Chapter 3 Structure and function in scallops

Multi-tissue metabolomic profiling reveals the crucial metabolites and pathways associated with scallop growth

BACKGROUND

Bivalves represent a vital economic resource in aquaculture for their high productivity and extensive market demand. Growth is one of the most important and desired aquaculture traits for bivalves, regulated by multiple levels, notably intricate metabolic processes. However, the understanding of the metabolic profiles that influence bivalve growth is limited, particularly from a multi-tissue perspective.

RESULTS

In this study, metabolic profiles of multiple tissues of Chlamys farreri with different growth performance were systematically investigated by ultraperformance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). Through comparing the metabolic variation between fast-growing (FG) scallops and slow-growing (SG) scallops, 613, 509, 105, and 192 significantly different metabolites (SDMs) were identified in the mantle, gill, adductor muscle, and digestive gland, respectively. Growth-related metabolic pathways including sphingolipid metabolism, fatty acid biosynthesis, and ABC transporter pathway, along with 11 SDMs associated with growth traits were identified in all four tissues, implying they were involved in the growth of multiple tissues in scallops. Tissue-specific metabolic profiling indicated that sulfur-containing amino acid metabolism in the mantle potentially contributed to shell growth, while the gill synergistically participated with the mantle through various metabolic processes, such as tyrosine metabolism, glycine, serine, and threonine metabolism and melanogenesis; energy metabolism was crucial for adductor muscle growth; and nutrients digestion and absorption in the digestive gland were linked to scallop growth.

CONCLUSIONS

Our results represent the first comprehensive analysis of the crucial pathways and metabolites associated with the growth of C. farreri, offering valuable insights for future bivalve aquaculture production.

Neuropeptides regulate shell growth in the Mediterranean mussel (Mytilus galloprovincialis)

In bivalves, which are molluscs enclosed in a biomineralized shell, a diversity of neuropeptide precursors has been described but their involvement in shell growth has been largely neglected. Here, using a symmetric marine bivalve, the Mediterranean mussel (Mytilus galloprovincialis), we uncover a role for the neuroendocrine system and neuropeptides in shell production. We demonstrate that the mantle is rich in neuropeptide precursors and that a complex network of neuropeptide-secreting fibres innervates the mantle edge a region highly involved in shell growth. We show that shell damage and shell repair significantly modify neuropeptide gene expression in the mantle edge and the nervous ganglia (cerebropleural ganglia, CPG). When the CPG nerve commissure was severed, shell production was impaired after shell damage, and modified neuropeptide gene expression, the spatial organization of nerve fibres in the ganglia and mantle and biomineralization enzyme activity in the mantle edge. Injection of CALCIa and CALCIIa peptides rescued the impaired shell repair phenotype providing further support for their role in biomineralization. We propose that the regulatory mechanisms identified are likely to be conserved across bivalves and other shelled molluscs since they all share a similar nervous system, a common mantle biomineralization toolbox, and shell structure.

Pulsative venous return from the branchial vessels to the heart of the bivalve Mytilus galloprovincialis supports the constant-volume mechanism

In bivalves and gastropods, ventricle contraction causes a negative pressure in the auricles and increases venous return from the afferent oblique vein (AOV): the constant-volume (CV) mechanism. The flow in the AOV should be a pulsative flow synchronized with the ventricular contraction. The flow in the heart and adjacent vessels of Mytilus galloprovincialis were measured by magnetic resonance imaging to confirm this hypothesis. Under a regular heartbeat, pulsative flows in the AOV and branchial vessels (BVs) were almost completely synchronized with the flow in the aorta, while filling of the ventricle was in the opposite phase. Flows in the BVs were directed to the posterior direction, and a pair of BVs in the gill axes (the efferent BVs) were connected to the AOV. Based on the images of the whole pathway of the AOV in an oblique slice, we confirmed that haemolymph flow was evoked from the efferent BVs and flow into the ventricle via the auricle was completed in a single heartbeat. Therefore, the walls of the AOV and BVs could resist negative transmural pressure caused by the ventricular contraction. In conclusion, the auricle, the AOV and the BVs, including the gill filaments, act as a suction pump. The pulsative venous return is driven by the negative pressure of the AOV as in the CV mechanism, and the negative pressure in the efferent BVs could draw haemolymph from the sinus via the gill and the afferent BVs. Therefore, Mytilus can start and stop its heartbeat as necessary.

Bivalve digestive epithelial virosis (DEV): a cause of disease or a natural process?

Epithelial hyperplasia and sloughing of the digestive gland in bivalve mollusks are a global phenomenon and occur in species of commercial interest and cultural significance to indigenous peoples. Where hemocytosis, hyperplasia, and necrosis of digestive tubule cells have been observed associated with electron-dense uncoated virus-like particles (VLPs) 25-45nm in diameter, the condition has been named digestive epithelial virosis (DEV). This condition has been associated with mortalities of some bivalve species in New Zealand. Similar digestive gland alterations, but without detection of associated VLPs, have been reported in other bivalve species worldwide and are termed "DEV-like" since no virus link has been demonstrated. It remains unclear if DEV is an infectious condition and whether associated VLPs are the cause, a contributor, or simply associated with the observed condition. It is also unclear whether DEV or DEV-like conditions pose a biosecurity or economic threat, or alternatively, whether they reflect a natural cyclic event that does not require disease management. In this mini-review, we summarize the history of digestive epithelial alteration with VLPs (i.e., DEV) or without observation of VLPs (i.e., DEV-like), and we examine the evidence for and against viral-like particles as the cause of DEV in bivalves. We also explore other viral afflictions of bivalves and non-infectious agents, such as harmful algae and xenotoxins, that could elicit similar tissue alterations. Future recommendations for approaches to identify key risk factors that lead to the development of digestive epithelial alterations such as DEV include histological characterization of the digestive gland of marine mollusks; the use of metagenome analysis to design primers that could be used for detection of VLPs and to study host microbiota; disease challenges demonstrating that DEV causes pathology and the relationship between DEV intensity and morbidity/mortality.

Distribution of Molecules Related to Neurotransmission in the Nervous System of the Mussel Crenomytilus grayanus

In bivalves neurotransmitters are involved in a variety of behaviors, but their diversity and distribution in the nervous system of these organisms remains somewhat unclear. Here, we first examined immunohistochemically the distributions of neurons containing different neurotransmitters, neuropeptides, and related enzymes, as well as the proliferative status of neurons in the ganglia of the mussel Crenomytilus grayanus. H-Phe-Met-Arg-Phe-NH2 (FMRFamide), choline acetyltransferase (ChAT), γ-aminobutyric acid (GABA) and tyrosine hydroxylase (TH) were found to be expressed by neurons in all the ganglia, whereas serotonin (5-HT) neurons were found only in the cerebropleural and pedal, but not visceral ganglia. Moreover, incubation of living mussels in the presence of a 5-HT precursor (5-HTP) confirmed the absence of 5-HT-containing neurons from the visceral ganglia, indicating that the "serotonin center" of the visceral nervous system is located in the cerebral ganglia. Furthermore, immunostaining of molecules related to neurotransmission together with α-acetylated tubulin demonstrated that this cytoskeletal protein may be a potential pan-neuronal marker in bivalves. Adult mussel neurons do not proliferate, but a population of proliferating PCNA-LIP cells which do not express any of the neurotransmitters examined, perhaps glia cells, was detected in the ganglia. These novel findings suggest that the nervous system of bivalves contains a broad variety of signal molecules most likely involved in the regulation of different physiological and behavioral processes. In addition, proliferating cells may maintain and renew glial cells and neurons throughout the lives of 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.

Magnetic resonance imaging analysis of water flow in the mantle cavity of live Mytilus galloprovincialis

Water flow inside the shell of Mytilus galloprovincialis was measured by phase-contrast magnetic resonance imaging (MRI). In seawater without algal cells at 23 °C, water approached the mussel from the posterior-ventral side, and entered through the inhalant aperture at a velocity of 40-20 mm s(-1). The flow rate in the lower mantle cavity decreased to 10-20 mm s(-1), the water flowed in the anterior-dorsal direction and approached the demibranches at a velocity of 5-10 mm s(-1). After passing through the lamellae to the upper mantle cavity, the water stretched the interlamellar cavity, turned to the posterior-dorsal direction and accumulated in the epibranchial cavity. The water flows came together at the ventral side of the posterior adductor muscle. The velocity increased more to than 50 mm s(-1) in the exhalant siphon, and exhaled out in the posterior-dorsal direction. The anterior-posterior direction of the flow was imaged every 1.92 s by the inflow effect of T1-weighted MRI. The flow seemed to be constant, and no cyclic motion of the mantles or the gills was detected. Spontaneous closure of the shells caused a quick drop of the flow in the mantle cavity. In the opening process of the shells, water flow in the interlamellar cavities increased before the opening, followed by an increase of flows in the exhalant siphon and inhalant aperture with minimum delay, reaching a plateau within 1 min of the shells opening. This provides direct evidence that the lateral cilia drive water in the mussel M. galloprovincialis.

Mantle margin morphogenesis in Nodipecten nodosus (Mollusca: Bivalvia): new insights into the development and the roles of bivalve pallial folds

BACKGROUND

Despite extensive knowledge on bivalve anatomy and development, the formation and differentiation of the mantle margin and its associated organs remain largely unclear. Bivalves from the family Pectinidae (scallops) are particularly promising to cast some light on these issues, because they exhibit a complex mantle margin and their developmental stages are easily obtained from scallop farms. We investigated the mantle margin of the scallop Nodipecten nodosus (L. 1758) during larval and postmetamorphic development.

METHODS

A thorough analysis of the mantle margin development in Nodipecten nodosus, from veliger larvae to mature adults, was conducted by means of integrative microscopy techniques, i.e., light, electron, and confocal microscopy.

RESULTS

Initially unfolded, the pallial margin is divided into distal and proximal regions by the periostracum-forming zone. The emergence of the pallial musculature and its neural innervation are crucial steps during bivalve larval development. By the late pediveliger stage, the margin becomes folded, resulting in a bilobed condition (i.e., outer and inner folds), a periostracal groove, and the development of different types of cilia. After metamorphosis, a second outgrowth process is responsible for emergence of the middle mantle fold from the outer surface of the inner fold. Once the three-folded condition is established, the general adult features are rapidly formed.

CONCLUSIONS

Our data show that the middle mantle fold forms from the outer surface of the inner fold after metamorphosis and that the initial unfolded mantle margin may represent a common condition among bivalves. The first outgrowth process, which gives rise to the outer and inner folds, and the emergence of the pallial musculature and innervation occur during larval stages, highlighting the importance of the larval period for mantle margin morphogenesis in Bivalvia.

Testing the constant-volume hypothesis by magnetic resonance imaging of Mytilus galloprovincialis heart

The constant-volume (CV) hypothesis was tested using the Mytilus galloprovincialis heart under two conditions. The volume of the ventricle, auricles and pericardium, and the flow in the heart and adjacent vessels were measured by magnetic resonance imaging. In synthetic seawater at 23°C (immersed condition), the end-diastolic volume (EDV), end-systolic volume (ESV) and stroke volume (SV) were 50%, 21% and 29% of the heart volume, respectively, and the auricle volume (VA) was maximized at end-systole. Assuming a constant volume of the heart, venous return to the auricles (IV) was constant, and out-flow from the pericardium to the kidney (IPK) was 2/3 of SV. During aerial exposure (emersed condition), EDV, ESV and SV decreased to 33%, 22% and 11%, respectively. VA was maximized at end-diastole and associated with the decrease of systolic IV to 1/2 of diastolic IV, while IPK remained at 80% of the immersed condition. Based on these results--in addition to two postulates of the CV hypothesis: (1) the total volume of the heart is always the same, and (2) ventricle contraction causes a decrease in pressure in the pericardium--we modified two postulates: (3) the low pericardial pressure maintains venous return from the anterior oblique vein to the auricle, and (4) the pressure difference between the auricle and the pericardium drives haemolymph filtration through the auricle walls. We also added a new postulate: (5) dilatation of the ventricle is associated with the haemolymph output to the kidney via the renopericardial canals.