Collected specimens of the neomeniomorph Helluoherpia aegiri (Aplacophora, Mollusca) from Bergen, Norway. (Copyright: Maik Scherholz)

Collected specimens of the neomeniomorph Helluoherpia aegiri (Aplacophora, Mollusca) from Bergen, Norway. (Copyright: Maik Scherholz)

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Evolution is not a one-way road towards complexity

18. Okt 13Mag. Veronika Schallhart | Original online publication Universität Wien | PDF Evolution is not a one-way road towards complexity_

Development of cryptic worms provides new insights into molluscan evolution.

There are still a lot of unanswered questions about mollusks, e.g. snails, slugs and mussels. The research group of Andreas Wanninger, Head of the Department of Integrative Zoology of the University of Vienna, took a detailed look at the development of cryptic worms. The larvae of the “wirenia argentea” hold a much more complex muscular architecture than their adults – they remodel during their metamorphosis. That’s a clue that the ancestors had a highly complex muscular bodyplan. Their findings are published in the current issue of the scientific journal “Current biology”.

Lateral view of the shell-less, 4 mm long, neomeniomorph Wirenia argentea (Aplacophora, Mollusca). Copyright: Thomas Schwaha)

Lateral view of the shell-less, 4 mm long, neomeniomorph Wirenia argentea (Aplacophora, Mollusca). Copyright: Thomas Schwaha)

With over 200.000 species described, the Mollusca – soft-bodies animals that, among others, include snails, slugs, mussels, and cephalopods – constitutes one of the most species-rich animal phyla. What makes them particularly interesting for evolutionary studies, however, is not the sheer number of their representatives, but rather their vast variety of body morphologies they exhibit. Ever since they have been unambiguously assigned to the phylum, a group of worm-like, shell-less mollusks whose body is entirely covered by spicules – the Aplacophora (“non-shell-bearers”, usually small animals in the mm-range that inhabit the seafloors from a few meters to abyssal depths) has been hotly debated as being the group of today’s living mollusks that most closely resembles the last common ancestor to all mollusks.

3D reconstruction of the larval musculature in the neomeniomorph Wirenia argentea (Aplacophora, Mollusca) with color coded major muscle units (ca. 0,2 mm length). (Copyright: Maik Scherholz)

3D reconstruction of the larval musculature in the neomeniomorph Wirenia argentea (Aplacophora, Mollusca) with color coded major muscle units (ca. 0,2 mm length). (Copyright: Maik Scherholz)

However, new studies on the development of a typical aplacophoran (Wirenia argentea, a species that was collected in 200 m depth off the coast of Bergen, Norway) tell a different story. Although their adult, worm-like body appears rather simple (hence the traditional assumption that they may constitute a basal molluscan group), their small, 0.1 to 0.3mm long larvae undergo a stage in which they show an extremely complex muscular architecture which is largely lost and remodeled during metamorphosis to become the simple muscular arrangement of the adult animal. The entire secret these animals hold only unravels if one takes a detailed look at the morphology of these tiny animals. In doing so, Andreas Wanninger, Head of the Department of Integrative Zoology of the University of Vienna, and colleagues found that the musculature of Wirenia larvae in detail resembles that of a quite different-looking mollusk, the so-called polyplacophorans or chitons (flat animals in the cm-range that bear 8 shell plates on their back). In contrast to the former, however, chitons do retain much of the larval muscles as adults. While it has been suspected for a long time that aplacophorans and chitons are closely related, it has often been argued that the aplacophoran morphology is closer to the ancestral molluscan condition than the polyplacophoran one. The current data paint a different picture: the fact that the highly complex larval muscular bodyplan is so similar in both groups but is only carried over into the adult stage in one of them – the chitons – strongly suggests that the common ancestor of both groups was of similar complexity; thereby implying that the worm-like groups lost these complex traits and became secondarily simplified over evolutionary time.

Top view of the eight-shelled chiton Acanthopleura japonica (Polyplacophora, Mollusca). (Copyright: Maik Scherholz)

Top view of the eight-shelled chiton Acanthopleura japonica (Polyplacophora, Mollusca). (Copyright: Maik Scherholz)

Interestingly, findings from the fossil record support this new developmental evidence. A recently described species from the Silurian – Kulindroplax perissokosmos – obviously had a mix of aplacophoran and polyplacophoran characters: while being long, slender, cylindrical in diameter, and covered by spicules – closely reminding us of today’s aplacophorans – it had seven shells on its back. Although, at an age of 425 myr, too young to be considered the long-sought ancestor of polyplacophorans, aplacophorans and maybe even all mollusks (the origin of the phylum is known to date back to at least the Cambrian Explosion some 540 myr ago), this relative of the distant past proves that evolution has widely played with the combination of the various morphological character sets in individual molluscan groups. Taking together the data currently available, a coherent scenario emerges that strongly suggests that today’s simple, wormy mollusks evolved from an ancestor that had a much more complex musculature (and probably overall internal anatomy) and was covered with protective shell plates.

Authors from left to right: Emanuel Redl, Tim Wollesen, Andreas Wanninger and Maik Scherholz (Department of Integrative Zoology, University of Vienna; not shown: Christiane Todt, University Museum Bergen, Norway). (Copyright: Andreas Wanninger)

Authors from left to right: Emanuel Redl, Tim Wollesen, Andreas Wanninger and Maik Scherholz (Department of Integrative Zoology, University of Vienna; not shown: Christiane Todt, University Museum Bergen, Norway). (Copyright: Andreas Wanninger)

Publication in “Current Biology”:
Aplacophoran Molluscs Evolved from Ancestors with Polyplacophoran-like Features. Maik Scherholz, Emanuel Redl, Tim Wollesen, Christiane Todt, Andreas Wanninger. Current Biology, October 2013.
DOI: http://dx.doi.org/10.1016/j.cub.2013.08.056

___Read straight from Universität Wien

( more informations, contacts, downloads, available )

Reference paper

Dcellpress

current biology

Aplacophoran Mollusks Evolved from Ancestors with Polyplacophoran-like Features

Original online publication Current Biology17 October 2013

DOI 10.1016/j.cub.2013.08.056

Maik ScherholzEmanuel RedlTim WollesenChristiane TodtAndreas Wanninger

Highlights

  • Myoanatomy of aplacophoran larvae is much more complex than that of adults
  • Homologous polyplacophoran and aplacophoran muscles support the Aculifera hypothesis
  • The last aculiferan ancestor had seven shell plates and associated dorsoventral muscles
  • The simple body plan of adult aplacophorans evolved by secondary simplification

Summary

Mollusca is an animal phylum with vast morphological diversity and includes worm-shaped aplacophorans, snails, bivalves, and the complex cephalopods.The interrelationships of these class-level taxa are still contentious, but recent phylogenomic analyses suggest a dichotomy at the base of Mollusca, resulting in a monophyletic Aculifera (comprising the shell-less, sclerite-bearing aplacophorans and the eight-shelled polyplacophorans) and Conchifera (all other, primarily univalved groups). The Aculifera concept has recently gained support via description of the fossil Kulindroplax, which shows both aplacophoran- and polyplacophoran-like features and suggests that the aplacophorans originated from a shelled ancestor, but the overall morphology of the last common aculiferan ancestor remains obscure. Here we show that larvae of the aplacophoran Wirenia argentea have several sets of muscles previously known only from polyplacophoran mollusks. Most of these are lost during metamorphosis, and we interpret them as ontogenetic remnants of an ancestor with a complex, polyplacophoran-like musculature. Moreover, we find that the first seven pairs of dorsoventral muscles develop synchronously in Wirenia, similar to juvenile polyplacophorans, which supports the conclusions based on the seven-shelled Kulindroplax. Accordingly, we argue that the simple body plan of recent aplacophorans is the result of simplification and does not represent a basal molluscan condition.

___Read straight from Current Biology

___PDF Aplacophoran Molluscs Evolved from Ancestors with Polyplacophoran-like Features

Further Reading and Links

a154

mollusks, snails, slugs and mussels
About.com | By Laura Klappenbach |PDF Molluscs – Mollusca – The Animal Encyclopedia

amlogo1

Snails, sea-slugs and squid, although very diverse in appearance and habit, all belong to the Phylum Mollusca. Molluscs also include chitons, clams, mussels, tusk shells and octopus. The study of molluscs is called malacology.

Molluscs

Although diverse, molluscs share several characteristics…

Bivalves

This group includes oysters, mussels, scallops and clams.

Sea Slug Forum

Ask questions, post information and find out more.

Gastropods

Snails and slugs comprise about 70% of all mollusc species.

Cephalopods

This group include octopuses, cuttlefishes, squids and nautiluses.

Chitons

Their most distinctive feature is their eight overlapping plates.

Molluscs in our Collections

The Malacology Collection holds a diverse array of mollusc specimens.

___Read straight from Australian Museum

frontiersinzoology

Of tests, trochs, shells, and spicules: Development of the basal mollusk Wirenia argentea (Solenogastres) and its bearing on the evolution of trochozoan larval key features

Christiane TodtAndreas Wanninger

Frontiers in Zoology 2010, 7:6 doi:10.1186/1742-9994-7-6

Abstract

Background

The phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed. Some authors propose the clade to represent the most basal branch within Mollusca, while others claim aplacophoran mollusks (Solenogastres and Caudofoveata) to be derived. Larval characters are central in these discussions, specifically the larval test (calymma, apical cap), the ontogeny of the epidermal scleritome, and the proposed absence of larval protonephridia. To date, developmental data are available for five solenogaster species, but most reports are incomplete and need confirmation.

Results

Wirenia argentea deposit small batches of uncleaved embryos that are tightly enclosed by a smooth and transparent egg hull. Cleavage is spiral and unequal. The ciliated larvae hatch about 45 hours after deposition and swim actively in the water column. Within 48-60 hours after hatching they become mushroom-shaped with a pronounced apical cap partly enclosing a posterior trunk. The cells covering the apical cap are large and cleavage arrested. On the apical cap there is a prominent prototrochal band of compound cilia and an apical ciliary tuft and the trunk bears a terminal ciliary band (telotroch). Obscured by the apical cap, a ciliary band originates in the stomodaeal pore and surrounds the trunk. As development is proceeding, the trunk elongates and becomes covered by cuticle with the exception of a ventral ciliary band, the future foot. The larvae have a pair of protonephridia. At 5 days after hatching they begin to settle and within the following 7-9 days the apical cap is gradually reduced. Scattered epidermal sclerites form under the cuticle.Wirenia argentea lack iterated groups of sclerites at any developmental stage. At 40 days after hatching, the postlarvae have a fully developed foregut, but the midgut and hindgut are not yet interconnected.

Conclusions

Solenogastres develop via a trochophore-like lecitotrophic larva with a preoral apical cap that at least partly represents an enlarged prototrochal area. Homology of this larval type (pericalymma larva) to test cell larvae of other spiralian clades is doubtful. The ontogeny of W. argentea does not provide any evidence for a derived status of Solenogastres within Mollusca.

___Read straight from Frontiers in Zoology 

___PDF Of tests, trochs, shells, and spicules (full research paper)

ucmp_shiny

The Aplacophora
The naked molluscs

Aplacophora is a relatively small group of molluscs that are characterized by their lack of shells and worm-like appearance. Instead of a shell, their mantle secretes tiny calcareous spicules which give them a beautiful shine. There are only about 320 species of these exclusively marine molluscs.

___Read straight from UC Museum of Paleontology

___PDF The Aplacophora

nature_logo

Figure 1: Oxford University Museum of Natural History (OUMNH) C.29641: holotype of Kulindroplax perissokomos.

From A Silurian armoured aplacophoran and implications for molluscan phylogeny

Mark D. Sutton, Derek E. G. Briggs, David J. Siveter, Derek J. Siveter, Julia D. Sigwart

 
a, b and e–m are ‘virtual’ reconstructions. a, Dorsal stereo pair. b, Ventral stereo pair (cuticle and spicules removed). c, Photograph of the specimen before serial grinding (that is, section along primary split). Dashed box indicates the position of d. d, Detail of c showing spicules. e, Lateral stereo pair. f, Dorsal stereo pair of gill array (cuticle, spicules, valves and body removed). g, Posterior view (cuticle and spicules removed). h, Ventral view. i, Detail of e (rotated) showing serrated margin of valve IV. j, Representative piece of spicule-bearing cuticle, from point labelled ‘ss’ in e. Posterolateral view (ventral up). k, Ventrolateral view of posterior of specimen (cuticle and spicules removed). l, Oblique (subventral) view (spicules removed). m, Sub-posterior view (ventral upright) of cuticle piece from j with all but three spicules removed. Scale bars, 2 mm (virtual reconstructions are perspective views and scale decreases away from viewer; where depth of object is substantial, the scale is calculated for the valve closest to the viewer). aa, apical area; bm, body mass; cu, cuticle; e1–e4, elements 1–4 of gill array; fs, fine-cut (~0.3 mm material removed); g, gut trace; pm, ‘pockmark’; ps, primary split (crack between part and counterpart); sc, saw-cut (~2 mm material removed); sp., spicules; ss, spicule sample; v1–v7, valves I (head)–VII (tail); vr, ventral ridge.

a, b and e–m are ‘virtual’ reconstructions. a, Dorsal stereo pair. b, Ventral stereo pair (cuticle and spicules removed). c, Photograph of the specimen before serial grinding (that is, section along primary split). Dashed box indicates the position of d. d, Detail of c showing spicules. e, Lateral stereo pair. f, Dorsal stereo pair of gill array (cuticle, spicules, valves and body removed). g, Posterior view (cuticle and spicules removed). h, Ventral view. i, Detail of e (rotated) showing serrated margin of valve IV. j, Representative piece of spicule-bearing cuticle, from point labelled ‘ss’ in e. Posterolateral view (ventral up). k, Ventrolateral view of posterior of specimen (cuticle and spicules removed). l, Oblique (subventral) view (spicules removed). m, Sub-posterior view (ventral upright) of cuticle piece from j with all but three spicules removed. Scale bars, 2 mm (virtual reconstructions are perspective views and scale decreases away from viewer; where depth of object is substantial, the scale is calculated for the valve closest to the viewer). aa, apical area; bm, body mass; cu, cuticle; e1–e4, elements 1–4 of gill array; fs, fine-cut (~0.3 mm material removed); g, gut trace; pm, ‘pockmark’; ps, primary split (crack between part and counterpart); sc, saw-cut (~2 mm material removed); sp., spicules; ss, spicule sample; v1–v7, valves I (head)–VII (tail); vr, ventral ridge.

___Read straight from Nature 490, 94–97 (04 October 2012) doi:10.1038/nature11328

Footnote to the book of Knowledge

 

_
Reference article

Evolution is not a one-way road towards complexity

18. Okt 13 | Mag. Veronika Schallhart | Original online publication Universität Wien

>

There are still a lot of unanswered questions about mollusks, e.g. snails, slugs and mussels. The research group of Andreas Wanninger, Head of the Department of Integrative Zoology of the University of Vienna, took a detailed look at the development of cryptic worms. The larvae of the “wirenia argentea” hold a much more complex muscular architecture than their adults – they remodel during their metamorphosis. That’s a clue that the ancestors had a highly complex muscular bodyplan.

Taking together the data currently available, a coherent scenario emerges that strongly suggests that today’s simple, wormy mollusks evolved from an ancestor that had a much more complex musculature (and probably overall internal anatomy) and was covered with protective shell plates.

_

Reference article

Aplacophoran Mollusks Evolved from Ancestors with Polyplacophoran-like Features

Original online publication Current Biology | 17 October 2013

DOI 10.1016/j.cub.2013.08.056

>

Highlights

  • Myoanatomy of aplacophoran larvae is much more complex than that of adults
  • Homologous polyplacophoran and aplacophoran muscles support the Aculifera hypothesis
  • The last aculiferan ancestor had seven shell plates and associated dorsoventral muscles
  • The simple body plan of adult aplacophorans evolved by secondary simplification

Summary

Mollusca is an animal phylum with vast morphological diversity and includes worm-shaped aplacophorans, snails, bivalves, and the complex cephalopods.The interrelationships of these class-level taxa are still contentious, but recent phylogenomic analyses suggest a dichotomy at the base of Mollusca, resulting in a monophyletic Aculifera (comprising the shell-less, sclerite-bearing aplacophorans and the eight-shelled polyplacophorans) and Conchifera (all other, primarily univalved groups). The Aculifera concept has recently gained support via description of the fossil Kulindroplax, which shows both aplacophoran- and polyplacophoran-like features and suggests that the aplacophorans originated from a shelled ancestor, but the overall morphology of the last common aculiferan ancestor remains obscure. Here we show that larvae of the aplacophoran Wirenia argentea have several sets of muscles previously known only from polyplacophoran mollusks. Most of these are lost during metamorphosis, and we interpret them as ontogenetic remnants of an ancestor with a complex, polyplacophoran-like musculature. Moreover, we find that the first seven pairs of dorsoventral muscles develop synchronously in Wirenia, similar to juvenile polyplacophorans, which supports the conclusions based on the seven-shelled Kulindroplax. Accordingly, we argue that the simple body plan of recent aplacophorans is the result of simplification and does not represent a basal molluscan condition.

 

Tracing Knowledge

page : Footnotes to the book of Knowledge

This footnote in pdf file : Evolution is not a one-way road towards complexity_TK_BK

_
Maik Scherholz, Emanuel Redl, Tim Wollesen, Christiane Todt, & Andreas Wanninger (2013). Aplacophoran Mollusks Evolved from Ancestors with Polyplacophoran-like Features Cell Press Current Biology DOI: 10.1016/j.cub.2013.08.056



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einsteinquotesimple

Avoid conclusions based on oversimplified  scientific research results

Reference article:  nature Shades of grey

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