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Site opening on 28 July 2022
Wonderland
Jellyfish came into the limelight in the life science world with the discovery of Dr. Osamu Shimomura, the 2008 Nobel Prize winner in Chemistry. The jellyfish was used as the source of the protein GFP, which emits green fluorescence. A photograph of this jellyfish graces the cover of Molecular Cloning 4th ed. Related species is found in Hakodate. The background photo is a shot of sample taken by Assistant Professor Tanaka.
I saw jellyfish display at the Shinagawa Aquarium. It gave a refreshingly cool feeling in the summertime. The aquarium was a wonderland showcasing jellyfish.
GFP has become an indispensable tool in the life sciences. I have used it to study the ecology of marine bacteria, and I will never forget the moment I saw living cells emitting a green fluorescence drifting into view in my microscope. It was truly a wonderland under the microscope.
Assistant Professor H. Tanaka, who leads the Faculty of Fisheries Sciences at Hokkaido University in the biochemistry and molecular biology of jellyfish, has prepared jellyfish contents for this jellyfish project. In addition, Associate Professor A. Yamaguchi of the Plankton Laboratory in the Faculty of Fisheries Sciences, Hokkaido University, introduced us to advanced filming equipment for ecological research of jellyfish. Please enjoy this detailed, up-to-date information on jellyfish research.
FoM Editorial
28 July 2022 posted
Jellyfish
There are two types of jellyfish: those belonging to the phylum Cnidaria, such as the moon jelly, Nomura’s jellyfish, and the sea wasp, and those belonging to the phylum Ctenophora, such as the comb jellyfish. Both are primitive invertebrates that first appeared on Earth more than 500 million years ago.
One of the evolutionary biological features of these jellyfish is that they were the first animals to acquire nerves and muscles. The acquisition of nerves and muscles enabled the animals to move efficiently even as they grew larger, and this is thought to have led to a diversity of behaviors such as predation, reproduction, competition, and symbiosis, in addition to migration, which in turn led to even greater evolutionary changes.
TANAKA Hiyoyuki・Faculty of Fisheries Sciences, Hokkaido University・Assistant Professor
28 July 2022 posted
Body of A Moon Jellyfish
The moonjelly is a member of the class Scyphozoa within the phylum Cnidaria. The body of an adult moonjelly is made up of more than 95% water, with a collagen gel surface covered by a microscopically thin cell layer. The functions of various organs such as muscles, nerves, digestive organs, and reproductive organs are concentrated in this thin cell layer.
The part that looks like a four-leaf clover is both the stomach and the gonad. From there, the water ducts (radial canal) radiate out, through which body fluids, including digested food, circulate throughout the body. The radial canal can be said to function as both an intestine and blood vessel. There is no heart, and circulation is powered by the movement of cilia on the inner wall of the radial canal. There are two types of radial canals: one in which luminal fluid flows from the stomach cavity to the limbus of the umbrella, and the other in which the fluid flows from the limbus back to the stomach cavity (Southward 1955) (Fig. 1).
Food is first captured by the nematocysts. The nematocysts are capsules containing venom needles, which are distributed throughout the body, but are especially abundant on the tentacles, oral-arms, and upper surface of the umbrella. The venom needles (stinging threads) in the nematocysts are hollow threads made up of geometrically precise folds of sheets of unique collagen of small molecular weight (Gold et al. 2019). Although the molecular-level mechanism by which the stinging threads are ejected is still unknown, it is believed that the pressure inside the stimulated nematocyst instantly increases to 15 MPa, causing the stinging threads to eject with great force from the ejection port. First, the triangular pyramidal spines emerge and pierce the prey. Next, a stinging filament is formed at the end of the spines, which expands while spraying venom through the hole. The stinging threads are extended while turning inside out, so that when they are fully extended, the sword-like spines are at their base (Tardent 1995) (Fig. 2). The venom is a mixture of neurotoxins and hemolytic poisons. The stinging threads do not penetrate the skin of the palm of the human hand, and it is neither painful nor itchy to touch the moonjelly.
The prey is entangled in the mucus on the body surface and collected by the movement of cilia on the edge of the umbrella, especially at the eight midpoints between the sensory organs. The oral-arms then ‘lick’ and collect the prey. The oral-arm resembles a ribbon folded in two lengthwise, and the food is carried through the inside of the fold to the mouth at the center of the underside of the umbrella. The food is then caught by the gastric filament, which are arranged in a horseshoe shape inside the stomach cavity, where it is digested. Undigested food is expelled, not simply in the reverse order of ingestion, but from a specific site near the base of theoral-arm.
The muscles of jellyfish, considered the most primitive of animals, are also categorized into striated and smooth muscles. When the striated muscle contract, the umbrella is squeezed into a squished shape and water is forced downward, while when it relaxes, the elasticity of the thick gel on the upper surface of the umbrella reverts it to its original shape. This process can be repeated for swimming (Fig. 3). On the other hand, the tentacles and oral-arms have smooth muscle fibers. Similar to the vertebrate muscles, jellyfish muscles contract when actin filaments and myosin filaments slide against each other. Its energy source is ATP, and myosin gradually tugs on the actin filaments while ATP is hydrolyzed into ADP and phosphate. Extracting and studying actomyosin from the striated muscle of the moonjelly, it was found that myosin is unique in that it has both striated and smooth muscle characteristics (Tanaka et al. 2018)4). The regulatory mechanism of contraction is also an enigma, as no component involved in the regulation is found that causes or stops contraction in response to stimulation from nerves (Fig. 3D).
TANAKA Hiroyuki・Faculty of Fisheries Sciences, Hokkaido University・Assistant Professor
References
Southward, A. J. (1995) Observations on the ciliary currents of the jelly-fish Aurelia aurita L. J. Mar. Biol. Assoc. United Kingdom 34:201–216.
Gold, D. A. et al. (2019) Mechanisms of cnidocyte development in the moon jellyfish Aurelia. Evol. Dev. 21:72–81.
Tardent, P. (1995) The cnidarian cnidocyte, a high-tech cellular weaponry. Bio essays 17:351–362.
Fig. 1. Body of moon jellyfish. A: Name of each part (underside). B: Schematic diagram showing the morphology of the radial canal. The radial canal is located on the underside of the umbrella; in fact, the distance from the underside to the radial canal is less than 1 mm in large specimens. There are two types of radial canals: one that drains fluid from the gastral cavity to the limbus of the umbrella (green), and the other that returns fluid from the limbus to the gastral cavity (purple). The underside of the umbrella is covered with myoepithelium containing muscle cells.
28 July 2022 posted
Life Cycle of the Moon Jellyfish
Jellyfish are either male or female, and they reproduce sexually to produce offspring. In the case of the moon jelly, the male releases a mass of sperm from its mouth (Fig. 4A), which is then taken into the body by the female, resulting in fertilization. In other words, internal fertilization occurs. The "mass" or "mass production" of many individuals of the jellyfish is sometimes seen as a problem, and it is thought that this density is intended to increase reproductive efficiency. From the fertilized egg to the first larva, the planula, development proceeds in the incubation sac that develops near the base of the female's oral-arm (Fig. 4C). The planula is about 0.2 mm long and rotates and moves by moving cilia that grow all over its body (Fig. 4D). They have no mouth and do not eat anything, but they already have nematocysts.
Planula eventually metamorphose into small polyps that attach to reefs, shells, and other substrates. The polyp looks like a small anemone with four to 16 tentacles around its mouth (Fig. 4E). At this time, muscle (smooth muscle) develops for the first time in the body, and the tentacles capture zooplankton and carry it to the mouth, where it is swallowed whole. A major characteristic of polyps is their ability to clonally proliferate infinitely (?) by division and budding, i.e., asexual reproduction. They also have the vitality to withstand months of fasting. Furthermore, even if a few polyps are grinded and dispersed down to the cellular level, the next day they give rise to spinning cell masses that eventually regenerate into hundreds of microscopic polyps (Fig. 5).
When the water temperature drops, the polyps of the moonjelly begin a metamorphosis called strobillation (Fig. 4F-H). Numerous nicks form below the polyp's tentacles, which eventually become deep slits that resemble many overlapping plates (strobilae). As the tentacles degenerate and the color turns brown and transparent, each ‘dish’ becomes an ephyrae larva that begins to beat from the apical side and eventually breaks free (Fig. 4I). About 7 to 14 ephyrae occur from a single polyp.
With strobillation, the striated muscles are generated for the first time, and the ephyra beat and swim using two types of the striated muscles: radial and circular muscles. The petal-like marginal lappet are usually eight in number, but occasionally individuals with ten or twelve occur. Ephyra develop into juvenile jellyfish (Fig. 4J) through a form called metephyra, which takes several months to reach adulthood. The beating cycle slows down as they grow. After a certain degree of growth, the reproductive organ develops, and the sexes are characterized as male and female. In the case of moonjelly, the mechanism of sex determination is unknown, but it is known for a species of the genus Chrysaora, also in the class Scyphozoa, that they are hermaphrodites. The life span of adults is often said to be a few months, but individuals collected from the sea and kept in aquariums have lived for three years. There is no clear moment of death; their food intake gradually decreases, they stop beating, their body size decreases, and they eventually simply disappear.
TANAKA Hiroyuki・Faculty of Fisheries Sciences, Hokkaido University・Assistant Professor
Fig. 4. Life cycles of a moon jelly. A: Spermatozoa released from the oral-arm of a male show sperm masses with tails entangled with each other and shimmering (scale bar 100 µm). B: Oocytes of various sizes are seen in the ovary of the female. The nucleus (appears somewhat brighter) is in an unbalanced position. C: Eggs during development collected with a pipette from the incubation sac of a female. D: Planula larva free from the incubation sac. E: Polyps. F-H: Polyps 26, 28, and 32 days after a 10°C reduction in rearing temperature, respectively. I: Free ephyrae. J: Juvenile jellyfish with an umbrella diameter of 1 to 1.5 cm.
28 July 2022 posted
Advanced Technology for Quantifying Jellyfish Density: Horizontal and Vertical Distribution of Large Jellyfish Using a Video Camera
Large jellyfish is a competitor with zooplankton-feeding fish and consume both fish eggs and larvae, and thus have a significant impact on fish stocks. There are several problems with methods for quantifying large jellyfish in the field. Because of their fragile bodies, jellyfishes are damaged by plankton and trawl nets, are cause a significant underestimation of their abundance, biomass, and diversity. To overcome this problem, quantifications without damage such as the use of acoustic cameras and video cameras mounted on remotely operated vehicles are developed recently. Vertical and horizontal distributions of large jellyfish species and changes with a year in the northern Bering Sea, where recent sea ice retreat has been significant, were revealed during the summers of 2017 and 2018 using a frame camera (Figure 1) with an underwater video camera mounted on the frame.
The comb jellyfish species Bolinopsis infundibulum and Beroe sp. and the cnidarian jellyfish Chrysaora melanaster were abundant in this area. B. infundibulum, the most dominant species, was distributed mainly in the northern and western waters of St. Lawrence Island. In the northern waters of St. Lawrence Island, B. infundibulum was distributed mostly in the upper layers of the water column, while in the southern waters of St. Lawrence Island, it was distributed in the deep layers of the water column (Figure 2).。
For yearly changes, the abundance of large jellyfish species was higher in 2017 and very low in 2018, about 1/20 (C. melanaster) - 1/90 (Beroe sp.) compared to 2017. Sea-ice retreat in 2018 was about a month earlier than in 2017. It is reported that the sea-ice retreat in 2018 was about one month earlier than in 2017. The early sea-ice retreat in 2018 delayed the formation of the stratification of the water column and the onset of the spring phytoplankton bloom, and zooplankton consisted mostly of small species, resulting in less production and therefore unfavorable food conditions for the growth and survival of polyp, the benthic generation of C. melanaster. This is thought to be the result of climate change. This climate change-induced interannual difference in the food supply could also be a possible reason for the sharp decline in 2018 of the comb jellyfish B. infundibulum and Beroe sp. which do not have a benthic generation.
YAMAGUCHI Atsushi・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
28 July 2022 posted
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