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Observations on the Swarming Habits and Lunar Periodicity Of<Emphasis OBSERVATIONS ON THE SWARMING HABITS AND LUNAR PERIODICITY OF PLATYNEREIS SP. FROM THE MADRAS HARBOUR. BY R. GOPALA AIYAR, M.A., M.Sc., AND N. KESAVA PANIKKAR, M.A., M.Sc. (From the University Zoological Laboratory, Madras.) Received April 20, 1937. Introduction. WHILE collecting specimens from the Madras I-Iarbour on a new moon night during March 1936, a number of worms were found swarming on the surface, causing brilliant phosphorescence in the water. Several specimens of this worm were collected with a tow-net, and later examined in the laboratory, which showed that they were heteronereid phases of Platynereis sp., a Poly- chmte belonging to the family Nereidm. Since the phenomenon of swarming is of considerable biological interest, regular observations were subsequently made, and a periodicity in relation to lunar phases was discovered in the behaviour of these Polyct~etes. So far as the authors are aware, periodic swarming of Heteronereids has not hitherto been recorded from the Indian coasts, though in a Syllid, Gravely (1927) once observed swarming and phos- phorescent display at Krusadi Islands. The following account of the I-Ieteronereid stage and the lunar periodicity in the swarming habits of this species is submitted with the hope that this will lead to further observations on similar phenomena that may occur also in other localities on the Indian coasts. Literature. Swarming behaviour of Polychmtes has received the attention of several investigators in Europe and America ; many Nereids and Eunicids are known to swim at the water surface in large numbers, when they are sexually mature and are ready to discharge their reproductive elements. Each worm is believed to take part in swarming only once, as in most cases it dies after the extru- sion of the ova or spermatozoa as the case may be. Most of the recorded instances of swarming take place during the night ; but instances of swarm- ing during daytime are not unknown (Verrill, 1873; tterpin, 1928). The swarming may be annual, occurring only once in the year, and that for a 245 246 R. Gopala Aiyar and N. Kesava Panikkar very restricted period, or it may occur several times during the year, between definite intervals. Thus Mayer (1908) found that the Atlantic Palolo (Eunice fucata) swarms only once in the year, the annual breeding season being only one to six days' duration, occurring in the third quarter of the moon between June and July. Izuka (1903), Friedlander (1898) and Horst (1905) have described a somewhat similar behaviour in three other species, Ceratocephale osawai (Japanese Palolo), Eunice viridis (Pacific Palolo) and Lysiclice ode (Wawo of Malay Archipelago) respectively. While in the Eunicids the reproductive period is more or less annual, in many of the Nereids, the re- productive season extends for a considerable part of the year, during which successive swarms take place after regular intervals. Hempelmann (1911) found the Heteronereids of Platynereis dumerilii swarming at Naples from the month of October to May, during the first and third quarter of the moon. At Woods Hole, both Nereis limbata (Lillie and Just, 1913) and Platynerds megalops (Just, 1914) are known to swarm regularly, the first especially for a fairly long period. In Japan, another Nereid, Nereis japonica swarms in large numbers during December (Izuka, 1906). Numerous other accounts of Nereids are given by Gravier (1901, 1923), Fage (1923, 1926), Fage and Legendre (1923), Rene Herpin (1923, 1928, 1929), Horst (1911), Ranzi (1931) and Gallien (1936) ; of F,unicids by Woodworth (1907), Whitmee (1875) and Treadwell (1909) ; and of Syllids by Fraser (1915), Galloway and Welch (1911), Herpin (1924) and Potts (1913). Amongst these, the only account of swarming of Nereids from the Indo-Malayan coasts is that of Horst (1911) on Platynereis polyscalma (vide Fauvel, 1932 for synonymy of the species). Fauvel (1932) in his report of the Annelida Polychmta of the Indian Museum, mentions that several epitokous individuals of Platynereis polyscalm~ have been collected from the water surface at Nankuari Harbour, and this is a sure indication that swarming of this species takes place in this locality. Description of Heteronereids. General.--The worms are reddish-brown when alive and are usually 3 to 5 cm. long. Most of the specimens collected were transparent since the body cavity was empty, the germ cells having been shed. The body of the worms consists of an anterior unspecialized part and a posterior and longer epitokous region with highly flattened parapodia bearing large oar-shaped setm. Head.--The head is prominent and the prostomium is obtusely rounded. There are two pairs of eyes ; they are considerably enlarged and are provided with lenses. The two pairs are situated quite close, but they do not touch. All the eyes are alike in structure, but the posterior pair is slightly the larger. Swarmizg Habits ~ Lu~mr Periodicity of Platynereis sp. 247 T~.xv-Fm. 1.--The head of the heterouereid of Platynereissp. • about 20. The eyes do not extend to the ventral surface and are not seen in a ventral view of the worm. In this respect, these specimens differ from Platynereis l)olyscalma and certain other species. The palps are present, but are not quite so prominent as in most heteronereids. The basal joints are swollen and are much longer than the terminal ones. All the four tentacular cirri are present in some individuals, but as they break away easily, most of the specimens examined do not have the full quota of cirri. The longest of them normally reaches the base of the fifth parapodium. Annulations are not visible on the tentacular cirri. The first or the uppermost pair is the smallest and reaches to the base of the second parapodium. The number of segments observed in six typical specimens is given in the following table :-- Number of Total number Anterior Posterior worm of segments atokous part epitokous part 1 56 25 31 2 60 20 40 3 67 22 45 4: 71 21 50 5 85 23 62 6 132 22 110 132 F 248 R. Gopala Aiyar and N. Kesava Panikkar The Atokous Region.--The anterior atokous region usually extends to 20th-25th segment. As mentioned previously, the parapodia are unspecialized here, and are insignificant as compared with those that follow in the epitokous part. The structure of the tenth parapodium which may be taken as more or less typical for this region, is as follows. The dorsal cirrus is long and exceeds the length of the dorsal and ventral lobes of the parapodium. It is slender, arched and not crenulated. The dorsal lobe bears three ligules and a single aciculum with 8-10 homogomph setm. The terminal portions of the blades are minutely serrated. Of the three ligules, the middle one is the smallest. The corresponding three ligules of the ventral lobe of the parapodium form a cluster with their tips standing widely apart. The dorsal part of the ventral setigerous bundle consists entirely of homogomph setm with their blades stouter than the corresponding ones of the dorsal lobe; and the edges of the blades are coarsely serrated. The setm are fewer in number in the ventral fascicle of the lobe, being about 7-8, and they consist entirely of heterogomphs, with shorter and coarsely serrated blades. The ventral cirrus stands apart from the rest of the parapodium. The Epitokous Region.--There is considerable difference in the structure of the parapodia of the different portions of the epitokous region. The parapodia from the 2nd, 80th and ll0th segments after the commence- ment of the epitokous transformation are described below from a complete worm. In the second segment, the dorsal ligule is broadly lanceolate. Into the base of the dorsal lobe the aciculum projects, and the lobe is longer than the dorsal ligule. Lamell~ are not found at the base of the ligule. The dorsal cirrus of this segment and those of the few nearby are not crenulated. The lower half of the parapodium consists of two parts: a well-developed ventral lobe, and extending beyond it, a heart-shaped lamella. The ventral ligule is quite distinct and lanceolate in shape. No appendage is seen at the dorsal side of its base. At the base of the ventral cirrus and especially on its dorsal aspect, there are two leaf-like processes. Ventrally, there is a lobed cordate structure, the bifurcated lamella of some authors. So far as the setee are concerned, there is not much difference between those occurr- ing in the upper and lower bundles. The blades start almost from the bases of the lobes, differing from what Gravier (1923) and Horst (1911) noted for other Heteronereids. In the parapodium from the 80th segment, which is about the middle of the epitokous region, the dorsal cirrus is distinctly crenulate, with five papillae on the inferior border. The cirrus projects beyond the notopodium Swarminr Habils ~ Lunar Periodicity o[ Platynereis s~. 249 Tgx~-Fm. 2.--Drawlngs of parapodia of the heteronereid of Platynereis sp. A.-Parapodium of the 10th segment from the atokous region. • 50. B.-Parapodium from the 80th segment, from about the middle of the epitokous region. • 40. for about a third of its length. Dorsal to its base is seen a rounded lamella. The ventral cirrus has a bifurcated process on the upper side, above which the ventral ligule projects freely. There is a large ear-shaped lamella below the ventral cirrus. This is much bigger than the corresponding one de- scribed for the 2nd segment of the epitokous region. The parapodium of the ll0th segment is very similar to that of the 80th. Only four papillm are noticed on the inferior border of the dorsal cirrus.
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