Larval Retractor Research Articles

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

AbstractShell‐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.

Morphogenesis of the digestive system during metamorphosis of the nudibranch Doridella steinbergae (Gastropoda): Conversion from phytoplanktivore to carnivore

The structural changes undergone by the digestive system of a phytoplanktotrophic nudibranch larva during metamorphosis into a benthic carnivore are described using histological and electron microscopic techniques. The relative positions of the stomach, digestive gland, and distal end of the intestine are rearranged at metamorphosis by the actions of the larval retractor muscle and the accessory pedal retractor muscle. Although the anus and distal end of the intestine are secondarily displaced to the posterior end of the gastropod, the stomach undergoes further torsional displacement at metamorphosis. The tissues of the larval stomach and distal end of the larval esophagus undergo drastic alteration at metamorphosis. The larval stomach consists of a ciliated vestibule, which receives the openings of the esophagus and left digestive gland, a gastric shield, a style sac, and an intestinal groove. All of these areas, except the vestibule, are destroyed by cell dissociation at metamorphosis. The vestibule becomes the ventral stomach of the benthic stage and the proximal end of the intestine becomes enlarged and muscularized to form the dorsal ‘stomach’ of the benthic stage. The metamorphic changes involving the distal end of the esophagus include the continued development of the radula and oral lip glands, which both appear in rudimentary form during the larval stage, and differentiation of the buccal pump, salivary glands, and oral lips.

Metamorphic Changes in the Velar Musculature, Head and Shell of some Prosobranch Veligers

The velum of Lacuna vincta (Montagu) and Nassarius incrassatus (Ström) is retracted into the shell by 2 dorsal and 2 ventral groups of muscles, bilaterally arranged, which are components of the larval retractor muscle. Each dorsal group passes forward from its area of origin on the shell to the dorsal part of the head where some fibres insert and adjacent ones enter the velar fold. The velum is bilobed in Lacuna and the muscles radiate through the dorsal half of the lobe to insert on the anterior and posterior surfaces and ciliated edge, whereas Nassarius has 4 long velar lobes, 2 dorsal and 2 ventral, and the muscles form a median bundle from which fibres radiate to similar insertions. Each ventral group of muscles passes through the nerve ring lateral to the developing radular sac in Lacuna and to the developing proboscis in Nassarius, and muscles accompanying them insert on the sac and proboscis wall. Anterior to the ring some muscles insert on the lips and the buccal wall posterior to the oral sphincter and alongside attachments of muscles from the foot. The majority pass to the mentum, a median protuberance of the lower lip with non-vibratile cilia, where right and left ventral cephalic retractors and muscles from the foot converge. The pedal muscles attach here and from this area muscles radiate to multiple insertions on the head and ventral half of the velum. This arrangement of the ventral muscles allows the increased angle of mobility required by the ventral areas of the velum in swimming and feeding and the intimate contact between velum and foot.

Direct development in a Nudibranch, Cadlina laevis, with a discussion of developmental processes in Opisthobranchia

Cadlina laevis (L.) is the first British species of nudibranch shown to possess direct development. The embryo leaves the egg-capsule approximately 50 days (at 10° C) after oviposition, and resembles the adult in the general form of the body. Sufficient food reserves remain to sustain life and apparent growth for more than 1 week of benthic life. A vestigial veliger phase is passed through within the egg-capsule. The shell, velum, mantle, and metapodium-rudiment resemble transitorily those organs of opisthobranch veliger larvae, but certain veliger structures (larval kidney, velar locomotor and pedal cilia) are vestigial in Cadlina, and others are altogether absent (operculum, larval retractor muscle, subvelar ridges, metapodial mucus-gland, pedal sensory cilia, and nephrocysts).In the Opisthobranchia three distinct development types can be recognized. Type 1 includes those species which possess planktotrophic veliger larvae. Type 2 comprises the species which have lecithotrophic larvae (in the sense employed by Ockelmann, 1965). Type 3 species have direct development, in which many veliger structures may be briefly recapitulated before hatching. Species of type 3 possess eggs of the largest sizes, species of type 2 in general have smaller eggs, while species of type 1 possess the smallest eggs. There is a positive correlation between egg size and the length of the embryonic period, and an inverse correlation between both and batch size. Within any development-type, the largest species tend to produce larger eggs in greater numbers. Within any species, the largest individuals tend to produce more eggs than do the smaller individuals; egg size varies little through the geographical range.

Studies on the ontogeny of Tritonia hombergi cuvier (gastropoda opisthobranchia)

Tritonia hombergi Cuvier is the largest British nudibranchiate mollusc; it is always found in association with the colonial cnidarian Alcyonium digitatum , in depths from 6 to 38 fm. Cleavage and development up to gastrulation resemble the pattern described for other nudibranchs. The heavily yolked stereoblastulae gastrulate by epiboly. As the veliger form is assumed, a slow movement of the anal cells (which predict the site of the proctodaeal invagination) represents the sole vestige of the morphogenetic processes of gastropod torsion and visceral flexure. This movement is completed before the differentiation of any recognizable muscle elements or of the organs of the visceral mass. Hatching occurs 36 to 38 days (at 10 °C) after oviposition; the lecithotrophic larvae are at first negatively geotactic but after 1 or 2 days this is reversed and searching behaviour begins. Searching larvae could be induced to metamorphose only if a live healthy colony of A. digitatum was provided. The shell and operculum are cast, the velar lobes resorbed, and reflexion and differential growth of the mantle fold bring about a movement of the anal complex which reverses the embryonic torsion and visceral flexure. The adult dorsal integument is derived from the layer of the mantle fold which lined the embryonic mantle cavity. It is concluded that the tritoniid nudibranchs probably arose via stages in which the ancestral shell was enclosed by folds of the mantle. Both right and left embryonic midgut diverticula contribute to the formation of the adult digestive gland. Larval metamorphosis involves also concentration of the main ganglia and the enlargement of the rudiments of the definitive kidney and pericardium, while the larval retractor muscle and the metapodial gland begin to atrophy. The young benthic stages browse on the tissues of the cnidarian. Thousands were reared in the laboratory for several months until, when some had reached a length of 3 cm, lack of sufficient food prevented their being cultured further. Shortly after metamorphosis, the histogenesis of the adult skin begins, the pleural ganglia make their first appearance, and the original embryonic otoliths are augmented. The larval kidney disappears and its functions are taken over by the definitive kidney. The ventricle and the auricles develop in the pericardium and, as the pallial branchiae are formed, the adult circulation of the blood is established. Changes in the radular dentition illustrate the taxonomic identity of T. hombergi Cuvier with T. alba Alder and Hancock. The primordia of the reproductive system are first detectable in post-larvae I 3 4 mm in length. The benthic stages feed exclusively on Alcyonium . Previous accounts of the feeding mechanism were incorrect in attributing to the jaws the function merely of prehension and in stating that the food is torn up by the radula. Digestion begins in the stomach after only a few minutes; the cnidarian spicules fall to the stomach floor from whence they are carried by cilia into the hindgut. In the juvenile T. hombergi partially digested matter is sucked into the lobules of the digestive gland by passive dilation following pulsating contractions of their muscle-sheaths; probably a similar mechanism exists in the adult tritoniid. In young animals absorption of the products of digestion, both particulate and liquid, takes place through the walls of the stomach, hindgut, and digestive gland. In older stages, particulate matter is taken up solely by the digestive gland, but fluids may be absorbed also through the stomach wall. Experiments with young individuals (in length 8 to 12 mm) at 15 °C showed that, while individual digestive cells pass through a sequence of changes, the complete digestive gland acts arhythmically under conditions of constant access to food. The conclusion is reached that Pelseneer was in error when he stated that all opisthobranchs undergo during their development a torsion identical with that shown by prosobranchs. Only vestiges of gastropod torsion remain both in veliger and adult stages of many (perhaps most) living Opisthobranchia.

THE DEVELOPMENT OF THE LAMELLIBRANCH CARDIUM EDULE L.

The development of Cordium edule is described from the fertilization of the eggs to the completion of metamorphosis. Cleavage and gastrulation are similar to those processes in other lamellibranchs. Differentiation of the major larval organs, the velum, the larval retractor muscles and the digestive system is completed within the first four days of life. Other organs are relatively undeveloped until the veliger is twenty‐two to twenty‐five days old. Differentiation is not complete until the spat have settled for several weeks. The larval retractor muscles are paired and are attached around the mouth and to the presumptive foot region as well as to the velum.

The natural history, embryology, larval biology and post-larval development of Adalaria proxima (Alder and Hancock) (Gastropoda Opisthobranchia)

The natural history, embryology, larval biology and post-larval development of <i>Adalaria proxima</i> (Alder and Hancock) (Gastropoda Opisthobranchia)

Muscle morphogenesis in primitive gastropods and its relation to torsion

Muscle morphogenesis in primitive gastropods and its relation to torsion