▪ Abstract Molecular tools have profoundly rearranged our understanding of metazoan phylogeny. Initially based on the nuclear small ribosomal subunit (SSU or 18S) gene, recent hypotheses have been corroborated by several sources of data (including the nuclear large ribosomal subunit, Hox genes, mitochondrial gene order, concatenated mitochondrial genes, and the myosin II heavy chain gene). Herein, the evidence supporting our current understanding is discussed on a clade by clade basis. Bilaterian animals consist of three clades: Deuterostomia, Lophotrochozoa, and Ecdysozoa. Each clade is supported by molecular and morphological data. Deuterostomia is smaller than traditionally recognized, consisting of hemichordates, echinoderms, chordates, and Xenoturbella (an enigmatic worm-like animal). Lophotrochozoa groups animals with a lophophore feeding apparatus (Brachiopoda, Bryozoa, and Phoronida) and trochophore larvae (e.g., annelids and mollusk), as well as several other recognized phyla (e.g., platyhelmin t...

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The New View of Animal Phylogeny

The structure of the nervous system.

We present an optogenetic illumination system capable of real-time light delivery with high spatial resolution to specified targets in freely moving Caenorhabditis elegans. A tracking microscope records the motion of an unrestrained worm expressing channelrhodopsin-2 or halorhodopsin in specific cell types. Image processing software analyzes the worm's position in each video frame, rapidly estimates the locations of targeted cells and instructs a digital micromirror device to illuminate targeted cells with laser light of the appropriate wavelengths to stimulate or inhibit activity. Because each cell in an unrestrained worm is a rapidly moving target, our system operates at high speed (∼50 frames per second) to provide high spatial resolution (∼30 μm). To test the accuracy, flexibility and utility of our system, we performed optogenetic analyses of the worm motor circuit, egg-laying circuit and mechanosensory circuits that have not been possible with previous methods.

/pdf/optogenetic-manipulation-of-neural-activity-in-freely-moving-1mrm3ustzw.pdf

Optogenetic manipulation of neural activity in freely moving Caenorhabditis elegans

This article reviews the literature on learning and memory in the soil-dwelling nematode Caenorhabditis elegans. Paradigms include nonassociative learning, associative learning, and imprinting, as worms have been shown to habituate to mechanical and chemical stimuli, as well as learn the smells, tastes, temperatures, and oxygen levels that predict aversive chemicals or the presence or absence of food. In each case, the neural circuit underlying the behavior has been at least partially described, and forward and reverse genetics are being used to elucidate the underlying cellular and molecular mechanisms. Several genes have been identified with no known role other than mediating behavior plasticity.

/pdf/an-elegant-mind-learning-and-memory-in-caenorhabditis-2jcrmmn9m0.pdf

An elegant mind: learning and memory in Caenorhabditis elegans.

The present work summarizes the progress attained in the study of sponge larval ecology since the state-of-the-art reviews performed in the 1970s and stresses the major weaknesses in our current understanding. Most available information on this subject comes from laboratory studies, with just occasional field observations or experiments. The data are also strongly biased because they are mostly derived from just one larval type out the eight types known in the phylum Porifera. Descriptive studies on larval histology are relatively abundant, but investigations directed at unravelling the cytological basis of the main larval behaviors are scarce. Most aspects of basic larval metabolism and sensing processes remain largely not investigated. Modelling of larval ecology is virtually lacking, with no serious attempt to investigate how the major features of larval ecology affect the structure and dynamics of sponge populations. In summary, the ecology of the sponge larva needs further research attention if we ar...

The ecology of the sponge larva

The calcareous sponge Leucosolenia laxa releases free-swimming hollow larvae called coeloblastulae that are the characteristic larvae of the subclass Calcinea. Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of cells: flagellated cells, bottle cells, vesicular cells, and free cells in a central cavity. The flagellated cells, the principal cell type of the larva, are arranged in a pseudostratified layer around a large central cavity. The larval flagellated cells characteristically have glutinous granules that are used as internal markers during metamorphosis. After a free-swimming period the larva settles on the substratum, and settlement apparently triggers the initiation of metamorphosis. The larval flagellated cells soon lose their flagellum and begin the process of dedifferentiation. Then the larva becomes a mass of dedifferentiated cells in which many autophagosomes are found. Within 18 h after settlement, the cells at the surface of the cell mass differentiate to pinacocytes. The cells beneath the pinacoderm differentiate to scleroblasts that form triradiate spicules. Finally, the cells of the inner cell mass differentiate to choanocytes and are arranged in a choanoderm that surrounds a newly formed large gastral cavity. We found glutinous granules in these three principal cell types of juvenile sponges, thus indicating the multipotency of the flagellated cells of the coeloblastula.

Metamorphosis of coeloblastula performed by multipotential larval flagellated cells in the calcareous sponge Leucosolenia laxa.

The intertidal sponge, Halichondria panicea, regularly begins releasing larvae shortly after dawn, and ejects most of them during morning hours under a natural light-dark (LD) cycle. Its diurnal periodicity was confirmed under an artificial LD 12:12h cycle. In search of a trigger that stimulates the sponge colonies to release larvae, the colonies were subjected to experimentally modified LD cycles. Under continuous darkness, only a single release peak was observed about fifteen hours after the beginning of darkness. Further, the colonies invariably released larvae about fifteen hours after the change from light to dark on the preceding day under all illumination regimes examined. The timing of their larval release was independent of both the tidal cycle and the daily cycle of seawater temperature. These results indicate that the trigger is a light signal: the onset of darkness (not onset of light) of the preceding day under natural illumination. Subsequent to this stimulus, H. panicea needs a period of fi...

Larval release in response to a light signal by the intertidal sponge halichondria panicea

The intertidal sponge Callyspongia ramosa releases larvae in the morning under natural light. The photic control ofthis morning release was studied under experimental light-dark (LD) cycles. Under LD 12: 12h cycles (light period, from 6:00 to 18:00), release peaked about 6:00. But the release was not stimulated by the illu mination in the morning because the sponge colonies re leased larvae even in the darkness. The experiments un der various light regimes showed that the photic stimulus is not the onset of darkness but, unexpectedly, the onset oflight the day before. In fact, C. ramosa colonies invari ably released larvae about 24 hours after the onset of light under all illumination regimes tested. The tidal cycle and the daily cycle of the seawater temperature did not in fluence the time of release. Therefore, in nature, the dawning light most likely stimulates larval release on the next day. This photoadaptation in the larval release of C. ramosa suggests that the morning release is advanta geous for their free-swimming larvae to seek out and set tle on the suitable substratum in the intertidal region within their short dispersive period.

Morning Release of Larvae Controlled by the Light in an Intertidal Sponge, Callyspongia ramosa

Summary Free-swimming larvae of two calcareous sponges were studied by electron microscopy. The larvae are composed of four kinds of cells, namely flagellated cells, granular cells, four cruciform cells, and several yolk-containing cells. In the anterior hemisphere of the larvae, the columnar flagellated cells are arranged in a single layer. Their nucleus and Golgi apparatus are located in close proximity to the flagellar rootlets. There are granular cells in the posterior hemisphere of the larvae. They have a nucleus with a nucleolus, large phagosomes, well-developed Golgi apparatus, and numerous RER cisternae. There is a cruciform cell in each quadrant of the larvae. From its characteristic arrangement of organelles, it is suggested but not concluded that the cruciform cells participate in photoreception. The yolk-containing cells are, most probably, nutritive cells derived from the mother sponge. The roles of these four kinds of cells in habitat selection and metamorphosis are discussed.

Metamorphosis of calcareous sponges I. Ultrastructure of free-swimming larvae

Summary The free-swimming larva of the calcareous sponge turns into a sessile juvenile during metamorphosis. Electron microscopic observations of metamorphosing larvae reveal the rearrangement and differentiation of larval cells. About 12 h after the larvae were released from a mother sponge, the settled larvae without flagella consist of an inner cell mass and an enveloping layer of pinacocytes. The inner mass cells have residual flagellar rootlets which clearly show the origin of the cells. On the other hand, the pinacocytes still show the intracellular profile characteristic of the granular cells of the swimming larva. One day after release, scleroblasts and other mesohyl cells differentiate in the peripheral region of the inner cell mass. Two days after release, the central cells of the inner cell mass differentiate into choanocytes. Three days after release, a large gastral cavity is formed and lined by a layer of choanocytes. These results demonstrate the cell lineage in the metamorphosis of the cal...

Shigetoyo Amano

Papers

Metamorphosis of coeloblastula performed by multipotential larval flagellated cells in the calcareous sponge Leucosolenia laxa.

Larval release in response to a light signal by the intertidal sponge halichondria panicea

Morning Release of Larvae Controlled by the Light in an Intertidal Sponge, Callyspongia ramosa

Metamorphosis of calcareous sponges I. Ultrastructure of free-swimming larvae

Metamorphosis of calcareous sponges II. Cell rearrangement and differentiation in metamorphosis