Notes on the Rearing of Chrysaora Isosceles in an Aquarium

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  • Notes on the Rearing of Chrysaora Isosceles in an AquariumAuthor(s): Maude J. DelapSource: The Irish Naturalist, Vol. 10, No. 2 (Feb., 1901), pp. 25-28Published by: Irish Naturalists' Journal Ltd.Stable URL: .Accessed: 18/06/2014 19:41

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    Jo ace, page 25.

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  • February, I90i. 25



    ON June 2 Ist, I 899, 1 picked up a damaged specimen of the jelly.

    fish Chtysaora isosceles on the shore of Valencia Harbour, and

    placed it in an aquarium for examination. On the following

    day I saw numbers of tiny particles moving about in the

    water, and found that they were ciliated planule, which had

    been liberated from the medusa. On June 27th the planule were attaching themselves to the 'glass and hanging down

    from the surface film of the water.

    Two days later the tentacles began to develop, showing that

    the free-swimming planula stage was over, and that a fixed

    hydroid-like stage, known as the Scyphistoma, had commenced.

    At first the Scyphistotnme had four tentacles, then four more

    appeared, one midway between each of the first tentacles, and

    later on eight others to make up the full number of sixteen.

    Some of the Scyphistomae had their full number by July 13th.

    I kept a large number of these Scyphistomae in an ordinary

    I2-inch bell-jar throughout the winter. About twice a week

    some fresh sea-water was put in. A supply of copepods was

    kept in the bell-jar, but the Scyphistomae, I found, preferred

    to feed upon small medusa, such as S'arsia, and little cteno

    phores-Pleurobrachia. In December a few of the Scyphis.

    tomae budded young ones from the base of the polyp. On April 3rd, I9oo, I saw an Ephyra swimming in the bell.

    jar containing the Scyphistomae, which I had reared from the

    Chrysaora taken during the previous summer, and on looking

    next day five more were found. It was evident that they bad

    only just been liberated from a Scyphistoma, as two specimens

    were still united. On April 14th I found a Scyphistoma just

    beginning to start the process of segmentation, and kept it

    under observation. The tentacles of the Scyphistoma were

    gradually absorbed, and the animal changed in colour from

    white to pink. A series of transverse rings next appeared,

    dividing the body into a number of segments. Each segment

    developed into an Ephyra. Segmentation began at the anterior

    end of the polyp, and proceeded downwards, so that the

    Ephyrne were liberated one after the other, but occasionally in A

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  • 26 The Irishi Naturalist. February,

    the struggle for freedom two or three came off together, and

    separated afterwards. A Scyphistoma in the process of segmentation is known as

    a Strobila. In the specimen under observation, Ephyre were

    liberated three days after the commencement of strobilization.

    The Ephyre when first liberated were pinkish, but soon

    changed to a translucent white. They measured aboult 2 mm.

    in diameter. The disc or umbrella was flat and divided into

    eight marginal lobes, each bearing a sense-organ (tentacu

    locyst). The next step was to try the experiment of rearing

    an Ephyra up to the adult stage, and to see how large the

    medusa would grow in confinement. On April 17th six

    Ephyre, liberated on 3rd and 4th April, were placed in a

    bell-jar containing some Sarsia tubulosa; the latter were soon

    eaten, in spite of their being much larger and more active

    swimmers. By April 22nd the largest Ephyra was beginning

    to assume its adult form.

    The umbrella measured half an inch in diameter, and four

    tentacles had developed on the margin. The tentacles when

    fully expanded were about 4 inches in length. The circular

    mouth of the Ephyra was now surrounded by four oral arms. Their food supply consisted chiefly of small medusee, which

    were greedily devoured. The largest of the Ephyrae

    (measured on April 22nd) increased in size more rapidly

    than its companions, and proved itself to be the survivor of the fittest by eating them. On May i6th the survivor measured

    Ji inches across the umbrella. The oral arms, now frilled,

    were 24 inches in length. Four more tentacles had appeared, and others were just visible, one on each side of every sense

    organ. By May 22nd it had reached the normal form of the adult

    (just seven weeks old) having 24 marginal tentacles. The

    umbrella measured 24 inches in diameter, and the frilled oral arms 5 inches in length. On June 4th the brown markings on

    the top of the umbrella began to appear. The umbrella now

    measured 31 inches across, and the oral arms 9 inches. By

    June 13th it had become too large for its bell-jar (12 inches

    across and 8 inches deep), so it was removed to another I4 inches across and I2 inches in depth, holding when full about

    4 gallons of water. At this stage the umbrella was 5 inches

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  • I9OI. DrmArP.-Rearing oj Chrysaora isosceles. 27

    in diameter. On June 2 iSt the umbrella had increased to 64 inches in diameter and 3 inches in thickness; the frilled

    oral arms extended to the bottom of the bell-jar. The colouring

    of the marginal lobes and the top of the umbrella was now

    as bright as in a specimen taken from the sea-a rich dark

    brown colour. About July 8th the medusa-13 weeks old-reached its

    maximum growth. The umbrella was 9 inches in diameter, the oral arms and tentacles extended to the bottom of thle bell-jar. The gonads were visible, showing quite yellow through the umbrella.

    The difficulty of obtaining a sufficient food supply owing to

    the stormy state of the weather, and the increased tempera

    ture of the water, gradually affected the health of the medusa.

    It began slowly to decrease in size, the umbrella became

    smaller in diameter, and the oral arms and tentacles shorter.

    It spent, too, a great deal of its time at the bottom of the bell

    jar bumpingr the top of the umbrella upon the gravel, instead

    of swimming round and round at the surface. By August 13th

    its condition became critical, and as it was not likely to live

    much longer I preserved it in a solution of formaline. The

    umbrella had decreased to 6 inches in diameter.

    The chief trouble connected with the rearing of this medusa

    was to obtain a sufficient supply of food; its appetite was

    enormous. I soon found out what kind of food Chrysaora

    preferred by placing different pelagic animals in the bell-jar,

    and watching the result.

    It had a great liking for small Antho-meduse and Lepto

    medusre, such as Corymorpha, Margelis, Sarsia, Amphinema,

    Phiaiidium, Laodice, Euchilota, &c.; also for the siphonophore

    Agalmopsis, and the ctenophores Pleurobrachia and Bolina. It had no objection to Tonzop/eris and Sagitta. There were,

    however, two animals it would not touch, even after a few days'

    starvation-the antho-medusa Tiara pileaa, and the cteno phore Beroe ova/a. It is well known that young fish are often

    found under the umbrella of the large Scypho-medusae,

    especially Pilema (Rhizostoma) octopus. One day I put a small

    fish, about an inch in length, into the bell-jar. The medusa

    caught it and held it for some time in the frills of the oral

    arms, but finally let it go without doing any harm to it. I

    kept afterwards two small fish for some weeks in the bell AZ2

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  • 23 The Irish Naturalist. February,

    jar along with Chrysaora, but it never attempted to catch

    them. Copepods and other crusta