Relation of form to life habit in free-living cupuladriid bryozoans

Abstract

Since the late Mesozoic, several bryozoan groups have occupied unstable soft-sediment habitats by adopting a free-living and motile mode of life. Today, the free-living bryozoans often dominate epibenthic faunal communities in these expansive habitats, yet their biology and ecology remain poorly understood. This study examines their unique mode of life by exploring the relationship between form and function in the free-living Cupuladriidae of tropical America. Cupuladriid species occupy distinct niches in water depth and sediment type, and these variables correlate with a variety of morphological traits amongst species, thereby helping formulate hypotheses on functional morphology. These hypotheses were tested with disturbance, burial and mobility experiments. Species deal both passively and actively with disturbance and burial. Colony size and shape passively influences colony stability on the sea floor, susceptibility to burial and the ability to emerge once buried. Movable mandibles (setae) assist in emergence following burial, actively increase colony stability, and deter the settlement and disruption of encrusting growth of other organisms. Mandibles also enable colonies to move over the sea floor, but such behaviour appears to be of minimal benefit to avoid predation or to influence dispersal. Disturbance on the sea floor is the most important factor in driving the convergent evolution of the mobile free-living form. Colony shape and size and the employment of mandibles to improve stability, return to the surface after burial, and remove epibiota are central to the post-Mesozoic success of free-living bryozoans. Colonies of free-living bryozoans dig to the surface using setae (blue) after being buried; this explains their long-term success on unstable sediments (scale bars: 500 µm).

Functional innovation through vestigialization in a modular marine invertebrate

“…Our behavioural observations are consistent with those made on avicularia of other species, which show a variety of ways in which different forms of avicularia can trap or deter potential predators or settling epibionts and that involve actions of capturing, impaling or sweeping (Kaufmann, 1968(Kaufmann, , 1971Cook & Chimonides, 1978;Chimonides & Cook, 1981;Winston, 1986Winston, , 2010O'Dea, 2009;Carter et al, 2010b). A range of functional needs are considered to be met by different avicularia across Cheilostome lineages (Winston, 1984) and the possibility that avicularia are multifunctional cannot be excluded.…”

“…The fact that avicularia are phenotypically variable is well documented (Darwin, 1859(Darwin, , 1872Harmer, 1909;Silén, 1938;Marcus, 1939;Hyman, 1959;Silén, 1977;Winston, 1984Winston, , 1986Winston, , 1991Winston, , 2010 supporting the view that avicularia, as vestigial autozooids, are evolving characters (Lidgard et al, in press). The morphological extremes of the mandible (operculum homologue) displayed by avicularia, from bristle-like mandibles to forms that resemble miniature bird beaks, correlate with functional diversity, where avicularia can function to clean colony surfaces from epibionts (O'Dea, 2009) or sand grains (Cook & Chimonides, 1978;Chimonides & Cook, 1981), or impale and trap passing invertebrates (Kaufmann, 1968(Kaufmann, , 1971Winston, 1984Winston, , 1986Winston, , 1991Winston, , 2010Carter et al, 2010b). Even other clonal invertebrates growing on the colony surface (e.g.…”

Environmental change prior to the K–T boundary inferred from temporal variation in the morphology of cheilostome bryozoans

“…In motile, freeliving species, radial symmetry is customary in all but a few Late Cretaceous species, even though all other aspects of morphology and patterns of growth vary greatly amongst species (O'Dea et al, 2008;O'Dea and Jackson, 2009), strongly supporting the idea that radial symmetry is highly adaptive. Indeed, experimental data have shown that deviating from radial symmetry is disadvantageous for motile freeliving species because: (1) asymmetrical colonies are more likely to be turned over by wave action and are less able to move up through sediment passively if they become buried (O'Dea, 2009), implying that radial symmetry confers hydrodynamic stability and better in-faunal coordination; (2) fragments that are always asymmetrical are more prone to burial than circular colonies of the same size (O'Dea, 2009); (3) when a free-living bryozoan is fragmented, initial regenerative growth is always concentrated in the central portions of the sides of the fragments (O'Dea, 2006;O'Dea et al, 2008), thereby ensuring that a circular, more stable shape is rapidly regained by the colony (Håkansson and Thomsen, 2001); (4) initial regenerative growth of asymmetrical fragments is five times more rapid than colony growth at any other time, implying an urgency in the need to create a circular colony (O'Dea, 2006); and (5) fragments that fail to achieve circularity demonstrate higher rates of mortality (O'Dea, 2006(O'Dea, , 2009.…”

“…They include vibracula which have stiff, whip-like setae that extend above the colony surface and whose functions include: (1) deterring epibiotic organisms from settling on colony surfaces; (2) cleaning fine sediment from the colony surface; and (3) removing or impeding the growth of epibionts (Winston, 1988;Carter et al, 2010). In the free-living bryozoans vibracula also physically lift the colony above the surface of the sea floor (Cook, 1963;Cook and Chimonides, 1978;Winston, 1988;O'Dea, 2009), and can be used to dig colonies out of the sediment if accidentally buried (Håkansson and Winston, 1985).…”

Secular changes in colony‐forms and bryozoan carbonate sediments through geological history

“…Feeding zooids open only on the upper, convex colony surface. Such ‘lunulitiform’ colonies are seldom more than a few centimetres in diameter (Cook & Chimonides, ; Rosso, ; O'Dea, ); they tend to show determinate growth, attaining a maximum size that is characteristic for each particular species. While some populations contain only sexual colonies produced from a larva, in others asexual, clonal colonies predominate.…”