Why do gastropods torsion

In , Walter Garstang, proposed a hypothesis both for how and why torsion occurs. Garstang was born in in England and ascended the ranks of academia to work on everything from slugs to sand crabs to sea gulls.

As noted in his obituary, in , he was a poet and lover of nature. And although noted for many things, Walter Garstang, is probably best remembered for his poems about form, function, and development in invertebrates. The veliger possesses two retractor muscles. One of these extends from the shell on right, over the gut, and attaches to the left side of head and foot. The other starts on the left and attaches to the right.

These are riverine species. These are marsh species. These are mud flat species. These are species that burrow in sand or mud in rivers or lakes.

These are deep water lake species. These are non-native species, introduced from other regions. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends.

Synapomorphy of the Bilateria. To cite this page: Myers, P. Burch Disclaimer: The Animal Diversity Web is an educational resource written largely by and for college students.

ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts.

While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control. Gastropoda Facebook. Sources : Hickman, C. Animal Diversity. Brown, Dubuque, IA. Great Lakes area freshwater snails These are species with general aquatic distribution in perennial waters These are species that inhabit quiet bays or ponds These are intermittant pool or intermittant stream species These are riverine species These are marsh species These are mud flat species These are species that burrow in sand or mud in rivers or lakes These are deep water lake species These are non-native species, introduced from other regions.

Glossary bilateral symmetry having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Read more A—D Scanning electron micrographs of 4 successive developmental stages viewed from the apical pole.

A Prior to formation of mantle cavity. B Mantle cavity arrowhead forms as a shallow cleft on the right side. C Mantle cavity arrowhead has enlarged and spread dorsally. D Mantle cavity arrowhead has expanded over the entire dorsal surface of the head. E Arrowhead indicates a fan of mantle muscles underlying the bottom of the shell; also note the larval retractor muscle lrm. H Mantle muscles arrowhead show no additional rotation. Phalloidin-labeled embryos were imaged from the posterior end but the images were flipped so that the position and orientation of muscles corresponds with the apical views shown in the scanning electron micrographs.

Adapted from Page In order to identify whether T. Scanning electron microscopy is inadequate for this purpose because the axial coordinates of the symmetrical, bowl-shaped shell at the outset of the process cannot be identified. Furthermore, damage to the fragile protoconch during preparation for scanning electron microscopy confounds efforts to ascertain its precise shape and orientation. However, it was possible to monitor change in shell orientation by tracking changes in orientation of shell-anchored muscles using fluorophore-tagged phalloidin Page Two sets of muscles are attached to the inner wall of the protoconch via specialized mantle epithelial cells: the larval retractor muscle with distal fibers extending into the head and base of the foot and a spray of mantle muscles that underlie the protoconch.

Fluorescence micrographs in Figure 4E—H show orientation of the mantle muscles in stages corresponding to those shown in the scanning electron micrographs. They suggest that shell rotation stops at the last stage when the mantle cavity is still confined to the right side of the embryo; at this stage the protoconch must be fully endogastric Fig.

Thus, similar to H. The Heterobranchia is a gastropod clade that includes opisthobranchs, pulmonates, and several additional groups Haszprunar ; Ponder and Lindberg Figure 5 shows an apical view of the planktotrophic larva of the notaspidean Pleurobranchaea californica Macfarland, The mantle cavity is located to the right of a mid-dorsal ridge of ciliated mantle fold epithelium.

In this species and in the pulmonate Amphibola crenata Little and others , the posterior part of the mantle cavity expands over to the left side during later larval development. According to Thompson organs and tissues of the opisthobranch visceropallium either differentiate in positions of partial rotation or undergo only modest displacements during embryogenesis.

Light micrograph of a larva of the notaspidean opisthobranch Pleurobranchaea californica , observed from the apical pole. The arrow indicates a ciliated ridge that borders the entrance to the mantle cavity on the right side.

Ruthensteiner described observations on developing larvae of a patellogastropod, a vetigastropod, and a cerithioid caenogastropod that are consistent with data reviewed here, because a right-sided mantle cavity is a common denominator during the development of all species examined.

Ruthensteiner argued that the lung of the pulmonates Ovatella myosotis Ellobiidae and Onchidium c. The foregoing survey of mantle cavity development among gastropods does not include developmental observations on the Patellogastropoda, despite the fact that patellogastropods have been placed as the most basal group of living gastropods Haszprunar ; Ponder and Lindberg Wanninger and colleagues have meticulously documented the fact that rotation between the cephalopodium and protoconch of Patella caerulea is a rapid, monophasic morphogenetic movement that is not subdivided into slow and rapid phases.

However, existing histological data on the morphogenesis of the mantle cavity in patellogastropods are currently inconsistent, possibly reflecting real differences among species. Smith and Lespinet and colleagues interpreted deep invaginations of epithelium on either side of the stomodeum, anterior to the foot rudiment, in pretorsional larvae of Patella vulgata Patellidae as rudiments of the mantle cavity. Alternatively, Wanninger and colleagues found that the pretorsional mantle cavity in P.

Ruthensteiner found that the initial mantle cavity of the patellogastropod Cellana sandwicensis Patellidae was an in-pocketing of mantle epithelium on the right side after ontogenetic torsion. These apparent discrepancies, together with the basal position of the Patellogastropoda, underline the need for a detailed study on mantle cavity morphogenesis within the Patellogastropoda.

Gastropod torsion, if defined as a suite of anatomical conditions in adult gastropods that includes an anterior position for the mantle cavity and anus, is unique to this clade of mollusks.

Therefore, outgoups are silent about the earliest stages in this body plan transformation and fossils of gastropod-like shells preserve only half of the torsion equation if torsion was indeed a rotation between 2 body regions. Lindberg and Ponder used outgroup comparison to conclude that a pair of pallial organs flanking a medial anus, as present in diotocardian mollusks, is ancestral for gastropods because other mollusks have pallial organs on either side of the anus.

However, the bilateral pairs of pallial organs in other mollusks develop synchronously on the left and right sides, whereas the 2 gills of haliotids develop asynchronously Crofts ; gill development in other diotocardians has not been described. For some authors, torsion as a synapomorphy of gastropods is best characterized as a developmental phenomenon Runnegar ; Signor However, even the few species surveyed here show that developmental events associated with ontogenetic torsion can be highly variable.

A conspicuous absence of detailed, comparative studies on ontogenetic torsion has perhaps led to a perception of greater uniformity in this process than actually exists. Despite the diversity of developmental processes involved in ontogenetic torsion and despite the ongoing difficulties in resolving the branching sequence along the trunk of the gastropod phylogenetic tree, comparative developmental data may nevertheless hold clues to the early evolution of the gastropod body plan.

If a specific organization of developmental morphology is seen to arise in multiple clades of distant relationship, then this organization may have occurred also in the first gastropods and may be highly informative about the nature of the body plan that pioneered the gastropod lineage.

In the absence of alternatives, a high degree of conservation can substitute for mapping diverse characteristics onto a highly resolved phylogeny to infer ancestral characteristic state. The conserved developmental stage in which the protoconch is endogastric and the mantle cavity lies to the right of the cephalopodium inspires an alternative proposal for the early evolution of the gastropod body plan.

With this proposal, I have abandoned the premise that pregastropods had a mantle cavity restricted to the posterior end and have instead assumed that the ancient conchiferans that eventually gave rise to gastropods had lateral mantle cavities that merged posteriorly, as in extant tryblidiid monoplacophorans.

Although fossil evidence for Yonge's hypothetical gastropod ancestor with a posterior mantle cavity is ambivalent at best, the fossil record strongly suggests that the gastropod shell evolved from the dorsal shield of monoplacophorans by elongating along its dorsoventral axis, which was accompanied by shell coiling and narrowing of the apertural gape of the shell. How did ancient gastropod predecessors reconcile progressive narrowing of the shell aperture with the need to efficiently aerate gas exchange epithelia within lateral mantle cavities, particularly when viscous forces must have significantly influenced water flow through the increasingly constricted mantle cavities of these small mollusks?

The conserved developmental state described here may be reminiscent of the response to this dilemma by the first gastropods: abandon the condition of increasingly constricted lateral mantle cavities on both the left and right sides for a single, unilateral mantle cavity of increased size. Under this hypothesis, an asymmetric mantle cavity 1 side only is the essential derived condition from which all descendant gastropod clades are derived, as illustrated in Figure 6A.

As increasing body size demanded ever more gill surface area, the response may have been to expand the unilateral mantle cavity the right cavity in dextral snails over the back of the head Fig. Illustration of the "asymmetry hypothesis"; dextral snails shown in this scenario. Ancient conchiferan mollusks that gave rise to gastropods had bilaterally symmetric, lateral mantle cavities, possibly with more than 1 gill in each lateral cavity not shown.

Garstang advocated that torsion is an adaptive feature and useful to the larvae veliger larva for protection of soft parts against enemies but of little direct use to the adult. But after torsion the mantle cavity is brought around the anterior end of the larva which provides the space for head and velum and the larva gives the greater protection of the head and associated structures.

Then the beating of cilia stops and the larva falls to the sea bottom. In this way they avoid the predators. This view is widely supported by Yonge , Barnes , Ruppert and Barnes and Anderson There are many pelagic larvae of lamellidens which are not twisted but still survive in pelagic larval life.

Primitive Gastropods were not twisted and the gills were attached posteriorly inside the mantle cavity. The twist brings the anus anterior, so there is some chance of interaction between the discharged faecal matter and respiratory current.

The respiratory surface in the mantle cavity which in some cases develops pallial gills, e. Morton emphasises the importance of anterior location of mantle cavity both in larval and adult molluscs. The anteriorly placed mantle cavity housing the head with sense organs, respiratory structures, etc. To maintain the balance of body the shell of the gastropods prolonged anteriorly. Stasek and Purchon have also supported that torsion is advantageous during adult stage. The head-foot complex retains its bilateral symmetry.

The visceral hump together with the protecting shell becomes coiled to economies the volume. The distortion means the reversion to the changes that have occurred during torsion. As a result of detorsion the pallial complex travels towards the posterior end along the right side. The ctenidia are pointed backwards and the auricles come behind the ventricle. The gills become exposed and subjected to external current.