Hidroemedusae from the middle and south Adriatic 1939 and 1940

Abstract

      Hydromedusae selected from trawl catches during fishery investigation in the middle and south Adriatic 1939 and 1940 have been classified and described.

      The first chapter contains the history of the research on the medusae of the Adriatic with a table IV. of all known hydromedusae hitherto found in this sea. In this table a division of the Adriatic in five regions is adopted, north Adriatic (off Trieste and west coast of Istria), Quarnero, middle Adri­atic (off Split), south Adriatic (off Dubrovnik and further southwards), high Adriatic (the deeper sea in the middle). There are 75 species of hydromedusae in the Adriatic as com­pared with 108 species in the whole Mediterranean. 15 species are known from the Adriatic, but not from Mediterranean.

       The methods used in plankton collecting and separating of the medusae are described. Petersens young fish trawl, Hensens fish egg net, Nansens net and the Helgoland net have been used. There are also data on conserving and preparing of medusae for microscopic specimens. The table V. gives the positions for trawl stations with some remarks on the charac­ter of the plankton.

       The medusae of Zanclea implexa  A l t m a n  and Cytaeis exigua                H a e c k e l  are described. A new variety of Bougainvillia autumnalis              H a r t l a u b  is described as var. magna with 8 mm wide and 7 mm high bell. For the Adriatic new species are: Laodicea undulata  F o r b e s  &           G o o d s i r, Eirene viridula  P é r o n  &  L e s u e u r  and Zygocanna sp. (only one very badly preserved fragment) .A new species Laodicea ocellata is described with the diagnosis: a smaller Laodicea, bell of globular form, diameter up to 3,5 mm, with 7-14 tentacles and 10-18 intertentacular bulbs without tentacles, with very large black ocelli on all bulbs, with short not undulated lips, with thick clubshaped gonads along the proximal parts of the radial canals. It is suggested that this small medusa with juve­nile characters but well developed gonads represents a neotenic form. Attention is called to the difficulty of identification of Obelia. There is a suggestion that our medusae belong to Laomedea (Obelia) dichotoma           (L i n n é). The very characte­ristic Obelia adriatica  N e p p i  has not been found. The taxo­nomic characters and the systematic position of Eirene viridula  P é r o n  &  L e s u e u r  and some similar medusae (Eirene pellucida  W i l l, Helgicirrha schulzei  H a r t l a u b) are discus­sed. Rhopalonema velatum  G e g e n b a u r  was found only in juvenile specimens which are here briefly described. A compa­rison with the descriptions of the immature form by previous authors points to the extreme probability that there might be in the Adriatic at least two hitherto not distinguished species under the name of Rhopalonema velatum. Aglaura hemistoma  P é r o n  &  L e s u e u r  is represented by small medusae which are distinct from the var. nausicaa  H a e c k e l  of the northern Adriatic. Liriope eurybia  H a e c k e l  is the most abundant and frequently found medusa in this material. Larger specimens resemble in the shape of gonads to the mediterranean Liriope mucronata, but the bells are typically thin-walled and flat. A series of observations shows, that with proceeding growth the gonads change in shape from egg-shaped to nearly heart­shaped and leaf-shaped. Larval medusae have been observed with 8 tentacles, never with 12 tentacles. A few abnormally developed specimens with 5 and 3 radiar-canals and gonads are described. In table III. and fig. 9 a reconstruction of the deve­lopment and growth-cycle of this species was attempted.

       In the systematic list  B r o c h’s  system has been followed with alterations proposed by  K r a m p  and  U c h i d a.

       The concluding chapter contains annotations on the biolo­gy, ecology and zoogeography of the adriatic hydromedusae. Attention is called to the temporary holoplanktonic life of some anthomedusae, in this material Podocoryne minuta. The most frequent holoplanktonic medusa is Liriope eurybia with a pro­bable time of propagation in the late autumn and early winter.

       It is suggested that the life cycle of the characteristic holoplanktonic medusa in the Adriatic Rhopalonema velatum is going on in a circle-current; beginning from the mature form in the high Adriatic, from where eggs and immature forms are conveyed by cold deep currents of a high salinity during spring and summer into the offshore waters. A similar life-cycle in circle-currents has been described before for some copepods in the Adriatic. There is a three months shift in the maxima of frequency of our holoplanktonic medusae as com­pared with the known maxima in the northern Adriatic, here occuring later in the year.

      Meroplanktonic medusae have been found in 21 species, of them 7 more frequently. Hydroids of these medusae are known in the Adriatic for 4 species only. Ecological explanation are given for the fact that some medusae found by previous authors in the Adriatic frequently on special shore formations are not contained in our material. Other common medusae li­ving in deeper waters were found as frequently as could be expected from the previous knowledge of their ecological value.

       Some ecological correlations have been found from the comparison of the data in the table 1 in 2. Maxima and minima of temperature and salinity in different depths, mean values of these data for all depths on the one side and maxima and minima of the^; frequency and abundancy of the observed medu­sae on the other side have been compared. Sudden falls of tem­perature and salinity were found to be associated with maxima of Stomotoca, Phialidium, Eutima, Podocoryne minuta and Rho­palonema. Temperature minima coincide with the minima of abundance for the common medusae. During the fast tempera­ture elevation in the spring attain Steenstrupia nutans and Laodicea undulata their maxima and Phialidium variabile its second maximum. The maximum of the plankton volume in summer was due to heavy rains, winds and waves; it is inter­resting that it was composed almost exclusively of hydrome­dusae (Eutima gegenbauri). Temperature maxima in summer show a close correlation with abundancy of Steenstrupia, Zanclea, Solmundella, Saphenia. Liriope eurybia appears in great numbers soon as the surface waters have been warmed up. The maximum of salinity in 40 m was associated in 1940 with the absolute maximum of plankton volume, which was mainly com­posed of salpae. The occurence of Zygocanna just then in this warm salted current might so find an easy explanation. Du­ring the summer an increase of temperature and salinity is accompanied by a marked decrease of whole plankton and medusae. Such minima have been observed also in the spring during a state of homohalinity due to heavy winds and vertical currents.

      There are some remarks on the size of the adriatic medu­sae. On page 57 the widths of umbrellae for 5 species are com­pared, being the smallest in our material, larger in the medu­sae from the gulf of Trieste and still larger in specimens from the northern seas. The reverse is true for Bougainvillia autumnalis, which is larger in the middle Adriatic and very small in the North Sea.

      The observed medusae belong to a typical warm water fauna. A few species, which are abundant in the gulf of Trieste, have not been found here. On the other side, there are most frequent medusae which are characteristic for a southern fauna, so Laodicea undulata and L. ocellata. There is also a group of species with a interesting discontinuity of distribu­tion, found hitherto in the Adriatic and in the Atlantic or in northern seas or in arctic waters but not in the Mediterranean. By previous authors no due attention was paid to this group of Adriatic medusae. A suggestion is made that at least a part of these might be considered as relics. The difficulties of a conclusive proof are also shown.

       In connection with this problem it is urged that much more research is required in the Adriatic on the ecology and biology of hydroids and medusae as a whole. Suggestions are given for the planning of this research which might be fruitful also for practical purposes, owing to the possibility of using medu­sae as leading forms for a rapid and precise characterisation of a given plankton.