Comparison of neurogenesis in bivalves with different types of development

The presence of different types of larvae within the same class suggests a broad ecological diversification. A clear comparison of bivalve larval nervous systems would give a broader view on evolutionary and ecological picture of the clade in question. The present study focused on the neurodevelopment in two bivalve species with different larval types: pericalymma of Acila insignis (Bivalvia: Protobranchia) and veliger of Spisula sybillae (Bivalvia: Autobranchia). It was shown that the pioneer dorsal and ventral neurons in S. sybillae appear at the trochophore stage. Subsequently, future three paired ganglia are developed on the nerve cords in pediveliger. In the pericalymma of A. insignis, serotonin- and FMRFamide-positive cells are found in the apical organ (AO), as well as two pairs of FMRFamide positive neurons are detected on dorsal and posterior part of pericalymma. A comparative analysis showed significant differences in the larval neuromorphology between veliger and pericalymma. In contrast to the S. sybillae veliger, the nervous system of the A. insignis pericalymma is simple, likely due to its different lifestyle. The larval nervous system in the species under study has features characteristic of Lophotrochozoa and Spiralia.

Hedgehog signaling is required for larval muscle development and larval metamorphosis of the mussel Mytilus coruscus

Understanding the developmental processes and signaling pathways involved in larval myogenesis and metamorphosis is crucial for comprehending the life history and adaptive strategies of marine organisms. In this study, we investigated the temporal and spatial patterns of myogenesis in the mussel Mytilus coruscus (Mc), focusing on the emergence and transformation of major muscle groups during different larval stages. We also explored the role of the Hedgehog (Hh) signaling pathway in regulating myogenesis and larval metamorphosis. The results revealed distinct developmental stages characterized by the emergence of specific muscular components, such as velum retractor muscles and anterior adductor muscles, in D-veliger and umbo larvae, which are responsible for the planktonic stage. In the pediveliger stage, posterior ventral, posterior adductor, and foot muscles appeared. After larval metamorphosis, the velum structure and its corresponding retractor muscles degenerate, indicating the transition from planktonic to benthic life. We observed a conserved pattern of larval musculature development and revealed a high degree of conservation across bivalve species, with comparable emergence times during myogenesis. Furthermore, exposure to the Hh signaling inhibitor cyclopamine impaired larval muscle development, reduced larval swimming activity, and inhibited larval metamorphosis in M. coruscus. Cyclopamine-mediated inhibition of Hh signaling led to reduced expression of four key genes within the Hh signaling pathway (McHh, McPtc, McSmo, and McGli) and the striated myosin heavy chain gene (McMHC). It is hypothesised that the abnormal larval muscle development in cyclopamine-treated groups may be an indirect effect due to disrupted McMHC expression. We provide evidence for the first time that cyclopamine treatment inhibited larval metamorphosis in bivalves, highlighting the potential involvement of Hh signaling in mediating larval muscle development and metamorphosis in M. coruscus. The present study provides insights into the dynamic nature of myogenesis and the regulatory role of the Hh signaling pathway during larval development and metamorphosis in M. coruscus. The results obtained in this study contribute to a better understanding of the evolutionary significance of Hh signaling in bivalves and shed light on the mechanisms underlying larval muscle development and metamorphosis in marine invertebrates.

Phalloidin fluorescence and confocal microscopy reveal the musculature development of clam Ruditapes philippinarum

Knowledge of early development in bivalves is of great importance to understand the function of animal organ systems and the evolution of phenotypic diversity. Manila clam (Ruditapes philippinarum) is an economically important bivalve living in marine intertidal zones. To determine the pattern of muscle development in the clams, we investigate the characteristics of musculature development utilizing phalloidin staining and confocal microscopy. Myofilaments first appear at the early trochophore stage, and gradually become orderly arranged during the transition from trochophore to veliger. For veliger, larval muscle system is mainly composed of dorsal velum retractors, medio-dorsal velum retractors, ventral velum retractors, ventral larval retractors and anterior and posterior adductor muscles. After metamorphosis, the muscle system of late veliger has been reconstructed, showing the irreversible shrink of velum retractor muscles, the rapid growth of wedge-shaped foot and mantle margins. One of the most striking changes in settled spats is the development of sophisticated architecture of foot musculature, which consists of transverse pedal muscles, anterior foot retractor and posterior foot retractor. These findings will not only provide the basis to understand the dynamic pattern of myogenesis in the burrowing bivalves, but also provide valuable information for comparative analysis of muscle development among bivalves.

Developmental dynamics of myogenesis in Yesso Scallop Patinopecten yessoensis

The development of bivalves has been extensively studied over the last 150 years. Despite this, the developmental dynamics of myogenesis in bivalves remains largely unknown, particularly at the early developmental stages. In the present study, we investigate the characteristics of muscle development of Yesso scallop Patinopecten yessoensis by phalloidin staining, light, electron and confocal microscopy. Myoblasts containing chaotic myofilaments are initially found in the blastocoel of trochophore, and become more organized during the transformation from trochophore into veliger. This is followed by a well-structured musculature including an anterior adductor, velum retractors and ventral retractors at the early veliger stage. With development into late veliger, larval muscle system is composed of the branched velum retractors and ventral retractors, anterior and posterior adductors. The most striking change for pediveliger is the development of foot retractor and mantle related muscles at this stage. During metamorphosis, the retractor muscles and anterior adductor undergo the irreversible shrink until vanishing completely towards the end of larval life, which coincide with the growth of foot retractor and mantle margin. All retractor muscles are found to be composed of striated fibres, whereas the adductor muscles have both smooth and striated components. The present study provides new evidences on the dynamic pattern of myogenesis during embryonic and larval development in scallops, which will greatly improve our understanding of scallop myogenesis and provide the basis for comparative analysis of muscle development in bivalves.

Inferring muscular ground patterns in Bivalvia: Myogenesis in the scallop Nodipecten nodosus

Background

Myogenesis is currently investigated in a number of invertebrate taxa using combined techniques, including fluorescence labeling, confocal microscopy, and 3D imaging, in order to understand anatomical and functional issues and to contribute to evolutionary questions. Although developmental studies on the gross morphology of bivalves have been extensively pursued, organogenesis including muscle development has been scarcely investigated so far.

Results

The present study describes in detail myogenesis in the scallop Nodipecten nodosus (Linnaeus, 1758) during larval and postmetamorphic stages by means of light, electron, and confocal microscopy. The veliger muscle system consists of an anterior adductor muscle, as well as four branched pairs of striated velum retractors and two pairs of striated ventral larval retractors. The pediveliger stage exhibits a considerably elaborated musculature comprising the velum retractors, the future adult foot retractor, mantle (pallial) muscles, and the anterior and posterior adductors, both composed of smooth and striated portions. During metamorphosis, all larval retractors together with the anterior adductor degenerate, resulting in the adult monomyarian condition, whereby the posterior adductor retains both myofiber types. Three muscle groups, i.e., the posterior adductor, foot retractor, and pallial muscles, have their origin prior to metamorphosis and are subsequently remodeled.

Conclusions

Our data suggest a dimyarian condition (i.e., the presence of an anterior and a posterior adductor in the adult) as the basal condition for pectinids. Comparative analysis of myogenesis across Bivalvia strongly argues for ontogenetic and evolutionary independence of larval retractors from the adult musculature, as well as a complex set of larval retractor muscles in the last common bivalve ancestor.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0125-x) contains supplementary material, which is available to authorized users.

Larval morphology of the brooding clam Lasaea adansonii (Gmelin, 1791) (Bivalvia, Heterodonta, Galeommatoidea)

The strongly modified mode of development of the small and brooding galeommatoid bivalve Lasaea adansonii (Gmelin, 1791) [syn. Lasaea rubra (Montagu, 1803)] has been studied by means of transmission and scanning electron microscopy and by fluorescent staining of the muscular system and of two neurotracers, FMRFamide and serotonin. In addition, two developmental stages were visualized using computer-aided 3D-reconstruction. All larval stages of L. adansonii lack ciliary rings. The apical organ appears invaginated: the base of the duct contacts the cerebral ganglia and opens on the preoral region. Larval protonephridia are lacking. The adult kidneys develop independently of the pericardial cavity and contain a protonephridial part that enables excretory function until the pericardium is formed. The larval muscular system is composed of smooth muscle fibers; striated fibers are lacking. Posteriorly and immediately below the ligament, a paired cell of unknown function is present that contains serotonin and FMRFamide. In summary, L. adansonii exhibits the direct mode of development. Only few truly larval structures (e.g., the modified apical organ) are elaborated.