Complexity by Subtraction

The eye and brain: standard thinking is that these devices are both complex and functional. They are complex in the sense of having many different types of parts, and functional in the sense of having capacities that promote survival and reproduction. Standard thinking says that the evolution of complex functionality proceeds by the addition of new parts, and that this build-up of complexity is driven by selection, by the functional advantages of complex design. The standard thinking could be right, even in general. But alternatives have not been much discussed or investigated, and the possibility remains open that other routes may not only exist but may be the norm. Our purpose here is to introduce a new route to functional complexity, a route in which complexity starts high, rising perhaps on account of the spontaneous tendency for parts to differentiate. Then, driven by selection for effective and efficient function, complexity decreases over time. Eventually, the result is a system that is highly functional and retains considerable residual complexity, enough to impress us. We try to raise this alternative route to the level of plausibility as a general mechanism in evolution by describing two cases, one from a computational model and one from the history of life.     

Ultrastructure of potential photoreceptor organs in the larva ofScrupocellaria bertholetti (bryozoa)

Summary Pigmented spots have been implicated as potential photoreceptors in many bryozoan larvae which display phototactic behavior. Larvae ofScrupocellaria bertholetti, initially photopositive on release from the brood chambers, have a pair of identical posterolateral pigmented spots and a third morphologically different spot in the anteromedian line. The presumed photoreceptoral organelle in each is composed of numerous unmodified cilia which have the typical 9+2 arrangement of microtubules with electron-dense arms extending from thea-microtubule of each peripheral pair. The posterolateral pigmented spot is composed of two modified coronal cells and a basal sensory cell. Cilia arising from the apical part of this basal cell are aligned vertically. Densely packed pigment vesicles in the three cells form a shield that restricts light entry to one direction. The anteromedian pigmented spot is composed of four cells, two lateral and two posterior. Cilia of the opposing lateral cells are horizontally aligned, whereas cilia of the posterior cells are vertical and curve outward from the oral margin of the pigmented spot. Pigment vesicles are present in all four cells to form a complete shield. Extensions of the larval nervous system are in direct contact with the four cells of the anteromedian spot and the basal cells of the posterolateral spots. The posterolateral pigmented spots share structural and topological similarities with the pigmented spots ofBugula neritina, the only other supposed photoreceptors in lophophorates which have been studied at the ultrastructural level. It is not yet possible to homologize these potential photoreceptors with those of other groups.     

Structure and occurrence of cyphonautes larvae (bryozoa,ectoprocta)

We have studied larvae of the freshwater ctenostome Hislopia malayensis with scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and LM of serial sections. Some additional observations on larvae of M. membranacea using SEM and CLSM are also reported. The overall configuration of muscles, nerves, and cilia of the two larvae are identical. However, the larva of H. malayensis is much smaller than that of M. membranacea, which may explain most of the differences observed. Although all major nerves and muscle strands are present in H. malayensis, they are generally composed of fewer fibers. The H. malayensis larva lacks the anterior and posterior intervalve cilia. Its pyriform organ is unciliated with only a small central depression. The adhesive epithelium is not invaginated as an adhesive sac and lacks the large muscles interpreted as adhesive sac muscles in the M. membranacea larva. The velum carries two rows of ciliated cells, though the lower “row” consists of only one or two cells. Both rows of ciliated cells are innervated by nerves, which have not been detected in the M. membranacea larva. The ciliated ridge of H. malayensis lacks the frontal cilia. The planktotrophic cyphonautes larvae in a number of ctenostome clades and in the “basal” cheilostome clade Malacostega (and probably in the earliest cheilostomes) support the idea that the cyphonautes larva is the ancestral larval type of the Eurystomata. It may even represent the ancestral larval type of the bryozoans (= ectoprocts). J. Morphol. 271:1094‐1109, 2010. © 2010 Wiley‐Liss, Inc.     

Solving Volterra-Lotka systems with diffusion by monotone iteration

In this paper we first present a theorem for a monotone iteration method, where the conditions for the considered operators are affine invariant. Then we apply this method to systems of nonlinear elliptic differential equations arising from the Volterra-Lotka model with diffusion. Finally an extended predator-prey system is considered. One numerical example is given.     

The degree of intransitivity and its measurement in an assemblage of encrusting cheilostome bryozoa

The outcome of spatial encounters between 12 species of encrusting bryozoans is largely dependent on the angle formed between the direction of growth of the two species. An index has been devised which describes the degree to which the pattern of interspecific overgrowth is an hierarchical, perfectly transitive one, where each species invariably overgrows those beneath it in the hierarchy or an intransitive one, where all of the species have an equal probability of being overgrown; these two states therefore form quantifiable and opposite ends of a continuum. Frontal encounters alone produced a value closer to the transitive end than did encounters from all directions. Since the latter are more likely in nature, the underlying pattern of overgrowth in assemblages of encrusting bryozoans will be a highly intransitive one.     

Growth and feeding of Fredericella sultana (bryozoa) in the outlet of a humic acid lake

Monthly observations on the bryozoa Fredericella sultana have been carried out in the outlet of a humic acid lake for more than two years. The growth of the colonies were rapid during algal bloom and in fall the colonies almost covered the free underside of nearly all stones on the bottom. In winter most of the colonies died out, but scattered individuals were also observed alive during this period. Statoblasts produced in summer were ‘hatching’ in March–April next year. F. sultana seems to feed on all particles that can pass through the mouth opening. A special study regarding this was made on the diatoms, by analysis of fecal pellets. It was found that young colonies of F. sultana filtrated most efficiently the smallest particles (<20 µm) while older individuals in addition were able to elevate the filtration of bigger particles (40–100 µm). The feeding takes place both by filtration by the cilia and by an active use of the tentacles themselves on the lophophore.     

Analysis of flicker fusion critical frequency by the iteration method

Two methods of the presentation of light stimuli for determining the flicker fusion critical frequency were compared: the classic method using a continuous increase in flicker frequency and a new iteration method in which impulses of fixed duration were presented step by step. The informative value of each method was also estimated under actual occupational conditions, where visual fatigue was estimated in users of video display terminals. Calculated values of the mean-square deviation showed that the iteration method provides a higher accuracy. This method proved to be more sensitive in studying visual fatigue.     

Morphogenesis in the individual and historical development of marine post-Paleozoic Tubuliporida (bryozoa,Stenolaemata)

The principal features of the morphogenesis in the individual and historical development of marine post-Paleozoic bryozoans of the order Tubuliporida (=Cyclostomata, part.) are discussed. Throughout their history (Ordovician–Recent), Tubuliporida retained the morphological type of the tubu-lar zooid with a terminal aperture. This may suggest similarities not only between the zooidal anatomy of fossil and living Tubuliporida but also between the ontogenetic processes in their ancestrulae and zooids and similarities in astogeny in general. After the two types of reproduction, sexual and vegetative, which lead to the formation of new colonies that could only grow by budding, had been developed, the develop-ment of different types of colonial organization was of special importance in the evolution of the post-Pale-ozoic Tubuliporida.     

Shear stress in cells generated by ultrasound

An experimental study using a mason horn of 21.4 kHz indicated that the threshold shear stress for cell sonoporation was 12±4 Pa for ultrasound exposure time up to 7 min. Numerical calculations have shown that shear stress associated with microstreaming surrounding encapsulated bubbles may be large enough to generate sonoporation at 0.1 MPa of 1 or 2 MHz ultrasound.     

A new species of microporella (bryozoa, cheilostomata) from alaska

Microporella speciosa sp. nov. is described from Alaska, USA. The new species is characterized by the following combination of features: 1) semicircular orifice with slightly curved and faintly denticu-late proximal edge having a straight, shallow groove, 2) two distal oral spines, 3) laterally directed avicularian rostrum with a broad, channeled tip, 4) open frontal pores, and 5) completely calcified basal walls. Opercula, avicularian mandibles, the ancestrula, and early ontogeny are also described.