Errant Polychaetes Research Articles

Intertidal zonation of hemichordates in soft sediments

AbstractIntertidal zonation of organisms is well studied on rocky shores but less so in soft sediment communities. On rocky shores, communities are two dimensional, with biotic factors such as competition and predation setting the lower bound of a zone, whereas abiotic factors such as desiccation set the upper bound. In soft sediment communities, these patterns persist, but with a dynamic three‐dimensional ecosystem occupied by mobile infaunal organisms, zonation can be more difficult to quantify and detect. Hemichordate worms, however, deposit fecal casts at the surface, which can be easily identified and counted, making them a potential model system for identifying zonation in soft‐sediment systems. Here, we describe the intertidal zonation of hemichordate worms at two sites in Maine and Virginia. In Virginia,Saccoglossus kowalevskiioccurs in the mid‐intertidal at densities up to 500 individuals per square meter, whereas a tube‐building polychaete,Spiochaetopterus oculatus, dominates the lower intertidal. In Maine, two hemichordate species,Saccoglossus bromophenolosusandProtoglossus graveolens, co‐occur at densities up to 100 individuals per square meter in the mid‐intertidal, whereas hermit crabs and errant polychaetes are numerically dominant in the low intertidal. Despite known chemical defenses, crustaceans readily consumed hemichordates in lab assays, although polychaetes did not. In a field tethering experiment, loss rates of hemichordate‐flavored agarose pellets increased as tidal height decreased, suggesting that the lower bound of the hemichordate zone could be set by biotic forces such as crustacean predation. Additionally, a field transplant experiment found low survivorship of hemichordates transplanted to the upper intertidal, suggesting that desiccation may set their upper bound. These results are broadly consistent with prior studies of zonation in soft sediments, and expand our limited understanding of basic hemichordate ecology.

Structure, function and cell dynamics during chaetogenesis of abdominal uncini in Sabellaria alveolata (Sabellariidae, Annelida).

BackgroundDynamic apical microvilli of a single cell, called the chaetoblast, inside an ectodermal invagination form the template of annelid chaetae. Changes in the pattern of microvilli are frozen in time by release of chitin, such that the structure of the definitive chaeta reflects its formation. Cellular interactions during chaetogenesis also influence the structure of the chaeta. Analysing chaetogenesis allows for testing hypotheses on the homology of certain chaetal types. We used this approach to test whether the unusual uncini in Sabellaria alveolata are homologous to apparently similar uncini in other annelid taxa.ResultsOur study reveals unexpected details of sabellariid uncini, which mechanically reinforce the neuropodia enabling their use as paddles. The final structure of the chaeta is caused by pulses of microvilli formation and dynamic interaction between the chaetoblast and adjoining follicle cells. Cell dynamics during chaetogenesis of the uncini in Sabellaria alveolata exceeds by far that reported in previous studies on the formation of this type of chaetae.ConclusionDespite the superficial similarity of uncini in sabellariids and other annelids, differences in structure and details of formation do not support the homology of this type of chaetae. Chaetogenesis of sabellariid uncini involves unexpected microvilli and cell dynamics, and provides evidence that interactions between cells play a larger role in chaetogenesis than previously expected. In addition to their function as anchors, uncini in Sabellaridae stabilize the paddle-shaped notopodia, as each uncinus possesses a long, thin rod that extends deeply into the notopodium. The rods of all uncini in a single row form a bundle inside the notopodium that additionally serves as a muscle attachment site and thus have a similar function to the inner chaeta (acicula) of errant polychaetes (Aciculata).

NERÉIDIDOS (NEREIDIDAE, POLYCHAETA, ANNELIDA) ASOCIADOS A RAÍCES DE MANGLE ROJO, RHIZOPHORA MANGLE, EN ISLAS SAN ANDRÉS Y PROVIDENCIA, CARIBE COLOMBIANO

The family Nereididae Blainville, 1818, is one of the most representative group of errant polychaete worms, with about 535 described species in 43 genera, with great biological and ecological importance. The aim of this research was to make a taxonomic analysis of the nereidids associated to red mangrove roots, Rhizophora mangle, in the San Andrés, Old Providence and Santa Catalina Archipelago. This archipelago is part of the Seaflwer Biosphere Reserve. San Andrés is the largest island, having 27 km2 of landmass area, while Old Providence Island, located 95 km to the north of San Andrés, has 17 km2; both islands have well developed mangrove forests, composed mainly by R. mangle. The nereidids, deposited in the Coleccion Estuarina y Marina de la Universidad de Antioquia (CEMUA), were identifid with updated keys, by performing dissections and extracting parts with taxonomic value. Four species of polychaetes from the family Nereididae were found: Neanthes acuminata, Neanthes succinea, Neanthes sp., and Platynereis mucronata. Only N. succinea and N. acuminata had been reported from the Colombian Caribbean, being N. succinea the only species previously reported from the archipielago. In addition, systematic, ecological and biogeographical considerations were added for each species.

Muscle Articulations: Flexible Jaw Joints Made of Soft Tissues.

This study surveys animals that use soft tissues rather than rigid links to build jaw joints. Hard biting elements are useful; they are used in piercing or shearing during feeding and interactive behaviors and can directly impact survival and reproduction. The best understood biting systems include biting elements that are mounted on rigid jaw links that form a joint capable of transmitting the bite reaction forces. As such, jaws must incorporate joints that resist compression. Many jaw joints are "sliding joints", in which jaw links come into direct contact and the shape of the sliding contact surfaces dictates possible motions. There are, however, organisms that have biting elements on jaws that are made of flexible muscle and connective tissues. If arranged as a muscular hydrostat, in which multiple orientations of the muscle fibers may co-contract to provide turgid skeletal support, the multifunctional joint not only (a) provides the force to move the biting elements, but also (b) creates repositionable pivots and (c) transmits bite reaction forces. Such flexible joints, termed "muscle articulations", may be important to a number of soft-bodied animals. In this survey, we review the function of previously described muscle articulations: the joints found between inarticulate brachiopods' valves, cephalopods' beaks, the hooks of kalyptorhynch flatworms, and errant polychaetes' jaws. We also review the morphology, physiology, and feeding behaviors of the hagfish as a putative muscle articulation in an effort to understand how this jawless craniate is capable of biting with surprising force, seemingly without the benefit of any obvious method of opposing the force of the dental plate that is used to remove portions of food. Initial analysis suggests that a muscle articulation may be a key feature in coordinating head and body movements to provide the leverage needed for strong "bites".

Expression of the Lhx genes apterous and lim1 in an errant polychaete: implications for bilaterian appendage evolution, neural development, and muscle diversification.

BackgroundArthropod and vertebrate appendages appear to have evolved via parallel co-option of a plesiomorphic gene regulatory network. Our previous work implies that annelids evolved unrelated appendage-forming mechanisms; we therefore found no support for homology of parapodia and arthropodia at the level of the whole appendage. We expand on that study here by asking whether expression of the LIM homeobox (Lhx) genes apterous and lim1 in the annelid Neanthes arenaceodentata supports homology of the dorsal branches as well as the proximodistal axes of parapodia and arthropodia. In addition, we explore whether the neural expression of apterous and lim1 in Neanthes supports the putative ancestral function of the Lhx gene family in regulating the differentiation and maintenance of neuronal subtypes.ResultsBoth genes exhibit continuous expression in specific portions of the developing central nervous system, from hatching to at least the 13-chaetiger stage. For example, nerve cord expression occurs in segmentally iterated patterns consisting of diffuse sets of many lim1-positive cells and comparatively fewer, clustered pairs of apterous-positive cells. Additionally, continuous apterous expression is observed in presumed neurosecretory ganglia of the posterior brain, while lim1 is continuously expressed in stomatogastric ganglia of the anterior brain. apterous is also expressed in the jaw sacs, dorsal parapodial muscles, and a presumed pair of cephalic sensory organs, whereas lim1 is expressed in multiple pharyngeal ganglia, the segmental peripheral nervous system, neuropodial chaetal sac muscles, and parapodial ligules.ConclusionsThe early and persistent nervous system expression of apterous and lim1 in Neanthes juveniles supports conservation of Lhx function in bilaterian neural differentiation and maintenance. Our results also suggest that diversification of parapodial muscle precursors involves a complementary LIM code similar to those generating distinct neuronal identities in fly and mouse nerve cords. Expression of apterous and lim1 in discrete components of developing parapodia is intriguing but does not map to comparable expression of these genes in developing arthropod appendages. Thus, annelid and arthropod appendage development apparently evolved, in part, via distinct co-option of the neuronal regulatory architecture. These divergent patterns of apterous and lim1 activity seemingly reflect de novo origins of parapodia and arthropodia, although we discuss alternative hypotheses.

Feeding habits of the apron ray,Discopyge tschudii(Elasmobranchii: Narcinidae), from off Uruguay and northern Argentina

The feeding habits of the apron ray,Discopyge tschudii, were investigated, off Uruguay and northern Argentina, and we tested the hypothesis that the diet changes with increasing body size, between sexes and seasons using a multiple-hypothesis modelling approach.Discopyge tschudiipreys mainly on polychaetes (88.77% index of relative importance (IRI)) followed by siphons of the clamAmiantis purpurata(8.13% IRI) and amphipods (3.08% IRI). Ontogenetic, sexual and seasonal changes were found. Larger individuals ofD. tschudiiconsumed buried polychaetes more often. The consumption of errant polychaetes was higher in males and in the cold season. Also, amphipods were preyed on more heavily by females and the number of siphons ofA. purpurataconsumed was higher in the cold season.

Structure of fouling communities formed by Halichondria panicea (Porifera: Demospongiae) in the White Sea

The structure of fouling communities formed by the sponge Halichondria panicea in the White Sea is described. On artificial substrates exposed in the sea for a long time, this sponge forms overgrowths on the colonies of long-lived organisms such as the mussel Mytilus edulis or the ascidian Styela rustica. The biomass of algae and sedentary animals (other than H. panicea) on the substrate is markedly smaller in areas colonized by this sponge than in neighboring sponge-free areas. Conversely, the biomass of both sedentary and errant polychaetes is greater in H. panicea colonies than in adjacent fouling communities.

Direction of evolution within Annelida and the definition of Pleistoannelida

The direction of evolution within Annelida has been a long and contentious debate since the middle of the last century. Based on functional morphology two hypotheses were predominantly discussed. The first hypothesis proposed an earthworm-like ancestor and the evolution of a segmented coelom as an aid for peristaltic burrowing, whereas the second one postulated that the ancestral annelid resembled more an errant polychaete and the segmented coelom evolved as a requirement for blood transportation to the parapodia. A recent phylogenomic study of annelids as well as new fossil records of early annelids provide new insights to this debate, which will be discussed in this paper. Based on these data the ancestral annelid is most likely a mixture of sedentary and errant annelid features indicating an evolutionary origin of Annelida as a microphagous surface deposit-feeder crawling upon and through soft-bottom habitats. Hence, the evolution of iterative parapodia as locomotive devices most likely drove the evolution of segmentation in Annelida. Moreover, as Struck et al. (2011) did not provide a proper definition and name for their clade 1 this clade shall be named Pleistoannelida and is defined by the last common ancestor of Sedentaria and Errantia sensu Struck et al. (2011), and all the descendants of that ancestor.

Confocal analysis of nervous system architecture in direct-developing juveniles of Neanthes arenaceodentata (Annelida, Nereididae)

BackgroundMembers of Family Nereididae have complex neural morphology exemplary of errant polychaetes and are leading research models in the investigation of annelid nervous systems. However, few studies focus on the development of their nervous system morphology. Such data are particularly relevant today, as nereidids are the subjects of a growing body of "evo-devo" work concerning bilaterian nervous systems, and detailed knowledge of their developing neuroanatomy facilitates the interpretation of gene expression analyses. In addition, new data are needed to resolve discrepancies between classic studies of nereidid neuroanatomy. We present a neuroanatomical overview based on acetylated α-tubulin labeling and confocal microscopy for post-embryonic stages of Neanthes arenaceodentata, a direct-developing nereidid.ResultsAt hatching (2-3 chaetigers), the nervous system has developed much of the complexity of the adult (large brain, circumesophageal connectives, nerve cords, segmental nerves), and the stomatogastric nervous system is partially formed. By the 5-chaetiger stage, the cephalic appendages and anal cirri are well innervated and have clear connections to the central nervous system. Within one week of hatching (9-chaetigers), cephalic sensory structures (e.g., nuchal organs, Langdon's organs) and brain substructures (e.g., corpora pedunculata, stomatogastric ganglia) are clearly differentiated. Additionally, the segmental-nerve architecture (including interconnections) matches descriptions of other, adult nereidids, and the pharynx has developed longitudinal nerves, nerve rings, and ganglia. All central roots of the stomatogastric nervous system are distinguishable in 12-chaetiger juveniles. Evidence was also found for two previously undescribed peripheral nerve interconnections and aspects of parapodial muscle innervation.ConclusionsN. arenaceodentata has apparently lost all essential trochophore characteristics typical of nereidids. Relative to the polychaete Capitella, brain separation from a distinct epidermis occurs later in N. arenaceodentata, indicating different mechanisms of prostomial development. Our observations of parapodial innervation and the absence of lateral nerves in N. arenaceodentata are similar to a 19th century study of Alitta virens (formerly Nereis/Neanthes virens) but contrast with a more recent study that describes a single parapodial nerve pattern and lateral nerve presence in A. virens and two other genera. The latter study apparently does not account for among-nereidid variation in these major neural features.

Comparative neuroanatomy suggests repeated reduction of neuroarchitectural complexity in Annelida.

BackgroundPaired mushroom bodies, an unpaired central complex, and bilaterally arranged clusters of olfactory glomeruli are among the most distinctive components of arthropod neuroarchitecture. Mushroom body neuropils, unpaired midline neuropils, and olfactory glomeruli also occur in the brains of some polychaete annelids, showing varying degrees of morphological similarity to their arthropod counterparts. Attempts to elucidate the evolutionary origin of these neuropils and to deduce an ancestral ground pattern of annelid cerebral complexity are impeded by the incomplete knowledge of annelid phylogeny and by a lack of comparative neuroanatomical data for this group. The present account aims to provide new morphological data for a broad range of annelid taxa in order to trace the occurrence and variability of higher brain centers in segmented worms.ResultsImmunohistochemically stained preparations provide comparative neuroanatomical data for representatives from 22 annelid species. The most prominent neuropil structures to be encountered in the annelid brain are the paired mushroom bodies that occur in a number of polychaete taxa. Mushroom bodies can in some cases be demonstrated to be closely associated with clusters of spheroid neuropils reminiscent of arthropod olfactory glomeruli. Less distinctive subcompartments of the annelid brain are unpaired midline neuropils that bear a remote resemblance to similar components in the arthropod brain. The occurrence of higher brain centers such as mushroom bodies, olfactory glomeruli, and unpaired midline neuropils seems to be restricted to errant polychaetes.ConclusionsThe implications of an assumed homology between annelid and arthropod mushroom bodies are discussed in light of the 'new animal phylogeny'. It is concluded that the apparent homology of mushroom bodies in distantly related groups has to be interpreted as a plesiomorphy, pointing towards a considerably complex neuroarchitecture inherited from the last common ancestor, Urbilateria. Within the annelid radiation, the lack of mushroom bodies in certain groups is explained by widespread secondary reductions owing to selective pressures unfavorable for the differentiation of elaborate brains. Evolutionary pathways of mushroom body neuropils in errant polychaetes remain enigmatic.

Cryptic Species Differentiated in Conus ebraeus, a Widespread Tropical Marine Gastropod

Anomalous mitochondrial and nuclear gene sequences in individuals of the widely distributed tropical marine gastropod Conus ebraeus that were not distinguishable by shell shape and color pattern characters suggested the presence of a second, cryptic species. We tested this hypothesis by genetic, morphological, and ecological comparisons of additional individuals from the site in Okinawa where the two forms co-occurred. Radular tooth size and shape, prey type in nature, and microhabitats utilized differed markedly between the two forms. Adults with typical C. ebraeus DNA and radular teeth preyed primarily on errant polychaetes (Eunicidae); those with anomalous DNA and teeth ate mainly sedentary capitellids. Juveniles (shell length <13 mm) had more similar teeth and ate primarily syllids. Radular teeth of the anomalous form agreed with those of Conus judaeus, distinguished from C. ebraeus by Rudolph Bergh in 1895 solely on tooth characters of one specimen from the Philippines. Samples from other widely scattered Pacific localities revealed only typical C. ebraeus gene sequences. Both forms occurred in Seychelles (western Indian Ocean), where their radular teeth and diets were consistent with the data from Okinawa, but DNA of available material was degraded. Although C. judaeus was long dismissed as an aberrant specimen and junior synonym of C. ebraeus, our results support its validity as a distinct species. These results highlight the importance of molecular and radular tooth characters relative to those of the shell. Moreover, cryptic species could well be important components of species richness in Conus specifically and marine molluscan biodiversity more generally.

About the Cover.

Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverThe illustration shows a juvenile Conus ebraeus foraging at night at a snail's pace on reef rock covered with a thin layer of sand and green algae, in Pago Bay, Guam. The front end of the foot (black, edged with pale red) is visible against the substrate at the right, as is the posterior end at the left. The long black siphon extending from the anterior end of the shell parallel to the substrate directs a stream of water into the mantle cavity, bringing oxygen to the gill for respiration and past a large chemoreceptor, the osphradium. During food seeking, the siphon swings from side to side, the osphradium receives chemical stimuli emanating from potential food and transmits this information to the brain, and the snail directs its movement toward the prey. Conus uses hollow, pointed radular teeth as hypodermic needles to inject neurotoxic venom into a victim that is quickly paralyzed and swallowed whole. C. ebraeus may have the broadest distribution of all benthic marine animals, ranging from the Red Sea across the tropical Indian and Pacific Oceans to Central America. Until the study by Thomas Duda, Alan Kohn, and Amber Matheny in this issue (pp. 292–305), it was considered one of more than 500 species in the hyperdiverse genus Conus. These species typically exhibit fixed differences in shell shape and color patterns that are sufficient to distinguish them. Genetic structure in C. ebraeus populations is minimal across much of its range,1 but mitochondrial and nuclear gene sequences revealed some specimens from Okinawa sufficiently distinct to suggest the presence of a second, cryptic species. Further investigation showed this species to co-occur with true C. ebraeus at Okinawa. Although the two species could not be distinguished by shell characters, study of radular tooth morphology showed striking differences, and these correlated exactly with the molecular distinctions. Conus species with different radular tooth sizes and shapes typically attack members of different prey taxa. Adult C. ebraeus prey exclusively on errant polychaetes of the families Nereididae and Eunicidae, but the cryptic second species discovered in this study eats mainly sedentary polychaetes of the family Capitellidae. Moreover, unlike most cryptic species discovered by genetic differences, this one was named and described in 1895. Rudolph Bergh described Conus judaeus after dissecting a single specimen whose radular teeth differed markedly from those of C. ebraeus, but whose shell and other internal anatomy were indistinguishable. Long synonymized as a freak C. ebraeus, C. judaeus, whose teeth are identical to those of the cryptic species and distinct from all other known Conus species, is now concluded to be a valid species. Additional heretofore unrecognized cryptic species likely exist in this and other species-rich marine genera, and their future discovery may profoundly affect our understanding of marine biodiversity. Photo credit: Thomas F. Duda, Jr., University of Michigan. Cover layout: Beth Liles, Marine Biological Laboratory. Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 217, Number 3December 2009 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv217n3cover Views: 199Total views on this site © 2009 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.

Invasive tunicates fouling mussel lines: evidence of their impact on native tunicates and other epifaunal invertebrates

Blue mussel Mytilus edulis aquaculture lines and associated gear provide habitat for sessile and mobile epifaunal fouling organisms. Due to food limitations and substrate space, these species are likely to interact among themselves and with mussels. In some areas of Prince Edward Island and elsewhere in Atlantic Canada, invasive sea squirts such as the vase tunicate, Ciona intestinalis, colonize mussel socks and rapidly become the dominant species in terms of abundance and biomass. A relevant question for these systems is which native epifauna are most affected by the growth of these invasive tunicates. Our study documents local variations in the abundance of vase tunicates and relates this information to the abundance patterns of three groups of native epifauna with distinctive levels of mobility: sessile tunicates of the genus Molgula, sedentary polychaetes and errant polychaetes. Following recruitment, large abundances of vase tunicates created a significant among-site variation pattern that was consistent over time irrespective of the season when the mussel socks were deployed. In contrast, native tunicates of the genus Molgula colonized the mussel socks in lower numbers and in a spatial pattern opposite to that of invasive tunicates. With the exception of one sampling period, sedentary polychaete colonization also displayed a negative relationship with invasive tunicates. Errant polychaetes displayed erratic patterns that were apparently unrelated to the other species studied. Overall, these results suggest that invasive tunicates have negative effects on many epifaunal species, particularly those that are sessile or have limited mobility.

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.

Correspondence between shape and feeding habit changes throughout ontogeny of gilthead sea bream Sparus aurata L., 1758

The shape development through ontogeny of the gilthead sea bream Sparus aurata was studied by means of geometric morphometry and elliptic Fourier analysis. In parallel, observations are reported on the feeding habits of the same specimens until the end of the larval stage. Data for later stages were obtained from the literature. Sparus aurata underwent ontogenetic shifts in diet from zooplankton, selected by larvae, to mysids, errant polychaete, amphipods and isopods for juveniles, and finally to hard prey (decapods, gastropods and bivalves) in adults. The aims were: (1) to provide a complete description of shape development, from hatching to the adult stage, (2) to identify distinguishable morphological stages during ontogenesis, (3) to search for correspondence between morphological development and diet shifts, and (4) to examine the functional effect connecting shape changes to diet shifts and habitat selection. The growth trajectory obtained reflected a saturating curve made up by different sections. Each section corresponded to a developmental shape stage characterized by different allometric trends of different body regions. Indeed, these shape stages matched those based on widespread features of development. Furthermore, the size‐defined limits of the shape stages closely match the feeding shifts in the development of dietary behaviour of gilthead sea bream. The different constraints underpinning this pattern of covariation are discussed by showing that the growth profile of early stages is mainly determined by organogenesis of important apparati, whereas that of later stages seems optimized to improve feeding on hard prey in complex microhabitat and swimming abilities, and to reduce predation risk.

Relationships between short-term variations in density of juvenile yellowfin goby Acanthogobius flavimanus and environmental variables on an estuarine mudflat

Weekly variations in density of juvenile yellowfin goby Acanthogobius flavimanus with a variety of environmental variables (e.g. water temperature, salinity and transparency, and prey density) were investigated on a tidal mudflat within the Tama River estuary, central Japan, from March to July 2001. Metamorphosing newly settled juveniles occurred from mid-March to late May. Metamorphosed benthich juveniles first appeared in late March, the density sharply increasing to a peak (67.0 ind./m2) in early May but rapidly decreasing to less than 10% of that two weeks later. No consistent relationships were apparent between short-term variations in fish density by developmental stages and water temperature, or salinity. In contrast, a weak negative relation was found between juvenile density and water transparency. Further, benthic juvenile density was positively related to short-term fluctuations of errant polychaetes, which is one of the main prey items.

Ontogenetic diet shift, feeding rhythm, and daily ration of juvenile yellowfin goby Acanthogobius flavimanus on a tidal mudflat in the Tama River estuary, central Japan

To ascertain the feeding habits of benthic juvenile yellowfin goby Acanthogobius flavimanus, the gut contents of 599 specimens (15–41 mm in standard length, SL), collected on a tidal mudflat in the Tama River estuary throughout the diel cycle, were examined. The major prey items changed from harpacticoid copepods to errant and sedentary polychaetes at ca. 20 mm SL. Prey width increased with fish size. Fish of 26–28 mm SL fed mainly from sunset to morning, with highest feeding intensity during twilight hours and/or high tide. Based on the gut evacuation rate estimated from a forced feeding experiment in the laboratory and data for the diel change of mean gut-content volume in the field, the daily ration of juvenile yellowfin goby (26–28 mm SL) was calculated to be 13.8 mm3 fish−1 day−1. This volume is approximately equivalent to 3.9 individuals of the errant polychaete Ceratonereis erythraeensis (9.7 mm in body length, BL) or 8.1 individuals of the sedentary polychaete Prionospio japonica (14.8 mm BL), both species occurring abundantly on the mudflat during the study.

Comparison of invertebrate abundance in a seagrass bed and adjacent coral and sand areas at Amitori Bay, Iriomote Island, Japan

To examine whether or not reef-associated seagrass beds harbor abundant food resources for resident and visiting fishes, the invertebrate density and biomass in a seagrass bed were compared with those in adjacent coral and sand areas at Amitori Bay, Iriomote Island, Japan, in June 2002. The vegetation within the bed was dominated by Enhalus acoroides, the coral area comprising primarily Acropora spp. Epifaunal density was greater on the seagrass than on the corals. whereas biomass was greater on the latter. Tanaids, chironomid larvae, errant polychaetes, and gammaridean amphipods were dominant taxa on the seagrass; larger crustaceans, such as crabs and shrimps, being abundant on the corals. The density of infauna was greatest in the seagrass bed, followed by the coral and sand areas, whereas biomass was greatest in the coral area, followed by the seagrass bed and sand area. Each of the three habitats was dominated by harpacticoid copepods and errant polychaetes, although the density of each taxonomic group differed among the habitats. Important food items of seagrass bed fishes, such as harpacticoid copepods, gammaridean amphipods, errant polychaetes, and tanaids, were abundant in the seagrass bed, the density of each being greater than in the other two habitats.

Anatomy and relationships of the Early Cambrian wormMyoscolex

Dzik, J. (2004). Anatomy and relationships of the Early Cambrian wormMyoscolex. —Zoologica Scripta, 33, 57–69.Numerous fossil specimens ofMyoscolex atelesGlaessner, 1979 from the late Early Cambrian Emu Bay shale of Kangaroo Island, South Australia with phosphatized organic matter‐rich tissues show its muscular body wall penetrated by rows of rod‐like structures — possible chaetae. The body wall was composed of an external layer with transverse (circular) fibres. This layer was thickest in lateral parts of the body and very thin dorsally. In the ventro‐lateral quarter of the body circumference, a belt of longitudinal fibres extended along the body. Longitudinal fibres also occurred in the dorsal region of the body. Along the venter extended a narrow longitudinal belt of probably oblique cords, crossing themselves perpendicularly. In having a virtually smooth, laterally flattened body,Myoscolexclosely resembles the slightly geologically youngerPikaiafrom the Burgess shale of British Columbia, generally believed to be one of the oldest chordates. Being the oldest probable annelid, at least superficially similar to the opheliid polychaetes,Myoscolexmay appear not too distant from the ancestor of the phylum. The lateral body flattening ofMyoscolexwas apparently an adaptation to swimming by undulation of the body in transverse plane, similar to today's errant polychaetes but without using chaetae or appendages in propulsion.

Development of pigment-cup eyes in the polychaete Platynereis dumerilii and evolutionary conservation of larval eyes in Bilateria.

The role of Pax6 in eye development in insects and vertebrates supports the view that their eyes evolved from simple pigment-cup ocelli present in their last common ancestors (Urbilateria). The cerebral eyes in errant polychaetes represent prototype invertebrate pigment-cup ocelli and thus resemble the presumed ancestral eyes. We have analysed expression of conserved eye specification genes in the early development of larval and adult pigment-cup eyes in Platynereis dumerilii (Polychaeta, Annelida, Lophotrochozoa). Both larval and adult eyes form in close vicinity of the optic anlagen on both sides of the developing brain ganglia. While pax6 is expressed in the larval, but not in the developing, adult eyes, expression of six1/2 from trochophora stages onwards specifically outlines the optic anlagen and thus covers both the developing larval and adult eyes. Using Platynereis rhabdomeric opsin as differentiation marker, we show that the first pair of adult eye photoreceptor cells is detected within bilateral clusters that transitorily express ath, the Platynereis atonal orthologue, thus resembling proneural sensory clusters. Our data indicate that--similar to insects, but different from the vertebrates--polychaete six1/2 expression outlines the entire visual system from early developmental stages onwards and ath-positive clusters generate the first photoreceptor cells to appear. We propose that pax6-, six1/2- and ath-positive larval eyes, as found in today's trochophora, were present already in Urbilateria.