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The Spawning of Echinus Esculentus and Some Changes in Gonad Composition by F

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The Spawning of Echinus Esculentus and Some Changes in Gonad Composition by F 133 THE SPAWNING OF ECHINUS ESCULENTUS AND SOME CHANGES IN GONAD COMPOSITION BY F. C. STOTT, B.SC. Herdman Research Scholar in the University of Liverpool. (Received z6th August, 1930.) (With Six Text-figures.) I. INTRODUCTION. THE gonads of Echinus esculentus, by reason of their size and ease of dissection, are excellent material for a study of the biochemical changes involved in the maturing process of development. Through frequent visits to the Biological Station, Port Erin, Isle of Man, extending from November 1929 to July 1930, an attempt has been made to follow the changes in the condition of the gonads and correlate them with the sea temperature cycle on the one hand, and the water, glycogen, fatty acid, and to some extent the total carbohydrate content on the other. Throughout the latter work the sexes have been examined separately with a view to finding if any marked sexual differences in metabolism existed. All the sea urchins examined were be- tween 9 and 12 cm. horizontal diameter. II. THE HABITAT AND MIGRATION OF ECHINUS ESCULENTUS • AT PORT ERIN, ISLE OF MAN. There are two well denned regions at Port Erin where this sea urchin can be obtained throughout the year: (1) On the concrete blocks of the ruined breakwater, where at low water spring tides a minimum of twenty to thirty can be obtained using a 3-foot hand net. (2) From the Scallop Beds, a gravel patch, about a mile offshore in 20 fathoms of water, where scores can always be obtained using a drag net. The animals used for biochemical work were all freshly collected off the breakwater. In 1930 an inshore migration of Echinus was well marked at the end of February; it was approximately gauged by taking counts of the number of sea urchins seen on the inner portion of the breakwater from a rowing boat at dead low water at various times throughout the year. Apart from the variability in the number present, there is a possibility that both the state of the sea surface and the depth of the tide would affect such counts. Hence both these features were noted at the time of observation. F. C. STOTT Six comparable sets of readings are given in Table I and are shown graphically in Fig. i. Since large quantities of sea urchins are collected each Easter off the break- water both by students and fishermen, the inshore migration is really greater in magnitude than that recorded; the June count should doubtless be increased for the same reason. Table I. Depth of tide Approximate Date in feet number seen Sea surface 1929. Nov. 6 19 5° Calm 1930. Jan. 14 21 100 Calm Feb.14 22 200 Calm „ 27 20 400 Calm Mar. 31 22 400 Calm after rough cold spell June 27 22 200 Calm The distribution of Echinus at Millport as described by Elmhirst (1922 b) closely resembles that at Port Erin. Elmhirst points out that a similar shoreward spawning migration is known to occur in eight other invertebrates, chiefly boreal species. Occasionally single individuals were obtained high and dry on the breakwater in November, December and January at dead low water springs. In February and March as many as forty stranded individuals have been seen on the portion of the breakwater kept under observation. A summary of records of the occurrence of Echinus on the foreshore in the British Isles is given by Orton (1929). E. esculentus is completely absent from the foreshore in the Plymouth district and north Cornish coast, whilst at Millport and Port Erin reliable records show its regular occurrence there. At Robin Hood's Bay, Echinus occurs low in the intertidal zone at equinoxial springs. Orton (1929) suggests that there may be a critical temperature above which natural spawning takes place. At Plymouth such a temperature is attained in March in offshore waters and, moreover, exposure of the foreshore at low water springs always occurs here about noon in the heat of the day. At Port Erin and Millport, by reason of their more northerly latitudes, offshore sea temperatures are lower than at Plymouth: inshore migration results, therefore, in the exposure of some animals to a zone of higher temperature particularly at low water springs, which here occur in the early morning and late afternoon, and at the same time avoids the high temperatures associated with the noonday sun. It is perhaps, however, significant that this inshore migration takes place in late February when the gonads are depleted of their chief carbohydrate reserve—glycogen—and provides an oppor- tunity for a more abundant carbohydrate diet, since the richer Laminarian zone is invaded. III. THE SEASONAL CONDITION OF THE GONADS AND SEA TEMPERATURE. In November, December and January it was impossible to tell at sight the sex of individuals. Sperm could always be obtained by teasing up a piece of testis in sea-water, but there was never sufficient present to give the characteristic white sperm suspension on puncture of the gonad. During this period the ovaries were The spawning of Echinus esculentus 135 firm and transverse section revealed the presence of oocytes embedded in a germinal matrix, each with a nucleus and vesicular space. In February, puncture of the testes resulted in the outflow of a little sperm suspension: the ovaries, too, were more mature, as shown by their gelatinous appearance and ease of fracture. Judging from general appearance the gonads were ripe at the end of February. The first recorded spawning occurred on February 27th—a female which had been kept in the tanks for 2 days. The resulting eggs were fertilised with sperm obtained by cutting a testis, and successful development took place to at least the gastrula stage. Throughout March and early April there were constant spawnings of individuals kept in the tanks: this can only be taken however as indicating the ripeness of the gonads. No spawnings of individuals on the breakwater were observed, and no spent individuals were obtained at this time. Prof. Orton informed me that between April 9th and 22nd the gonads of all Echinus examined by the Easter Class were full, i.e. natural spawning had not taken place. All gonads of a total of sixty individuals examined at the end of June and beginning of July were spent. It was at this time extremely difficult to distinguish the sexes, since very few active sperms were present in the testes and numerous clear gonocytes, which at first sight looked like young oocytes, were present. In the spent ovaries there were a few eggs, together with numerous small cells resembling senescent spermatozoa and larger clear spherical cells. It was therefore concluded that natural spawning had taken place in late April, May and perhaps early June. Routine sea (taken from the breakwater steps) and air temperature observations made each day by the Biological Station staff at 9 a.m. and 4 p.m. were recorded. By averaging each group of fourteen weekly observations the accompanying Table II and Fig. 1 were drawn up. Table II. Weekly means, sea and air temperatures, Port Erin Bay, 1929-30. Week Air Sea Week Air Sea ended (°F.) (°F.) ended (°F.) (°F.) Nov. 2 496 53-2 Mar. 1 41-8 44'S 9 500 53-i 8 45-2 45° 16 43 9 5i-4 IS 407 44-8 23 479 5i-3 22 39-3 437 3° 482 5i-3 29 466 447 Dec. 7 47-4 51-2 Apr. s 468 45'1 14 466 50-2 12 467 45'2 21 44-1 497 19 467 457 28 429 487 26 511 460 Jan. 4 459 485 May 3 55-6 463 11 437 48-1 10 52-8 471 18 43 7 471 17 SS-6 479 25 44-5 474 24 S7-i 49'4 Feb. 1 39'2 464 3i 64-2 U 8 39'4 45-3 677 5 4 June 7 C xI "C 15 J J 39-6 446 14 643 C2" 1T 22 387 44'5 21 687 5* 28 649 529 54-6 Since no observations were made from early April until the end of June, the time of natural spawning, i.e. of the first occurrence of spent individuals, is not known. 136 F. C. STOTT Elmhirst (1922 a) states that spawning continues until the end of August at Millport but not only were the gonads practically free from eggs and active sperm in all individuals examined at the end of June at Port Erin but their water and glycogen contents were vastly different from those of ripe individuals. Orton (1920) has shown that a broad correlation exists between sea temperature and breeding in some marine animals. The European oyster—Ostrea edulis—begins to spawn at a mean temperature of i5°-i6° C. throughout its geographical range, and continues to produce sexual products so long as the temperature remains above that figure. In the case of the mackerel, Scomber scomber, spawning occurs above 12-130 C. throughout its geographical range. Amirthalingam (1928) has shown that in Pecten opercularis, at Plymouth, spawning takes place periodically whilst the temperature is below the critical temperature of n° C. c 70 r400 COUNTS of ECHINUS on 65° .200 BREAKWATER 60° 55C MINIMUM 50c SEA TEMP 45C 40< NOV. JUNE Fig. 1. Weekly means, sea and air temperature, Port Erin Bay, 1929-30, and spring inshore migration of Echinus on the breakwater. In Echinus, natural spawning occurred at Port Erin during 1930 in late April, May and perhaps early June, when the sea temperature is rapidly rising between 460 and 510 F.
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