GABA is an inhibitory neurotransmitter in the neural circuit regulating metamorphosis in a marine snail

The marine mud snail, Tritia (=Ilyanassa) obsoleta, displays a biphasic life cycle. During the initial phase of early development, embryos hatch from benthic egg capsules to become weakly swimming veliger larvae. In the second phase, adult T. obsoleta are facultative carnivores and major agents of community disturbance. Metamorphosis is the irreversible developmental event that links these two life history stages. When physiologically competent, larvae can respond to appropriate environmental cues by settling onto their mudflat habitat and transforming themselves into miniature adult snails. Two neurotransmitters—serotonin and nitric oxide—have opposing effects on the metamorphic process in this species. In multiple other species of gastropod and bivalve molluscs, a third neurotransmitter, the classically inhibitory compound γ‐aminobutyric acid (GABA), can induce settlement or metamorphosis upon external application to competent larvae. In this situation, GABA is presumed to mimic the action of ligands from the juvenile environment that bind to larval chemosensory receptors and activate the metamorphic pathway. Results of our experiments contradict this commonly reported action of GABA on molluscan larvae. External application of GABA to competent larvae of T. obsoleta elicited no response, but instead attenuated the action of serotonin (5‐HT), a metamorphic inducer. Our investigations into the responses of larval T. obsoleta to multiple GABAergic reagents support our hypothesis that GABA functions internally as a neurotransmitter in the pathway that controls the initiation of metamorphosis. Our results also suggest that GABA acts directly on or downstream from serotonergic neurons to regulate the metamorphosis‐inducing effects of this neurotransmitter. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 736–753, 2018     

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.     

温度、盐度对龟足胚胎发育和幼虫生长的联合影响

龟足(Capitulum mitella Linnaeus)在我国主要分布于长江口以南海浪剧烈冲击的暴露型岩相海岸的中、高潮区,是一种颇具养殖潜力和市场前景的新品种。研究温度(24、27、30、33℃)和盐度(28,31,34)对龟足胚胎发育和幼虫生长的协同影响,可为龟足的人工育苗提供依据。结果如下(1):33℃-28温盐度组合胚胎发育时间最短144h,27℃-28温盐度组合胚胎相对孵化率最高。温度与盐度对胚胎发育时间没有显著影响;但温度和盐度对胚胎孵化率有极显著影响,温度与盐度间的交互作用显著。胚胎发育最适宜的温盐度组合是27℃-28。(2):27℃的3个盐度组、30℃-31温盐度组合无节幼虫持续时间最短。在同一盐度条件下以27℃的存活率较高,在同一温度条件下以盐度31的存活率较高,其中以27℃-31温盐度组合的存活率最高;存活率1和存活率2分别高达99.0%、90.7%。27℃-28、27℃-31温盐度组合变态率最高,变态率分别为81.8%、73.7%。34高盐组幼虫的存活率和变态率均很低甚至为零。温度和盐度对幼虫存活率和变态率有极显著影响,两者的交互作用极为显著。综合无节幼虫持续时间、存活和变态情况,27℃-31温盐度组合为幼虫生长发育的最佳组合条件。龟足胚胎发育、无节幼虫的生长和变态对温度盐度的敏感性有所不同,这是由龟足的自身繁殖特点及生活环境决定的。     

Neurogenesis in the developing crab brain: Postembryonic generation of neurons persists beyond metamorphosis

A considerable amount of information is available about the structure and function of the central nervous system in adult crustaceans. However, little effort has been directed toward understanding embryonic and larval neurogenesis in these animals. In the present study we recorded neurogenesis in the brain of laboratory-reared larvae of the spider crab Hyas araneus. Proliferating cells were detected immunocytochemically after in vivo labeling with 5-bromo-2′-deoxyuridine. This method has already been used to study the proliferation of neuroblasts in the ventral nerve cord of spider crab larvae. In the brain, a set of mitotically highly active neuroblasts was found in newly hatched zoea 1 larvae. These neuroblasts are individually identifiable due to their position and therefore a schematic map of the cerebral neuroblasts could be established. The number of active neuroblasts is high from hatching throughout the molt to the zoea 2. This proliferative action then decreases dramatically and has ceased at the time of first metamorphosis toward the megalopa larva. However, many ganglion mother cells born by unequal division of neuroblasts then go through their final division throughout the subsequent megalopa stage. In the brain, all mitotic activity has ceased at the time of second metamorphosis with the exception of a cluster of labeled nuclei within the olfactory lobe cells. In this cluster, the generation of neurons persists beyond the second metamorphosis into the crab 1 stage. Meanwhile, the neuropil volume of the olfactory lobes increases 10-fold from hatching to the crab 1. These results are discussed with regard to reports on neuronal proliferation during adult life in insects and rodents. © 1996 John Wiley & Sons, Inc.     

A new internal structure of nauplius larvae: A “ghostly” support sling for cypris y left within the exuviae of nauplius y after metamorphosis (Crustacea: Thecostraca: Facetotecta)

Facetotecta, or crustacean “y-larvae,” occur in all the world's oceans although the adult forms remain completely unknown. At the metamorphic molt from the last naupliar instar to the terminal cypris larval stage a free carapace, six pairs of natatory thoracopods, and a segmented thorax and abdomen all develop anew. Unlike in earlier molts, the cephalic shield and the so-called “faciotruncal integument” usually remain together at this last naupliar molt, and the posterior “trunk” portion of the exuviae, while hollow, is not empty. In mounted preparations examined by phase contrast or differential interference contrast (DIC) microscopy, a ghost-like image of part of the cypris thorax, particularly the thoracopods and even their setae, is commonly visible inside the naupliar exuviae, and may be universally present in the Facetotecta. To investigate this “ghost,” we used DIC and digital photographic stacking, and also scanning electron microscopy, on slide or stub-mounted final naupliar exuviae of an assortment of undescribed species of Facetotecta that had been reared from planktonic lecithotropic nauplii to the cypris stage at Sesoko Island, Okinawa, Japan, and at Keelung and Green Island, Taiwan. These techniques showed that the “ghost” is a delicate, three-dimensional, fibrous structure, essentially a sling-like mold or matrix with struts attached to the outer cuticle and pairs of deep pockets that previously held the thoracopods of the developing cypris y. Whether it is endoskeletal in nature, the (partial) exuvia of an additional instar, remnants of apoptosis, or something else is currently unknown. Nothing similar has been reported in other thecostracans, or in other crustaceans that undergo a similarly abrupt metamorphosis at the last naupliar molt.     

A developmental study of serotonin-immunoreactive neurons in the larval central nervous system of the spider crabHyas araneus (Decapoda,Brachyura)

Larval development in crabs is characterized by a striking double metamorphosis in the course of which the animals change from a pelagic to a benthic life style. The larval central nervous system has to provide an adequate behavioural repertoire during this transition. Thus, processes of neuronal reorganization and refinement of the early larval nervous system could be expected to occur in the metamorphosing animal. In order to follow identified sets of neurons throughout metamorphosis, whole mount preparations of the brain and ventral nerve cord of laboratory reared spider crab larvae (Hyas araneus) were labelled with an antibody against the neurotransmitter serotonin. The system of serotonin-immunoreactive cell bodies, fibres and neuropils is well-developed in newly hatched larvae. Most immunoreative structures are located in the protocerebrum, with fewer in the suboesophaegeal ganglia, while the thoracic and abdominal ganglia initially comprise only a small number of serotonergic neurons and fibres. However, there are significant alterations in the staining pattern through larval development, some of which are correlated to metamorphic events. Accordingly, new serotonin-immunoreactive cells are added to the early larval set and the system of immunoreactive fibres is refined. These results are compared to the serotonergic innervation in other decapod crustaceans.     

Developmental modularity and phenotypic novelty within a biphasic life cycle: morphogenesis of a cone snail venom gland

The venom gland of predatory cone snails (Conus spp.), which secretes neurotoxic peptides that rapidly immobilize prey, is a proposed key innovation for facilitating the extraordinary feeding behaviour of these gastropod molluscs. Nevertheless, the unusual morphology of this gland has generated controversy about its evolutionary origin and possible homologues in other gastropods. I cultured feeding larvae of Conus lividus and cut serial histological sections through the developing foregut during larval and metamorphic stages to examine the development of the venom gland. Results support the hypothesis of homology between the venom gland and the mid-oesophageal gland of other gastropods. They also suggest that the mid-region of the gastropod foregut, like the anterior region, is divisible into dorsal and ventral developmental modules that have different morphological, functional and ontogenetic fates. In larvae of C. lividus, the ventral module of the middle foregut transformed into the anatomically novel venom gland of the post-metamorphic stage by rapidly pinching-off from the main dorsal channel of the mid-oesophagus, an epithelial remodelling process that may be similar to other cases where epithelial tubes and vesicles arise from a pre-existing epithelial sheet. The developmental remodelling mechanism could have facilitated an abrupt evolutionary transition to the derived morphology of this important gastropod feeding innovation.     

The auricularia-to-doliolaria transformation in two aspidochirote holothurians, Holothuria mexicana and Stichopus californicus

Abstract. The fragmentation and rearrangement of the ciliary bands that occurs during the auricularia-to-doliolaria transformation is described for the non-feeding auricularia larva of Holothuria mexicana and the more typical planktotrophic auricularia of Stichopus californicus. The ciliary band of the auricularia larva runs along a series of ridges that project from the sides of the body. Fragmentation results from a loss of ciliary band cells from the zones between the ridges. The remaining fragments then reorient, elongate, and fuse to form the 5 circumferential bands of the doliolaria. The fate of the band cells lost during this process could not be determined with certainty, but they disappear after being sequestered beneath the epithelium for a time, probably through histolysis. Cell counts indicate that significant numbers of cells are also lost from the ridges. Normal swimming ceases just before transformation begins, probably because the nerve supply to all or parts of the band is disrupted, and this may play a role in initiating morphogenesis.