Development in Berthella californica (Gastropoda: Opisthobranchia) with comparative observations on phylogenetically relevant larval characters among nudipleuran opisthobranchs

Abstract. Morphological criteria defining the Nudipleura and sister group relationships among the three nudipleuran subgroups (pleurobranchoideans, anthobranch nudibranchs, and cladobranch nudibranchs) have been controversial. Analysis of larval stages may help resolve these uncertainties by identifying additional phylogenetically informative characters, but existing information on pleurobranchoidean larvae is meager. We studied larval development and metamorphosis of the pleurobranchoidean Berthella californica using histological sections, scanning and transmission electron microscopy, and immunolabeling of neurons within the larval apical ganglion. We also provide comparative data on other nudipleuran larvae that may be useful for phylogenetic reconstruction. Berthella californica fills a previously unoccupied place within an evolutionary scenario that derives nudibranchs from pleurobranchoideans, two groups in which the larval mantle fold forms the post‐metamorphic notum (dorsal epidermis). In B. californica, reflection of the mantle fold epithelium to form the notum begins at metamorphosis, as also occurs in nudibranchs, whereas mantle reflection in other pleurobranchoideans begins well before metamorphosis. Dissolution of overgrown shell walls inside the protoconch and formation of the post‐metamorphic notum from the inner epithelium of the larval mantle fold may be synapomorphies of the Nudipleura. The larval shell in B. californica is additionally noteworthy because it acquires bilateral symmetry later in development, which is very unusual among larval opisthobranchs. We demonstrate an osphradium in the larvae of two pleurobranchoideans and one anthobranch nudibranch, although adults lack this trait. We also identified an autapomorphy of cladobranch nudibranchs in the form of five ampullary neurons within the larval apical ganglion, whereas other planktotrophic opisthobranch larvae have only four of these neurons. Although our data provide morphological criteria defining both the Nudipleura and the cladobranch nudibranchs, they are insufficient to resolve sister group relationships within the Nudipleura.     

Molluscan Metamorphosis: A Study in Tissue Transformation

Metamorphosis of the pelagic larvae of benthic marine invertebratesis often a cataclysmic event in which a rapid loss of organsspecialized for larval life occurs simultaneously with the renewalor increased rate of development of potential adult organs.In the nudibranch gastropod Phestilla sibogae this change involvesloss of the velum, shell, operculum, larval kidney, some retractormuscles, and some of the pedal mucous glands. Exit from thelarval shell at metamorphosis is rapid and is correlated withthe spread of epidermis from the larval foot over the visceralmass as the visceral mass emerges from the shell aperature.This spreading of epipodial epidermis to cover the entire bodyhas not been previously reported for other nudibranchs. Neithercell proliferation nor active cell motility are responsiblefor this epidermal migration. Rather it appears that the actionof larval muscles pulls the visceral organs out of the shelland simultaneously causes the epipodial epidermis to cover thevisceral mass. This epidermis becomes the definitive adult epidermis.     

Similarities in Form and Developmental Sequence for Three Larval Shell Muscles in Nudibranch Gastropods

Shell-anchored muscles that extend into the cephalopodium of five species of planktotrophic nudibranch larvae were studied by ultrastructural examination of sequential larval developmental stages. All species, regardless of larval shell type (inflated or non-inflated), showed a similar basic pattern of shell muscles. The larval retractor muscle (LRM) differentiates prior to hatching and its fibres insert on epithelia of the velum, apical plate, stomodeal region, or mantle fold. Many fibres also connect with subepithelial intrinsic muscles of the cephalopodium. Most but not all LRM fibres Project to left-sided targets and are innervated from the left cerebral ganglion. Two pedal muscles, which are innervated from the pedal ganglia, differentiate during the post-hatching larval stage and both insert primarily on pedal epithelium attached to the operculum. The left pedal muscle is anchored to the shell immediately adjacent to the attachment plaque of the LRM and consists of basal and distal tiers of muscle cells. The right pedal muscle arises on the ventral rim of the shell aperture and consists of a single tier of muscle cells. Ontogenic changes in larval retraction behaviour correlate with developmental change in the muscle effectors. Although some interspecific differences were noted, the presence of a common ground plan for larval shell muscles in these five species contrasts with previous indications of marked variability for nudibranch larval shell muscles.     

Véligère planctotrophe du doridien Aegires punctilucens (D'Orbigny) (Mollusca: Nudibranchia: Notodorididae): Description et métamorphose

Different larval planktonic stages of the nudibranch Aegires punctilucens (D'Orbigny) are described. The youngest has a shell of the protoconch type 1 (Thompson) and a bilobed velum. After loss of the shell, the next stage is characterized by a large velum with broad and thick lobes. The mantel covers the body and has tubercles which grow progressively. Spicules appear in the mantle and in the foot; they are simple, triradiate or cross-shaped. A zone of hyaline denticles are present in the stomach lumen.Metamorphosis has been obtained under laboratory rearing. After the gradual resorption of the velum, the animal looks like a small dorid and is grey with white spots. The foot is slender and there are 14 tubercles always arranged in the same way and bristled with spicules.After discussing the species identification, the veliger is compared with other nudibranch larvae. The development of Aegires is very unusual with a two-stage metamorphosis, the first at the time of loss of the shell, and the second at loss of the velum cast. The intermediate stage between those two stages is planktonic.     

Development,metamorphosis, and natural history of the nudibranch Doridella obscura Verrill (Corambidae: Opisthobranchia)

The doridacean nudibranch Doridella obscura Verrill was raised through one complete generation in laboratory culture, and spawning behavior monitored for a year at monthly intervals in Barnegat Bay, New Jersey.The nudibranch deposited egg masses throughout the year in Barnegat Bay, and the larvae remained viable at temperatures ranging from 1.5 to 28 °C. At 25 °C the eggs hatch 4 days after oviposition, and the planktotrophic veliger larvae swim and feed for 9 days before they metamorphose. Settlement occurs specifically on the bryozoan Electro crustulenta (Pallas). The spirally coiled larval shell grows rapidly until the dorsal mantle fold is retracted from the aperture 5–6 days after hatching. Although starved larvae grow only slightly and do not metamorphose, they resume normal development on introduction of suitable food. Newly metamorphosed juveniles consume algae and debris on the surface of the bryozoan until they grow large enough to attack the living zooids of E. crustulenta.The life cycle of Doridella obscura is short (26 days at 25 °C), allowing the nudibranchs to take advantage of short-lived Electra crustulenta colonies in unstable habitats in bays and estuaries.     

Complete development of the northeast Pacific arminacean nudibranch Janolus fuscus

Early life-history stages of the nudibranch Janolus fuscus (superfamily Arminoidea) were reared in the laboratory and collected from the field to document embryology, larval morphology, and pre-and post-metamorphic growth. This species produces Type B egg masses containing an average of 23,063 viable embryos. The spiral holoblastic cleavage pattern resembled that of other nudibranchs, with early cell divisions occurring roughly at 4-h intervals at 11-13 °C. Embryos progress through a gastrula stage with cellular extensions covering the blastopore and a trochophore-like stage before hatching as veliger larvae after 10-18 days. Veligers ranged in size from 125-153.8 μm at hatching, with size being positively correlated with the duration of encapsulated development. After a 36-41-day larval period, some veligers ceased growing at 266 μm and showed signs of approaching competence. Four larvae settled and two metamorphosed into 280-μm juveniles on the bryozoan prey of the adults, Bugula pacifica, 46 and 54 days post-hatching. Average growth of pre-ovipositional (<19 mm) J. fuscus was more rapid (8.79% per day) than growth of ovipositional individuals (3.52% per day). Growth continued to a maximum of 57 mm in the laboratory, with an estimated total lifespan of about 5 months. These data, which agree with concurrent field observations, suggest a subannual life cycle.     

Decline in diversity of early Palaeozoic loosely coiled gastropod protoconchs

Quantitative data on molluscan larval conch fossil assemblages of ages ranging from the Ordovician (Argentina and the Baltic region), through Silurian (Austria), Devonian (Poland) to Carboniferous (Texas) supplement knowledge of early planktonic gastropods communities transformations. They show that larval shells of the bilaterally symmetrical bellerophontids and dextrally coiled gastropods with a hook-like straight apical portion of the first whorl initially dominated. Their relative frequency, as well as that of the sinistrally coiled ‘paragastropods’, diminished during the Ordovician and Silurian to virtually disappear in the Late Devonian and Early Carboniferous. Already during the Ordovician, diversity of larvae with gently loosely coiled first whorl increased, to be replaced then with more and more tightly coiled forms. Both the aperture constrictions and mortality peaks, probably connected with hatching and metamorphosis, indicate that the Ordovician protoconchs with hook-like first coil represent both the stage of an embryo developing within the egg envelope and a planktonic larva. The similarity of the straight apex to larval conchs of hyoliths and advanced thecosome pteropods is superficial, as these were not homologous stages in early development.     

Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods

The early ontogeny of gastropods (i.e., planktotrophic vs. nonplanktotrophic) may be inferable from the morphology of the protoconch in adult shells. The protoconch consists of both embryonic and larval shells in species with planktotrophic development; the embryonic shell forms in the intracapsular period and the succeeding larval shell gradually develops during the larval period. In nonplanktotrophic species, on the other hand, there is no additional growth of the larval shell and the protoconch consists exclusively of a relatively large embryonic shell formed prior to hatching. This "shell apex theory" has been applied to many species of shell-bearing gastropods, but biotic and abiotic erosion of the apex often prevents detailed examination of the protoconch and subsequent inferences about ontogeny. I examined the gastropod operculum to test its utility for predicting developmental mode, drawing on the Neritimorpha as model taxa. Most aquatic members of Neritimorpha were found to bear an operculum with a clearly demarcated nucleus; SEM observations reveal four types of nuclei, which correspond to different types of protoconch morphologies and observed ontogenies for the study species. The nucleus is secreted before metamorphosis, fits into the shell aperture of the larva, and reflects early ontogeny as morphology, as does the protoconch. Moreover, the apparently organic (rather than calcareous) composition of the nucleus makes it nearly invulnerable to erosion and very advantageous, compared to the protoconch, in this ecologically diverse group, whose habitats range from freshwater streams and mangrove swamps to rocky shores and deep-sea hydrothermal vents. The measurements of the nucleus are also valuable for taxonomic purposes, especially in the species identification of veliger larvae and juvenile snails. On the other hand, the opercular nuclei of the Caenogastropoda and Heterobranchia are often eroded away in adult individuals; even if present, the morphology of the nuclei does not seem to clearly reflect early ontogeny in those groups.     

Veligers from the nudibranch <Emphasis Type="Italic">Dendronotus</Emphasis><Emphasis Type="Italic">frondosus</Emphasis> show shell growth and extended planktonic period in laboratory culture

The planktonic period of planktotrophic veliger larvae from the nudibranch Dendronotus frondosus was characterized by laboratory culture methods. Larvae in culture successfully metamorphosed at 73–86 days after hatching. These veligers have Type 2 (Thompson) larval shells that significantly increased in length over the first 7–14 days after hatching. Direct observations of the development of nudibranch larvae with Type 2 protoconchs are limited, and these data help clarify previous attempts to correlate shell type and growth with minimum planktonic periods. Although these are not absolute values for the planktonic period of D. frondosus larvae, these data show the potential for extended larval dispersal and may help explain reports of an extensive geographic range in north-temperate waters for this species.     

Ontogenetic Torsion and Protoconch Form in the Archaeogastropod Haliotis kamtschatkana: Evolutionary Implications

Ontogenetic torsion in archaeogastropods has strongly influenced theories about early gastropod evolution, but the seminal studies by Crofts (1937, 1955) remain the major source of information about tissue movements during this developmental process. Computer-generated reconstructions of histological sections indicate that the cephalopodium of Haliotis kamtschatkana Jonas, 1845 rotates by a full 180° relative to the shell and visceral lobe during the first quarter of pre-metamorphic development. However, a portion of pallial epithelium, including some of the shell field, accompanies the rotating cephalopodium; a process facilitated by detachment of the pallium from the apertural rim of the protoconch. Transmission electron microscopy indicates that a tract of the larval retractor muscle, which Crofts (1955) implicated in the mechanism of torsion, inserts on both pedal and pallial cells. A deep invagination of shell field epithelium is a major focus of rotational torque. As a result of pallial deformation during cephalopodial rotation, the anus and gill rudiment are restricted to the right half of the larval body for 2 days after cephalopodial rotation by 180° has been completed. Scanning and transmission electron microscopy indicate that grooves in the lateral flanks of the protoconch correspond to the deep invagination of shell field epithelium. The grooves are not created by a coiling type of accrelionary shell growth or by flexion of the protoconch. A calcareous shelf is secondarily added to the periostracal template of the protoconch along its visceral apertural rim. Morphogenetic movements during ontogenetic torsion in this species are more complex than a simple rotation between cephalopodium and visceropallium and the protoconch shows no evidence of exogastric coiling. © 1997 Published by Elsevier Science Ltd on behalf of The Royal Swedish Academy of Sciences.     

Induction of Settlement and Metamorphosis of the Veliger Larvae of the Nudibranch,Onchidoris bilamellata

Summary

Onchidoris bilamellata veligers were reared in the laboratory on a combination of phytoñagellates and diatoms. They attained metamorphic competence after a period of 28 to 32 days at 11°C, or 60 to 80 days at a temperature averaging 7.5° C.

Experimental evidence suggests that settlement is stimulated by a diffusible chemical emanating from living barnacles, whereas metamorphosis is induced by a chemical or mechanochemical cue, which is also associated with barnacles. Settlement and metamorphosis are considered to be separable events in O. bilamellata. The settlement response is reversible and can be repeated; it involves a characteristic behavioral repertoire including descent to the bottom, foot contortions and crawling on the pedal sole. Settlement occurs only in seawater that contains, or had previously contained, living barnacles. Metamorphosis is irreversible and involves the resorption of the velum, loss of the larval shell, and incorporation of the visceral mass into the cephalopedal mass. Metamorphosis is triggered only when physical contact with living or dead barnacles is made (dead barnacles refers to shell and tissue fragments which are only effective in inducing metamorphosis when they are used in combination with seawater that had previously contained living barnacles).

Settlement and metamorphosis in O. bilamellata is compared with that of other nudibranch species, and its unique settlement response is discussed.     

The expression of an engrailed protein during embryonic shell formation of the tusk-shell, Antalis entalis (Mollusca, Scaphopoda)

SUMMARY This study presents the first detailed account of the larval and early post-metamorphic development of a scaphopod species, Antalis entalis , since 1883. Special reference is given to the expression pattern of an engrailed protein during the formation of the embryonic (protoconch) and adult shell (teleoconch). We found that in the trochophore-like larva the engrailed protein is expressed in shell-secreting cells at the margin of the protoconch close to the mantle edge. During metamorphosis the growth of the protoconch and expression of the engrailed protein along its margin stop and the teleoconch starts to form. These data suggest a different genetic background regarding protoconch and teleoconch formation in the Scaphopoda and possibly all Conchifera, thus inferring a different evolutionary origin of both organs. The single anlage of the scaphopod protoconch contradicts earlier hypotheses of a monophyletic taxon Diasoma (Scaphopoda + Bivalvia), which has been mainly based on the assumption of a primarily bilobed shell in both taxa. Comparative data on engrailed expression patterns suggest nervous system patterning as the basic function of engrailed in the Bilateria. However, there are several independent gain-of-function events, namely segment compartmentation in the Annelida and Arthropoda, protoconch formation in the Mollusca, skeletogenesis in the Echinodermata, and limb formation in vertebrates. These findings provide further evidence that homologous genes may act in very different pathways of bilaterian body plan formation in various animal phyla.     

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.     

Larval and juvenile gastropods from a Carboniferous black shale: palaeoecology and implications for the evolution of the Gastropoda

A horizon in the late Visean Ruddle Shale from Arkansas contains the oldest well-preserved gastropod protoconchs known from the Americas. The gastropod fauna consists of a diverse larval shell assemblage and a low diversity assemblage of juvenile gastropods that probably had a benthic life habit. Gastropod larval shells are always isolated, i.e. the gastropods did not complete their life cycle (no metamorphosis) and were unable to become benthic. This was caused by unfavorable environmental conditions on the soft muddy bottom that was probably due to anaerobic to exaerobic conditions. The absence or scarcity of bioturbation caused by invertebrate detritus or sediment feeders in both shale and concretions (formed before compaction) favored preservation of the delicate larval shells. The lack or scarcity of infauna and bioturbation as well as the low diversity of the presumed benthos supports an interpretation of a quasi-anaerobic to exaerobic benthic environment. The superbly preserved larval shells demonstrate that there are more caenogastropod clades present in the late Palaeozoic than suggested previously. Some larval shell types have an openly coiled first whorl followed by a planktotrophic larval shell; openly coiled initial whorls are unknown from modern caenogastropods. The vetigastropods have a smooth protoconch of two whorls clearly demarked from the following whorls - a pattern unknown in modern vetigastropods which have a protoconch of less than one whorl and build no larval shell during their planktonic stage. This could indicate a link between Palaeozoic vetigastropods and the caenogastropods.     

Sequential predator effects across three life stages of the African tree frog, <Emphasis Type="Italic">Hyperolius spinigularis</Emphasis>

While theoretical studies of the timing of key switch points in complex life cycles such as hatching and metamorphosis have stressed the importance of considering multiple stages, most empirical work has focused on a single life stage. However, the relationship between the fitness components of different life stages may be complex. Ontogenetic switch points such as hatching and metamorphosis do not represent new beginnings—carryover effects across stages can arise when environmental effects on the density and/or traits of early ontogenetic stages subsequently alter mortality or growth in later stages. In this study, I examine the effects of egg- and larval-stage predators on larval performance, size at metamorphosis, and post-metamorphic predation in the African tree frog Hyperolius spinigularis. I monitored the density and survival of arboreal H. spinigularis clutches in the field to estimate how much egg-stage predation reduced the input of tadpoles into the pond. I then conducted experiments to determine: (1) how reductions in initial larval density due to egg predators affect larval survival and mass and age at metamorphosis in the presence and absence of aquatic larval predators, dragonfly larvae, and (2) how differences in mass or age at metamorphosis arising from predation in the embryonic and larval environments affect encounters with post-metamorphic predators, fishing spiders. Reduction in larval densities due to egg predation tended to increase per capita larval survival, decrease larval duration and increase mass at metamorphosis. Larval predators decreased larval survival and had density-dependent effects on larval duration and mass at metamorphosis. The combined effects of embryonic and larval-stage predators increased mass at metamorphosis of survivors by 91%. Larger mass at metamorphosis may have immediate fitness benefits, as larger metamorphs had higher survival in encounters with fishing spiders. Thus, the effects of predators early in ontogeny can alter predation risk even two life stages later.     

An extraordinarily long larval duration of 4.5 years from hatching to metamorphosis for teleplanic veligers of Fusitriton oregonensis

Veliger larvae of the NE Pacific snail Fusitriton oregonensis were reared in culture for 4.5 to 4.6 years from hatching to metamorphosis and through postlarval growth to reproduction. Larval shells grew in length from 0.20 to 3.9 mm. Late veligers grew slowly, but shell sizes increased even in the 4th and 5th years. Widths of larval shells at late stages equaled or exceeded those of the protoconchs of two juveniles from the field. Cultured larvae did not metamorphose until presented with subtidal rocks and associated biota. There was no indication of larval senescence: the first 2 years of postmetamorphic shell growth were slightly faster, and time from metamorphosis to first reproduction (3.3 years) was slightly less than for an individual that had developed to metamorphic competence in the plankton. A 4.5-year larval phase exceeds previous estimates for teleplanic larval durations and greatly exceeds estimates of the time for transport across oceans. This extraordinarily long larval period may exceed the usual duration in nature but shows that larval periods can be much longer than previously suspected without complete stasis in growth and with little if any loss of viability.     

Carry-over effects of the larval environment on post-metamorphic performance in two hylid frogs

Life history theory and empirical studies suggest that large size or earlier metamorphosis are suitable proxies for increased lifetime fitness. Thus, across a gradient of larval habitat quality, individuals with similar phenotypes for these traits should exhibit similar post-metamorphic performance. Here we examine this paradigm by testing for differences in post-metamorphic growth and survival independent of metamorphic size in a temperate (spring peeper, Pseudacris crucifer) and tropical (red-eyed treefrog, Agalychnis callidryas) anuran reared under differing larval conditions. For spring peepers, increased food in the larval environment increased post-metamorphic growth efficiency more than predicted by metamorphic phenotype and led to increased mass. Similarly, red-eyed treefrogs reared at low larval density ended the experiment at a higher mass than predicted by metamorphic phenotype. These results show that larval environments can have delayed effects not captured by examining only metamorphic phenotype. These delayed effects for the larval environment link larval and juvenile life history stages and could be important in the population dynamics of organisms with complex life cycles.     

Laboratory culture of the larvae of Conus textile Linné (Gastropoda: Toxoglossa)

The larvae of the Indo-Pacific gastropod Conns textile Linné were reared in the laboratory from hatching through metamorphosis. Larvae fed a mixed phytoplankton culture of Isochrysis galbana and Phaeodactylum tricornutum grew at a rate of 0.06 mm/day and began metamorphosing 16 days after hatching. Unfed control cultures yielded no metamorphically competent larvae. Laboratory-reared larvae metamorphosed spontaneously on the walls of the fïberglass rearing tanks when their average shell length was 1.5 mm. Measurements made on field-collected Conns textile juveniles indicate that the larvae metamorphose at the same size in the laboratory as they do in nature.Rates of larval shell length increase and dry weight increase paralleled each other until metamorphosis. At this point, shell growth slowed while dry weight increased suddenly. It is suggested that this weight increase reflects calcification and strengthening of the fragile larval shell upon entering the benthic environment.     

The apical sensory organ of a gastropod veliger is a receptor for settlement cues

On the basis of anatomy and larval behavior, the apical sensory organ (ASO) of gastropod veliger larvae has been implicated as the site of perception of cues for settlement and metamorphosis. Until now, there have been no experimental data to support this hypothesis. In this study, cells in the ASO of veliger larvae of the tropical nudibranch Phestilla sibogae were stained with the styryl vital dye DASPEI and then irradiated with a narrow excitatory light beam on a fluorescence microscope. When its ASO cells were bleached by irradiation for 20 min or longer, an otherwise healthy larva was no longer able to respond to the usual metamorphic cue, a soluble metabolite from a coral prey of the adult nudibranch. The irradiated cells absorbed the dye acridine orange, suggesting that they were dying. When larvae not stained with DASPEI were similarly irradiated, or when stained larvae were irradiated with the light beam focused on other parts of the body, there was no loss of ability to metamorphose. Together these data provide strong support for the hypothesis. Potassium and cesium ions, known to induce metamorphosis in larvae of many marine-invertebrate phyla, continue to induce metamorphosis in larvae that have lost the ability to respond to the coral inducer due to staining and irradiation. These results demonstrate that (1) the ASO-ablated larvae have not lost the ability to metamorphose and (2) the ions do not act only on the metamorphic-signal receptor cells, but at other sites downstream in the metamorphic signal transduction pathway.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.