Functional morphology and taxonomy of branch dimorphism in the Paleozoic bryozoan genus Rhabdomeson

Silicified maria of the widespread cylindrical genus Rhabdomeson have been discovered that bear conical branches traditionally assignable to Coeloconus . Other, conical stems gave rise to cylindrical branches. That the dimorphic branches are not encrustations is shown by the absence of overgrowths and the regular alignment of apertures between parent stem and branch. A similar relationship is true of the type species of Coeloconus , which is synonymized with Rhabdomeson . Both conical and cylindrical branches were broken but survived to resume growth; this is shown by basal healing patterns and growth reversals. Colony fragmentation has been described in certain Cenozoic and modern bryozoans as a mode of increase, and is inferred to have been important in Rhabdomeson . The presence of fragmentation in taxonomically and stratigraphically widely separated bryozoans suggests that this mode of increase may be more important than realized.     

Evolution and dynamics of branching colonial form in marine modular cnidarians: gorgonian octocorals

Multi-branched arborescent networks are common patterns for many sessile marine modular organisms but no clear understanding of their development is yet available. This paper reviews new findings in the theoretical and comparative biology of branching modular organisms (e.g. Octocorallia Cnidaria) and new hypotheses on the evolution of form are discussed. A particular characteristic of branching Caribbean gorgonian octocorals is a morphologic integration at two levels of colonial organization based on whether the traits are at the module or colony level. This revealed an emergent level of integration and modularity produced by the branching process itself and not entirely by the module replication. In essence, not just a few changes at the module level could generate changes in colony architecture, suggesting uncoupled developmental patterning for the polyp and branch level traits. Therefore, the evolution of colony form in octocorals seems to be related to the changes affecting the process of branching. Branching in these organisms is sub-apical, coming from mother branches, and the highly self-organized form is the product of a dynamic process maintaining a constant ratio between mother and daughter branches. Colony growth preserves shape but is a logistic growth-like event due to branch interference and/or allometry. The qualitative branching patterns in octocorals (e.g. sea feathers, fans, sausages, and candelabra) occurred multiple times when compared with recent molecular phylogenies, suggesting independence of common ancestry to achieve these forms. A number of species with different colony forms, particularly alternate species (e.g. sea candelabrum), shared the same value for an important branching parameter (the ratio of mother to total branches). According to the way gorgonians branch and achieve form, it is hypothesized that the diversity of alternate species sharing the same narrow variance in that critical parameter for growth might be the product of canalization (or a developmental constraint), where uniform change in growth rates and maximum colony size might explain colony differences among species. If the parameter preserving shape in the colonies is fixed but colonies differ in their growth rates and maximum sizes, heterochrony could be responsible for the evolution among some gorgonian corals with alternate branching.     

Colony Structure in Flustra foliacea (Linnaeus) (Bryozoa,Chéilostomata)

Encrusting forms presumably represent the original mode of growth in the bryozoans. They usually exhibit a relatively low degree of colony integration. The numerous different erect colony types, certainly independently derived from this stage, show a higher degree of integration. Eventually forms arise with a strict pattern only little influenced by the environment. The Flustra foliacea colony holds an intermediate position in this respect. It forms an initial crust on a foreign substratum such as rock. The crust forms marginal lobes. The lobes collide two and two. The two components of each collision then rise back to back, owing to lack of space, into bilaminar fronds. The two zooid layers composing these erect branches and mutually encrusting the “auto-substratum” of their respective basal walls, grow much more rapidly than the initial crust on the “heterosubstratum” of the rock to finally comprise more than 95 % of the colony volume. The different rate of growth on “auto-” and “heterosubstratum” respectively is interpreted as indicating a preference for the “autosubstratum” and as the main force in the formation of erect branches. The Fl. foliacea case indicates one of the pathways from encrusting to erect colonies in the bryozoans. It is probably valid for at least some of the erect bilaminar colonies appearing in different parts of the bryozoan system.     

Determinate growth and modularity in a gorgonian octocoral

Growth rates of branches of colonies of the gorgonian Pseudopterogorgia elisabethae were monitored for 2 years on a reef at San Salvador Island, Bahamas. Images of 261 colonies were made at 6-month intervals and colony and branch growth analyzed. Branch growth rates differed between colonies and between the time intervals in which the measurements were made. Colonies developed a plumelike morphology through a pattern of branch origination and determinate growth in which branch growth rates were greatest at the time the branch originated and branches seldom grew beyond a length of 8 cm. A small number of branches had greater growth rates, did not stop growing, and were sites for the origination of subsequent "generations" of branches. The rate of branch origination decreased with each generation of branching, and branch growth rates were lower on larger colonies, leading to determinate colony growth. Although colonial invertebrates like P. elisabethae grow through the addition of polyps, branches behave as modules with determinate growth. Colony form and size is generated by the iterative addition of branches.     

Larger foraminifera as a substratum for encrusting bryozoans (Late Oligocene,Tethyan Seaway,Iran)

Considering the diversity and abundance of larger foraminifera examined from a wide range of Late Oligocene to Early Miocene palaeoenvironments in the Tethyan Seaway, encrusting bryozoans make extremely little use of their tests as substratum. Significant encrustations by bryozoans were exclusively found on large (ø c. 6 cm), undulating tests of Lepidocyclina spp., on which, however, a remarkable 34 taxa of encrusting bryozoans were recorded. This shallow-water fauna of Chattian age was analyzed in respect of the bryozoan taxa present, colony growth type, and mode of budding, colony size, as well as onset of reproduction. Taxic and morphological similarities between the fossil assemblage and modern faunas encrusting mobile substrata indicate a long history of bryozoans as part of the interstitial habitat, while the tests of certain larger foraminifera may have played a significant role in the evolution of shallow-water bryozoans by providing substrata for encrusting species in otherwise unfavorable environments.     

Astogenese vonAulopora cf.enodis Klaamann 1966Visby-Mergel, Silur von Gotland

This paper deals with bifurcation angle of a reptant, dichotomously branchingAulopora cf.enodis colony (Silurian, Gotland). It can be demonstrated, that during early astogenetic growth the branching angle between offsetting corallites decreases rapidly from 165° to 100° averagely during later growth stages. This growth pattern resembles development of similar dichotomous branching uniseral bryozoans, e.g. PaleozoicCorynotrypa species or MesozoicStomatopora species.     

Branching and self-organization in marine modular colonial organisms: a model

Despite the universality of branching patterns in marine modular colonial organisms, there is neither a clear explanation about the growth of their branching forms nor an understanding of how these organisms conserve their shape during development. This study develops a model of branching and colony growth using parameters and variables related to actual modular structures (e.g., branches) in Caribbean gorgonian corals (Cnidaria). Gorgonians exhibiting treelike networks branch subapically, creating hierarchical mother-daughter relationships among branches. We modeled both the intrinsic subapical branching along with an ecological-physiological limit to growth or maximum number of mother branches (k). Shape is preserved by maintaining a constant ratio (c) between the total number of branches and the mother branches. The size frequency distribution of mother branches follows a scaling power law suggesting self-organized criticality. Differences in branching among species with the same k values are determined by r (branching rate) and c. Species with rr/2 or c>r>0). Ecological/physiological constraints limit growth without altering colony form or the interaction between r and c. The model described the branching dynamics giving the form to colonies and how colony growth declines over time without altering the branching pattern. This model provides a theoretical basis to study branching as a simple function of the number of branches independently of ordering- and bifurcation-based schemes.     

Growth pattern and modular reiteration of a hardy coloniser Polytrichum commune Hedw.

Heathlands in Brittany are subject to recurrent summer burning. Recolonisation after large fires begins with bryophytes, but species composition during succession may be altered by colonies of Polytrichum commune Hedw. which appears to be able to prevent reestablishment of the typical heathland formation. Three complementary levels of investigation were adopted in attempting to understand how P. commune, manages to persist and succeed in space open to rapid saturation.Analysis at the level of the local plant community, showed radical changes in the distribution and composition of the herbaceous and muscinal layers over two years. Between May 1992 and May 1994, P. commune cover increased from 13% to 77% accompanied by an appreciable decrease in other pioneer species.We compared individual colony growth of P. commune with that of Polytrichum piliferum, another pioneer moss. In both species, horizontal and vertical growth occurred in spring and autumn. During 1995, P. commune colony diameter had increased by a factor of 4.0 compared with 3.6 for P. piliferum. Over the same interval, colony height increased by a factor of 1.3 for P. commune compared with 1.8 for P. piliferum.Analysis at the level of the module showed that 79% of experimentally cultured stems produced branches, 25% producing a single branch, 32% two branches, and 25% 4 to 7 branches. The lower the location of new branches on culture stems, the longer their leafed section and the greater their overall length. These new branches were morphologically and functionally new ramets.These observations on P. commune conform to the inhibition model of community succession (Connell & Slatyer 1977). Successional patterns recorded in the present study could be interpreted in terms of differences in species colony growth ability. Dense colony growth would allow a species to saturate one of the more important ecosystem compartment. Polytric capacity to grow simultaneously and efficiently in the few centimetres in and above the soil surface could prevent seedling establishment and decrease fitness of coexisting Poaceae.     

Critical Evaluation of Branch Polarity and Apical Dominance as Dictators of Colony Astogeny in a Branching Coral

The high morphological resemblance between branching corals and trees, can lead to comparative studies on pattern formation traits, best exemplified in plants and in some cnidarians. Here, 81 branches of similar size of the hermatypic coral Stylophora pistillata were lopped of three different genets, their skeletons marked with alizarin red-S, and divided haphazardly into three morphometric treatment groups: (I) upright position; (II) horizontal position, intact tip; and (III) horizontal position, cut tip. After 1 y of in-situ growth, the 45 surviving ramets were brought to the laboratory, their tissues removed and their architectures analyzed by 22 morphological parameters (MPs). We found that within 1 y, isolated branches developed into small coral colonies by growing new branches from all branch termini, in all directions. No architectural dissimilarity was assigned among the three studied genets of treatment I colonies. However, a major architectural disparity between treatment I colonies and colonies of treatments II and III was documented as the development of mirror structures from both sides of treatments II and III settings as compared to tip-borne architectures in treatment I colonies. We did not observe apical dominance since fragments grew equally from all branch sides without documented dominant polarity along branch axis. In treatment II colonies, no MP for new branches originating either from tips or from branch bases differed significantly. In treatment III colonies, growth from the cut tip areas was significantly lower compared to the base, again, suggesting lack of apical dominance in this species. Changes in branch polarity revealed genet associated plasticity, which in one of the studied genets, led to enhanced growth. Different genets exhibited canalization flexibility of growth patterns towards either lateral growth, or branch axis extension (skeletal weight and not porosity was measured). This study revealed that colony astogeny in S. pistillata is a regulated process expressed through programmed events and not directly related to simple energy trade-off principles or to environmental conditions, and that branch polarity and apical dominance do not dictate colony astogeny. Therefore, plasticity and astogenic disparities encompass a diversity of genetic (fixed and flexible) induced responses.     

Evolution of erect helical colony form in the Bryozoa: phylogenetic, functional, and ecological factors

Erect helical colony forms have evolved at least six separate times within the Bryozoa, but only among those in which branches are composed of a single layer of feeding zooids. The four best known genera with helical colony forms evolved independently, and each occupied different benthic marine environments, achieved different growth habits, and utilized different aspects of an array of functional potentials resulting from the radially symmetrical colonies. Examination of the distribution of these four genera ( Archimedes , Bugula , Crisidmonea , and Retiflustra ) within a theoretical morphospace of hypothetical helical colony form reveals that each occupies its own characteristic region of morphospace, broadly overlapping in some dimensions but separated in others. The genera differ markedly in the branching densities within their filtration-sheet whorls, reflecting their phylogenetic legacies rather than constructional or functional constraints associated with helical growth. In contrast, all tend toward helices in which the radiating whorls of the unilaminate branches are held at an average of 50–60° to the central axis of the colony, and this may reflect a common functional optimum associated with the cilia-driven, auto-generated currents within the helix. The region of morphospace characterized by high values of surface area – i.e. helical geometries with branches orientated at very low angles to the central axis, and with very closely spaced whorls along the axis – is entirely empty of bryozoans, and these geometries apparently represent functionally unrealistic colony forms.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 235–260.     

Structure of Adnate Colony Portions in Crisiidae (Bryozoa Cyclostomata)

The “rhizoidsrdquo; surrounding the base of the erect “colony” emanating from the ancestrula in the Crisiidae, especially the simple species Crisidia cornuta (L.), are a regular adnate system of autozooids. Each autozooid is composed of a proximal adnate part and a distal peristome (in some species kenozooids are possibly intercalated). The autozooid peristomes support erect branches identical in budding and structure with the branch emanating from the erect peristome of the ancestrula. Thus, the complete crisiid colony consists of an adnate system of ancestrula and autozooids, which form erect branches from their peristomes. The adnate zooid system is comparable in autozooid morphology and budding pattern with simple uniserial stomatoporids. The tentative hypothesis is proposed that the crisiid group has developed from primitive stomatoporids; the adnate zooid system of the stomatoporids apparently evolved peristomial budding to produce the erect colony branches characteristic of crisiids.     

Evolution of the colonial growth habit in the ordovician bryozoans of the class stenolaemata: Feeding adaptations (Leningrad Region,Russia)

Based on the study of the growth habits and the relief of the colony surface in bryozoans of the class Stenolaemata from the Lower (Latorp horizon) and Middle (Volchov and Kunda horizons) Ordovician of the Leningrad Region, these bryozoans are shown to develop from the simple, unilaminate colonies (B_I^β) to the massive colonies with a nodular surface and smooth columnar colonies (B_II^α), which subsequently evolved into the columnar-spiral (B_II^β) and more complex erect branching and fenestrate constructions (B_II^γ), and subsequently into the branching, articulate colonies (B_III^α). The apertures of autozooecia and the character of their arrangement on the colony surface changed correlatively from the circular (B_I^β) to polygonal and roundedpolygonal, randomly arranged apertures, and subsequently to the oval apertures (B_III^α) arranged in strictly regular longitudinal or longitudinal-diagonal rows or in a quincuncial pattern. Thus, the development of growth habits in the bryozoans under consideration has a progressive character. It is expressed in the progressive increase in the complexity of growth habits of colonies and in the more regular arrangement of apertures and other structures on the colony surface. The directionality of morphological changes in the growth habits of colonies of Ordovician bryozoans was apparently closely associated with the development of more complex environmental interactions of these bryozoans, especially with water currents supplying food particles. It is suggested that the high competitive ability of bryozoans of the class Stenolaemata at early stages of its development in the basin of Baltoscandia was apparently due to the better use of food resources.     

Branch architecture,light interception and crown development in saplings of a plagiotropically branching tropical tree,Polyalthia jenkinsii (Annonaceae)

To investigate crown development patterns, branch architecture, branch-level light interception, and leaf and branch dynamics were studied in saplings of a plagiotropically branching tree species, Polyalthia jenkinsii Hk. f. & Thoms. (Annonaceae) in a Malaysian rain forest. Lengths of branches and parts of the branches lacking leaves ('bare' branches) were smaller in upper branches than in lower branches within crowns, whereas lengths of 'leafy' parts and the number of leaves per branch were larger in intermediate than in upper and lower branches. Maximum diffuse light absorption (DLA) of individual leaves was not related to sapling height or branch position within crowns, whereas minimum DLA was lower in tall saplings. Accordingly, branch-level light interception was higher in intermediate than in upper and lower branches. The leaf production rate was higher and leaf loss rate was smaller in upper than in intermediate and lower branches. Moreover, the branch production rate of new first-order branches was larger in the upper crowns. Thus, leaf and branch dynamics do not correspond to branch-level light interception in the different canopy zones. As a result of architectural constraints, branches at different vertical positions experience predictable light microenvironments in plagiotropic species. Accordingly, this pattern of carbon allocation among branches might be particularly important for growth and crown development in plagiotropic species.     

Wachstums- und Knospungsstrategie vonGrammothoa filifera Voigt & Hillmer (Bryozoa,Cheilostomata, Ob. Kreide)

The genusGrammothoa, growing on the interior walls ofThalassinoides burrows in the Maastrichtian Chalk-tuff, has developed a special growth strategy for spatial competition with other encrusting species which has not been previously recorded in bryozoans. The zoarium consists of auto-zooecia, gynozooecia and chains of smaller zooeciules, the latter forming stolon-like outgrowths from the distal edge of the colony. New areas of substratum are explored and colonized by the growth of these zooeciules. At the branching points of the zooeciules, autozooecia and gynozooecia are budded and may coalesce to form the main part of the colony.     

Branching structure in arborescent animals: Models of relative growth

Many invertebrate animals belonging to diverse phyla grow as regularly branching structures with the general appearance of miniature trees. If it is assumed that regularity of branching implies regularity in growth, models can be mathematically derived to depict growth of such a structure as a set of changing morphologic properties. Modes of growth, branching properties, and growth models can be expected to differ markedly from one major taxonomic group to another. Nevertheless, these properties can furnish a useful basis for comparing adaptive morphologies and underlying mechanical designs not only among arborescent animals, but with arborescent plants as well.Branching structures of some cheilostome bryozoans with rigidly erect, arborescent growth habits are inferred to result from continuous growth at steadily increasing numbers of growing tips through a process of repeated bifurcation and lengthening. In a model of continuous growth, the pattern by which the number of growing tips increases can be shown to be a generalized mathematical series, of which the Fibonacci series and a geometric series are two special cases. The quantities which determine the series can be calculated from measurable properties of the branching structure: lengths of paired branch portions ending in growing tips (relative growth ratio), lengths of paired branch portions between bifurcations (mean link length and link-length ratio), and numbers of branch portions belonging to different orders (branching ratio). Data for eight species of cheilostome bryozoans indicate, with high levels of confidence, that measurable branching properties and the models of relative growth inferred from them are species-specific. This specificity and a tendency to adhere to characteristic values of branching properties during growth are apparently direct expressions of internal control in these bryozoans.     

Bryozoans are returning home: recolonization of freshwater ecosystems inferred from phylogenetic relationships

Bryozoans are aquatic invertebrates that inhabit all types of aquatic ecosystems. They are small animals that form large colonies by asexual budding. Colonies can reach the size of several tens of centimeters, while individual units within a colony are the size of a few millimeters. Each individual within a colony works as a separate zooid and is genetically identical to each other individual within the same colony. Most freshwater species of bryozoans belong to the Phylactolaemata class, while several species that tolerate brackish water belong to the Gymnolaemata class. Tissue samples for this study were collected in the rivers of Adriatic and Danube basin and in the wetland areas in the continental part of Croatia (Europe). Freshwater and brackish taxons of bryozoans were genetically analyzed for the purpose of creating phylogenetic relationships between freshwater and brackish taxons of the Phylactolaemata and Gymnolaemata classes and determining the role of brackish species in colonizing freshwater and marine ecosystems. Phylogenetic relationships inferred on the genes for 18S rRNA, 28S rRNA, COI, and ITS2 region confirmed Phylactolaemata bryozoans as radix bryozoan group. Phylogenetic analysis proved Phylactolaemata bryozoan's close relations with taxons from Phoronida phylum as well as the separation of the Lophopodidae family from other families within the Plumatellida genus. Comparative analysis of existing knowledge about the phylogeny of bryozoans and the expansion of known evolutionary hypotheses is proposed with the model of settlement of marine and freshwater ecosystems by the bryozoans group during their evolutionary past. In this case study, brackish bryozoan taxons represent a link for this ecological phylogenetic hypothesis. Comparison of brackish bryozoan species Lophopus crystallinus and Conopeum seurati confirmed a dual colonization of freshwater ecosystems throughout evolution of this group of animals.     

Ecology of cryptic coral reef communities. IV. Community development and life histories of encrusting cheilostome bryozoa

Life histories of encrusting cheilostome species from the cryptic reef community at Rio Bueno, Jamaica, were studied on fouling panels over 3 yr. Recruitment and growth were generally slow compared with those reported for temperate cheilostomes. Most species that became abundant and persisted throughout the study did so through relatively rapid growth to a large size by a few successful colonies, rather than by accumulating great numbers of small colonies. This pattern, which reflects a striking increase in maximum survival with increasing colony size, is the basis for the extremely patchy distributions of bryozoans under corals. Reproduction in these species is delayed, and only a few long-lived, large colonies ever reproduce. Only one species, Drepanophora tuberculatum (Osburn), approached the characteristic opportunistic pattern of high recruitment, small colony size, and early reproduction.Grazing and nesting activities of one yellowtail damselfish greatly affected distributions of major taxa and cheilostome species on one set of panels. Species more abundant on grazed panels are more heavily calcified and showed other protective features, compared with species more abundant elsewhere. Despite intensive grazing by the fish, overgrowth interactions occurred frequently on both sets of panels. The fish affected what organisms were present, but did not obviously reduce the amount of overgrowth.     

Relationships between branch spacing, growth rate and light in tropical forest saplings

1. The spacing of branches along central stems was related to growth rate and light level in forest saplings and trees in tropical moist forest on Barro Colorado Island (BCI), Panama. The study included 14 species with tiers of plagiotropic branches (having planar leaf arrangements) and four species with continuous distributions of plagiotropic branches.
2. All species showed increases in branch spacing with increasing light and growth rate of diameter, similar to the patterns in leaf spacing noted previously in species which initially bear large leaves on unbranched stems.
3. Non-tiered species had shorter internodes than tiered species but because the latter bear more branches per node, both groups had similar numbers of branches per unit stem length, when compared at similar growth rates.
4. Differences in the relationship between internode length and growth rate among tiered species were related to demographic characteristics, suggesting that tree architecture may influence forest composition.
5. The strong correlation observed between branch spacing and growth rate suggests that branch spacing may be used to estimate past growth histories of forest tree species with plagiotropic branches.     

Epifaunal associates of Fucus serratus at Dale, south-west Wales

The macrofauna associated with Fucus serratus at Dale in south-west Wales contained 30 taxa of which five were common: these comprised four encrusting bryozoans and a hydroid. The bryozoans Flustrellidra hispida and Electra pilosa showed significant variations in abundance according to the size and structure of plants colonised. Electra was most abundant on plants where other bryozoans were least numerous. Colony growth in Electra was proportional to the amount of space provided by individual plant segments whilst growth in competitively superior species appeared to be independent of the size of the plant segments colonised. Dynamena pumila was larger and more abundant on larger segment faces where competition was presumably weaker. Each species showed a specific pattern of zonation along the fronds. The dominant species were generally more abundant on the concave surfaces of the plants. There was some evidence for the tendency for certain taxa to co-occur but the degree of association between species was weak and probably of limited biological significance. Species diversity varied with plant structure. More species occurred on concave surfaces at all plant levels but the overall pattern of diversity on the two segment faces was similar; diversity was greatest in the shrubbier mid-frond regions. Differential distribution of the dominant encrusting taxa within this Fucus community results in a considerable degree of niche segregation and ecological isolation.     

Theoretical morphology of colonial meshworks in the Fenestrata (Bryozoa): modelling hydraulic-resistance constraints and minimum fenestrule widths

Ancient fenestrate bryozoans (Fenestellidae, Polyporidae) exhibit a range of fenestrule widths that can be investigated to gain insights about constraints on meshwork morphology. Theoretical morphospace analyses of branch width and spacing reveal that there is a minimum fenestrule width common to both groups. We propose that the minimum fenestrule width observed in fenestrate colonies was constrained by hydraulic resistance (the hydraulic-resistance-constraint hypothesis). Modelling that considers both the viscous and inertial components of fluid-flow resistance through hypothetical fenestrate meshworks corroborates this hypothesis for the smaller fenestrate meshworks of the biserial fenestellids and for polyporids with branch widths less than about 0.6 mm. However, the geometry of the polyporid meshworks in species with branches wider than 0.6 mm does not appear to have been determined by a hydraulic-resistance constraint. Instead, maximum branch densities in the larger-branched polyserial polyporid colonies appear to have been constrained by a limit on the number of zooid rows that these colonies could develop on their branches (the zooid-row-limit hypothesis). The observed decrease in the minimum fenestrule width in these colonies is the indirect geometric result of the inability of the larger polyserial polyporid colonies to develop meshworks with high branch densities.