Protoconch

An SEM image of the trochophore of Haliotis asinina 11 hours post fertilization, with calcified protoconch (pc) prior to torsion.

Protoconch of Haliotis asinina when the torsion is complete.

A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older text it is also called "nucleus". The protoconch may sometimes consist of several whorls, but when this is the case, the whorls show no growth lines.

The whorls of the adult shell, which are formed after the protoconch, are known as the teleoconch. The teleoconch starts forming when the larval gastropod settles and becomes a juvenile. Quite often there is a visible line of demarcation where the protoconch ends and the teleoconch begins, and there may be a noticeable change in sculpture, or a sudden appearance of sculpture at that point.

In some gastropod groups (such as the Architectonicidae), the teleoconch whorls actually spiral in the opposite direction to the protoconch. In those cases the shell is called heterostrophic.

In species which have a veliger or swimming larval stage which hatches out of egg capsules, there are two parts to the protoconch. The first part of the protoconch (which is formed within the embryonic egg capsule) is called protoconch 1, while the part that is formed after the larva has hatched is called protoconch 2. There is often different sculpture or ornamentation on protoconch 1 compared with protoconch 2, and this can be distinguished under the microscope.

The structure of the protoconch has been widely used as a discriminating feature in gastropod systematics.

The homologous structure in bivalves (clams) is called the prodissoconch.

Examples

Comparison of the whole shell and the protoconch of Atlanta lesueurii:

Apical view of the whole shell.

Detail of apical view of the same shell showing clearly visible protoconch, that has 2¼ whorls

References

Cephalopod anatomy
Shell	Shell types External shells: Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal shells: Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet External features Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal features Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls
Mantle	External anatomy Funnel Dermal structures Fins (wings) Mantle locking-apparatus Internal anatomy Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands
Head and limbs	Brachial crown Arms Hectocotylus Tentacles Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Other parts Eyes Statocysts
General	Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)