Research Study 1: Larval defenses

All marine gastropods undergo
Page 1997 Acta Zoologica 78: 227
Research Study 2: Larval defenses

There has been much conjecture about the function of torsion, and opinion is divided as to whether it benefits primarily the larva or the adult. The major consequence to the veliger larva is that it can now retract its head and foot into the shell and seal it with the operculum (the operculum is produced at the same time as torsion occurs). Since this ability (see Right-hand drawing) carries over into the adult and provides protection for the adult against predators, drying, UV irradiation, and so on, there is no quarrel as to its post-metamorphic importance. The intitial hypothesis for its function, proposed in the 1920s, is that it primarily benefits the larva (rather than the adult) by enabling it to protect its sensitive soft parts (head and velum) from being nipped off by planktonic predators. But is this realistic? Wouldn’t most predators either swallow veligers whole or crush their weak shells and then swallow the parts? The idea for a larval function for torsion has lasted many decades and has many contemporary adherents.
Research Study 3: Larval defenses


Shown on upper Left is normal torsion in a veliger larva of the trochid snail Calliostoma ligatum (the larva is about 300µm in shell diameter). The retractor muscles attach the larval body to the shell and allow the body to be pulled in (usually only partly, before torsion). In the Pre-Torsion drawing, the foot is facing to the Right. At Mid-Torsion the foot has rotated 90o, while at Final Torsion the foot has rotated almost 180o and an operculum has formed. The larva is now able to pull into its shell and close off the opening. The scientist who originated the larval-defense theory, Walter Garstang, thought that a sudden mutation imposed an asymmetry on these muscles in an otherwise symmetrical ancestral larva, thus torquing the larval body and producing torsion. In this theory, the condition, with its presumed benefits, produced a new evolutionary line leading to present-day gastropods. However, several scientists have questioned the role of the retractor muscles in torsion.
In the study considered here, antibiotics employed to control bacterial infection during embryonic culture of keyhole limpets Diodora aspera and trochid snails Calliostoma ligatum are found to interfere with the attachment of the retractor muscles to the inner wall of the larval shell, yet torsion still occurs. Such observations, and others, place into question the fundamental tenents of Garstang’s theory regarding the larval function of torsion. For information on torsion in other gastropods, including the caenogastropods Amphissa columbiana and Euspira lewisii see Bondar & Page 2003 Invert Biol 122: 28.
Bondar & Page 2003 Invert Biol 122: 28
Research Study 4: Larval defenses

In adddition to the retractor-muscle issue, other studies cast doubt on Garstang’s hypothesis. For example, torsion in the west-coast species Margarites pupillus could not be caused by contraction of the retractor muscles because the larval shell is not fully calcified at this time and could not hold up to the stress.
Research Study 5: Larval defenses

Note in the drawings that on withdrawal into the shell, the mantle fold "slips" down, making room for the velum and foot. The implication of this is that this space would be available for the larva to withdraw its “fragile” head/velum into regardless of whether torsion happened or not; thus, according to this author, the position and volume of the mantle cavity is irrelevant. Further, on microscopic examination the head and velum appear no more “fragile” than the foot; thus, there is no need to invoke the “special” protective strategy conferred by torsion. The observations in these and other Research studies question several basic tenents of Garstang’s theory. None of these
Goodhart 1987 J Moll Stud 53: 33
Research Study 6: Larval defenses

What about a test of the hypothesis? The first and possibly only
Research Study 7: Larval defenses
As noted above, there are other objections to the larval-benefit theory and other ideas regarding torsion, but only the single experimental test. The authors of the Haliotis study acknowledge the possibility that torsion may protect veligers from adverse physical conditions, but do not explain. One such example may relate to the behaviour of swimming veligers that have undergone torsion. When the larvae bump into something, they immediately withdraw into their shells and
Veliger’s a lively tar, the liveliest afloat,
A whirling wheel on either side propels his little boat;
But when the danger signal warns his bustling submarine,
He stops the engine, shuts the port, and drops below unseen.
Research Study 8: Larval defenses

A feature that seems to be ignored in this veliger/torsion research, or at least not given the attention that it deserves, is the operculum. It appears in post-torsional larvae and, while perhaps not so important (it seems) for survival of the veliger stage, is of critical importance for survival of juvenile and adult stages. The shell and operculum are arguably the two most important morphological features enabling colonisation of intertidal and land habitats by gastropod molluscs.
Research Study 9: Larval defenses


In addition to being knowledgeable about marine-invertebrate larvae and fervently interested in the role of larvae in evolution of invertebrates, Walter Garstang had a delightful knack of presenting his ideas, and some quite serious ones at that, in the form of light-hearted verse. These are collected in a book of poetry, published in 1962. In addition to the perennial favourite “The ballad of Veliger...”, are the equally enjoyable and evolutionarily provocative poems "The amphiblastula and the origin of sponges", "The invaginate gastrula and the planula","Mülleria and the ctenophore", "Tornaria’s water-works",and many others. Because the book is several decades out of print and may not be readily available, the poem most germane to this section of the ODYSSEY is presented in full, with identical spelling to the original:
The ballad of Veliger, or how the gastropod got its twist:
Veliger's a lively tar, the liveliest afloat,
A whirling wheel on either side propels his little boat;
But when the danger signal warns his bustling submarine,
He stops the engine, shuts the port, and drops below unseen.
He's witnessed several changes in pelagic motor-craft;
The first he sailed was just a tub, with a tiny cabin aft.
An Archi-mollusk fashioned it, according to his kind,
He'd always stowed his gills and things in a mantle-sac behind.
Young Archi-mollusks went to sea with nothing but a velum -
A sort of autocycling hoop, instead of pram - to wheel 'em;
And, spinning round, they one by one acquired parental features,
A shell above, a foot below - the queerest little creatures.
But when by chance they brushed against their neighbours in the briny,
Coelenterates with stinging threads and Arthropods so spiny,
By one weak spot betrayed, alas, they fell an easy prey -
Their soft preoral lobes in front could not be tucked away!
Their feet, you see, amidships, next the cuddy-hole abaft,
Drew in at once, and left their heads exposed to every shaft.
So Archi-mollusks dwindled, and the race was sinking fast,
When by the merest accident salvation came at last.
A fleet of fry turned out one day, eventful in the sequel,
Whose left and right retractors on the two sides were unequal:
Their starboard halliards fixed astern alone supplied the head,
While those set aport were spreadabeam and served the back instead.
Predaceous foes, still drifting by in numbers unabated,
Were baffled now by tactics which their dining plans frustrated.
Their prey upon alarm collapsed, but promptly turned about,
With the tender morsal safe within and the horny foot without!
This manoeuvre (vide Lamarck) speeded up with repetition,
Until the parts affected gained a rhythmical condition,
And torsion, needing now no more a stimulating stab,
Will take its predetermined course in a watchglass in the lab.
In this way, then, Veliger, triumphantly askew,
Acquired his cabin for'ard, holding all his sailing crew -
A Trochosphere in armour cased, with a foot to work the hatch,
And double screws to drive ahead with smartness and despatch.
But when the first new Veligers came home again to shore,
And settled down as Gastropods with mantle-sac afore,
The Archi-mollusk sought a cleft his shame and grief to hide,
Crunched horribly his horny teeth, gave up the ghost, and died.
- Walter Garstang 1928
Summary comment:
Although torsion may seem more suited as a topic for philosophical discussions in turn-of-the-century drawing rooms, it is actually one of the more dynamic and interesting controversies in marine-invertebrate study. A dialogue of opinion and argument has been on-going for 80 years! Students interested in such things (and in torsion) should additionally read the references cited in Pennington & Chia (1985) and Page (2002)
Pennington & Chia 1985 Biol Bull 169: 391
Page 2002 Evol & Develop 4: 212