zooids 2–3, occasionally 4, in breadth. This type of zooid formation is therefore similar to the “common bud” or, better-named, “multizooidal budding” characteristic of the stenoleamates but has certainly evolved independently as a special modification of the usual gymnolaemate budding.     

Unconventional signalling in tunicates

The zooids in colonial tunicates do not appear to be directly interconnected by nerves, but this has not prevented the evolution of coordinated behaviour in several groups. In Botryllus and other colonial styelid asci‐dians the endothelium lining the blood vessels is excitable and transmits action potentials from cell to cell via gap junctions. These signals mediate protective contractions of the zooids and synchronize contractions of the vascular ampullae. In didemnid ascidians such as Diplosoma a network of myocytes in the tunic serves to transmit excitation and to cause contractions of the cloacal apertures. Individual zooids of Pyrosoma protect themselves by closing their siphons and arresting their branchial cilia when stimulated. At the same time a flash of light is emitted. Neighbouring zooids sense the flash with their photoreceptors and respond in turn with protective responses and light emission. Protective responses thus spread by photic signalling and propagate from zooid to zooid through the colony in a saltatory manner. In chains of Salpafusifortnis, changes in the direction and/or speed of swimming are transmitted from zooid to zooid via adhesion plaques. When a zooid is stimulated, its body‐wall epithelium conducts action potentials to the plaque connecting it to the next zooid, exciting receptor neurons in that zooid. These receptors have sensory processes that bridge the gap between the two zooids. The sensory neurons so excited in the second zooid conduct impulses to the brain where they alter the motor output pattern, and at the same time generate epithelial action potentials that travel to the next zooid in line, where the same thing happens.

It is not clear why these unconventional signalling methods have evolved but the tunic may be an inhospitable environment for nerves, making conventional nervous links impossible.     

Keratin-like fibres in the hemichordate Rhabdopleura compacta

Summary The coenecium of Rhabdopleura is a tube that surrounds the zooid. It is secreted by the cephalic shield of the zooid and contains three sorts of fibres in an electron lucent matrix. One of the fibre types contains a double helix of fine fibrils. Preliminary histochemical investigations suggest that the fibres may be keratinous.I wish to thank Professor J. Z. Young F.R.S. for enthusiastic advice and encouragement. Dr. R. Bellairs generously provided the facilities for electron microscopy. Dr. A. J. Southward and Dr. A. Stebbing of the Plymouth Marine Biological Laboratory generously gave of their time and expertise, and helped me to obtain and identify the specimens. Dr. R. Willis and Miss Marion Dennison assisted with the preliminary stereoscopic electron microscopy. Mr. R. Moss gave excellent technical and photographic assistance.     

Relative size predicts competitive outcome through 2 million years

Competition is an important biotic interaction that influences survival and reproduction. While competition on ecological timescales has received great attention, little is known about competition on evolutionary timescales. Do competitive abilities change over hundreds of thousands to millions of years? Can we predict competitive outcomes using phenotypic traits? How much do traits that confer competitive advantage and competitive outcomes change? Here we show, using communities of encrusting marine bryozoans spanning more than 2 million years, that size is a significant determinant of overgrowth outcomes: colonies with larger zooids tend to overgrow colonies with smaller zooids. We also detected temporally coordinated changes in average zooid sizes, suggesting that different species responded to a common external driver. Although species‐specific average zooid sizes change over evolutionary timescales, species‐specific competitive abilities seem relatively stable, suggesting that traits other than zooid size also control overgrowth outcomes and/or that evolutionary constraints are involved.     

The Mediterranean bryozoan Myriapora truncata (Pallas, 1766): a potential indicator of (palaeo‐) environmental conditions

Fossil and Recent specimens of the Mediterranean bryozoan Myriapora truncata show considerable intra‐ and intercolonial differences in branch diameter and zooid size. Statistically significant variability occurs within colonies, between colonies within sites, and between sampled sites, while the presence of intracolonial variability clearly shows that branch diameter is largely controlled by environmental parameters. The three structural traits measured (branch diameter, zooid size and zooid depth) do not correlate, thus indicating a disconnection between the controls on overall zooid size and branch diameter. Possible environmental parameters that may have an influence on morphology are temperature, food supply or current energy. Whereas current energy has an effect on the colony branching pattern (branch spacing), there are indications that temperature may be the main, but not the only, parameter controlling zooid size, and it is suggested that food supply largely determines the branch diameter in M. truncata. However, the identification of the decisive factors and quantification of the relationships between environmental and morphological change is beyond the scope of this study. The results nevertheless show that, if the control factors of morphological variability can be ascertained in Recent M. truncata, this species may prove to be an indicator of environmental conditions and their change at different spatial and temporal scales in Cenozoic to Recent Mediterranean habitats.     

Comparative study of zooid and non-zooid forming strains ofScenedesmus obliquus. Physiology and cytomorphology

Two zooid forming strains and four non-zooid strains of the green chlorococcal alga Scenedesmus obliquus were compared in terms of growth, morphological and physiological characteristics. Large differences were observed among the strains grown under various growth conditions (light and temperature). The assumption that the zooid forming strains may be similar was not confirmed. Since they considerably differed in daughter cells morphology, photosynthesis, growth rate in batch culture or commitment to cellular division. Molecular-genetic comparison of 18S RNA/DNA might distinguish zooid forming strains from non-zooid ones.     

Diversity of brood chambers in calloporid bryozoans (Gymnolaemata,Cheilostomata): comparative anatomy and evolutionary trends

Comparative anatomical studies of 12 species from 10 genera (Callopora, Tegella, Amphiblestrum, Parellisina, Corbulella, Crassimarginatella, Valdemunitella, Bryocalyx, Concertina, Cauloramphus) belonging to one of the largest and most diverse bryozoan taxa, the Calloporidae, and one species from the genus Akatopora belonging to the related taxon Antroporidae, were undertaken to elucidate the morphological diversity of brooding structures and to recognize main trends in their evolution. Most of the species studied possess ovicells (specialized brooding receptacles) formed by the distal and maternal (egg-producing) autozooids. The distal zooid can be an autozooid, a vicarious avicularium or a kenozooid. The calcified protective hood (ooecium) is an outgrowth from the distal zooid. Hyperstomial or prominent ovicells are most common. They were found in species of the genera Callopora, Tegella, Amphiblestrum, Parellisina, Corbulella, Bryocalyx and Concertina. Subimmersed ovicells were found in Valdemunitella, and immersed ovicells in Crassimarginatella and Akatopora. Cauloramphus has an internal brooding sac and a vestigial kenozooidal ooecium, budded by the maternal zooid. Based on the structure of the brooding organs, the following evolutionary trends can be recognized within the group: (1) reduction of the distal (ooecium-producing) zooid, (2) immersion of the brooding cavity correlated with a reduction of the ooecium and ooecial vesicle and with changes in the ovicell closure and the structure of the brood chamber floor, (3) reduction of the calcification of the ectooecium, and (4) transition from bilobate to entire ooecium. The trend towards immersion of the brooding cavity could have evolved repeatedly within the Calloporidae. Transition from bilobate to entire ooecium is characteristic of the related taxon Cribrilinidae, showing a good example of parallel evolution of the ooecium in two closely related clades. Possible causes for the transformations described are discussed.     

Material transport within specialised ciliary shafts on Rhabdopleura zooids

Summary The surface of the Rhabdopleura zooid is ciliated. The cilia of the cephalic shield and tentacles have paddle-like swellings of the shaft. These swellings are usually about 0.6–1 m in diameter and most frequently found in the distal 1–2 m of the ciliary shaft. Others are found in other positions along the length of the cilium and it is suggested that at least some of these swellings represent material transport within the cilium.Paddle shaped cilia are probably more efficient than normal cilia in moving water and food particles. If these cilia are involved in the building of the tubular coenecium then their distribution suggests that the tentacles as well as the cephalic shield are actively involved in tube building.I should like to thank the director and staff of the Marine Biological Laboratory, Plymouth, for collecting the material and the generous loan of facilities during the preparation of the material. Mr. R. Moss provided skillful technical and photographic assistance     

Contraction of protoplasm. IV. Cinematographic analysis of the contraction of some peritrichs

The contraction and relaxation of Vorticella difficilis, V. campanula and Carchesium sp. were studied by high speed cinematography. In Vorticella it was shown that coiling of the stalk usually started near the zooid and spread downwards; the point of initiation bore no relation to the position of the stimulating electrodes. Contraction took about 5 msec to complete, and the fully contracted animals were 29 ± 3.9% of their original lengths. The zooids were 66 ± 5.0% and the stalks 14 ± 6.0% of their original lengths (V. difficilis). The shortening of the stalk was mostly in the form of coiling. Measurement of the myoneme length demonstrated that its real shortening was less than 10%. Thus the contraction is virtually isometric, producing a helical deformation of the stalk. As the stalk contracts it takes the form of a steeply pitched helix. This change in shape should produce rotational forces on the zooid (torque). Physical models of similar proportions produced about 1.5 revolutions of torque for similar changes in pitch. However during contraction no turning of the zooid was detected, though rotation did occur after the completion of contraction. In Carchesium the contraction is not so isometric, the myoneme apparently shortening by 20%. While the coiled shape of the contracted Vorticella stalk can be explained by its acentric structures, the stalk of Carchesium is much more symmetrical in cross-section, demonstrating that a high acentricity is not necessary for helical coiling. In all three species there seems to be some separation of the control of zooid and stalk contraction.     

武汉东湖周丛原生动物六新种

本文报道武汉东湖周丛原生动物的6种纤毛虫新种:绿累枝虫Epistylis chlorelligerum sp. nov.、斗鞘居虫Vaginicola cupulata sp. nov.、弯鞘居虫Vaginicola curvata sp. nov.、褶鞘居虫Vaginicola plicata sp. nov.、大靴纤虫Cothurnia magna sp. nov.、多纹平鞘虫Platycola multistriata sp. nov.。       

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.     

The induction of reproductive cell formation of Ulva pertusa Kjellman (Ulvales, Ulvophyceae)

Synchronous zooid formation in Ulva pertusa Kjellman was induced in excised disks maintained in sterilized seawater at 20°C, 12:12 h L:D cycle and fluorescent light at 100 μmol photons m ²s ¹. Zooids were reieased from mature disk tissue on the morning of the second or the third day after excision. The degree of zooid formation was found to be dependent on disk size and the region of the mother thalius from which the disk tissue was excised. Zooid formation was induced in more than 90% of small disks (0.9 mm in diameter) which were taken from the margins of the Ulva thalli. When disks were incubated together with a perforated mother thalius, the disks remained sterile. The presence of maturation inhibitors in vegetative thalli is suggested.     

Description of Zoothamnium foissneri n. sp. and redescription of Z. duplicatum Kahl, 1933 and Z. mucedo Entz, 1884, three species of marine peritrichous ciliates

Three marine peritrichs, Zoothamnium foissneri n. sp., Z. duplicatum Kahl, 1933 and Z. mucedo Entz, 1884, were isolated from the littoral area of Qingdao, China. The morphology, infraciliature and silverline system were studied on living and silver-impregnated specimens. Z. foissneri is distinguished from congeners by its slender zooid shape, size, appearance of peristomial border, the form of peniculus 3, colony form and habitat. Based on the Qingdao populations, supplementary information is given for two known species showing that Z. duplicatum can be distinguished by the zooid shape and size, the number of silverlines between the aboral trochal band and the scopula, and the form of peniculus 3, while Z. mucedo can be distinguished from other species of similar zooid shape and size by the form of peniculus 3 and the number of silverlines between the oral area and the aboral trochal band.     

Quantitative observations on asexual reproduction of Aeolosoma viride (Annelida,Aphanoneura)

Agametic reproductive activity (via paratomy) of Aeolosoma viride was analyzed throughout the life cycle in individually reared specimens. Aeolosoma viride is organized in linear chains of 3–4 zooids; the main zooid is anterior, and the secondary zooids are positioned posterior to the main zooid in inverse order with respect to their degree of growth, the most advanced being at the posterior end, and those less advanced nearer the main zooid. On average, worms lived 66±10 d and produced 57±6 offspring. A budding area located in the sub‐terminal part of the main zooid produced chaetigers that formed the origin of the secondary zooids. A growth zone was located in the posterior end of each secondary zooids. Fission occurred between the penultimate and the last zooid of the chain. Just before fission, the growth zone of each secondary zooid became a budding area. Agametic reproduction was via multiple paratomy with linear succession of the secondary zooid and terminal fission. The structure of the chain was therefore modulated by the interaction of the processes of budding, growth, cephalic differentiation, and fission, which occurred continuously and on different timescales. Values of parameters describing paratomic activity (interval between origin of the zooids, time to produce a chaetiger, growth time of the zooids, and interval between the fission of the filial chains) are low early in an individual's life, but increase during senescence. Due to its relatively rapid lifecycle and high reproductive activity, A. viride is a convenient experimental organism for the study of agametic reproduction.