Hannerz, Lennart. (1956). Larval development of the polychaete families Spionidae Sars, Disomidae Mesnil, and Poecilochaetidae n. fam. in the Gullmar Fjord (Sweden). Zoologiska bidrag från Uppsala. 31: 1-204.

50136

Hannerz, Lennart

1956

31: 1-204

Publication

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Summary: "1. The investigations have been carried out at the Gullmar Fjord on the west coast of Sweden. 2. The greater part of the examined larvae have been obtained from plankton. They were caught in plankton nets of different construction, fitted with filter cloth or varying coarseness. Vertical nettings have been carried out, and the number of individuals of different species has been counted in order to get an idea of the relative frequency during different days and seasons. 3. A number of experiments have been carried out with rearing from the egg to the adult stage. 4. Larvae and young benthonic stages have been figured from living specimens. 5. The larvae have been reared in small culture-dishes until their metamorphosis was obtained. The young benthonic stages were allowed to live until identification of the species was possible. 6. The pelagic larvae of Nerine cirratulus, N.foliosa, Nerinides tridentata, and two heretofore unidentified Nerine larvae are described. 7. The pelagic larva of Laonice cirrata is described. It is shown that the Laonice larva is identical with Häckers (1896) "Chaetosphaera" larva. 8. The pelagic larvae of Aonides paucibranchiata and Aonides oxycephala as well as the young benthonic stage of the latter are described. The larvae have a developmental type which differs from that of the other Spionid larvae by their abundance of yolk. 9. The pelagic larvae and the young benthonic stages of Spiophanes bombyx and Spiophanes kroyeri are described. 10. The pelagic larvae and the young bottom stages of Prionospio steenstrupi, Prionospio cirrifera, and Prionospio malmgreni are described. In Prionospio malmgreni it has been possible to follow the entire development from the egg to the adult stage. 11. The pelagic larvae and the young benthonic stages of Scolelepis ciliata and Scolelepis girardi are described. A comparison is made between the larvae of the two closely related species S. fuliginosa and S. ciliata. 12. The justification in separating Spio and Microspio into two different genera is discussed. The author does not consider that Söderstroms's (1920) basis for distinction is adequate. Further investigations must, however, be carried out before a definite conclusion can be reached on this point. 13. The pelagic larvae and the young benthonic stages of Spio filicornis, Spio multioculata, Spio theeli, Microspio atlantica, and two different larval types of Spio martinensis are described. 14. It is shown that the fusion of Spio martinensis and Spio filicornis into one species, as proposed by Söderstrom (1920), cannot be upheld. It is discussed whether or not Spio martinensis should be divided into two species. These would be distinguished from each other through differences in regard to reproduction. 15. The development of Pygospio elegans is described. It is demonstrated that this species has a brood protection which varies in duration according to season. A relatively short period of parental care and a long pelagic stage occur in the late winter-early spring. During the summer the period of parental care is prolonged, and the pelagic stage is gradually shortened. Contrary to what has been assumed of late it has been found that this species has larvae of only one morphologic type. 16. The pelagic larvae and the young benthonic stages of Polydora ligni, Polydora flava, Polydora caulleryi, Polydora coeca, and the new species Polydora hermaphroditica are described. From Polydora (Carazzia) antennata the variety pulchra is removed as a separate species on the basis of the relatively great differences in regard to the larvae. The pelagic larvae and the young benthonic stages of these species are described. A peculiar type of hermaphroditism combined with neoteny has been observed in P. hermaphroditica. A comparison is made between the pelagic larvae of P. ligni and P. ciliata. In P. ligni, P. ciliata, and P. hermaphroditica a hitherto undescribed gland has been observed in connection with the modified bristles in segment 5. This gland is assumed to be of importance for the larva when it bores its first tube. It may produce an acidic secretion which dissolves the lime, facilitating thereby the work in boring. 17. Pelagic stages of Poecilochaetus serpens are described. It could be ascertained that the pelagic life of this species is divisible into two phases. During the first a ciliation and larval setae are found, during the second these organs are lost. The phases are called the metatrochophore stage and the nectosoma stage. 18. The pelagic larva and the young benthonic stage of Disoma multisetosum are described. 19. Sexual products, type of reproduction, and pelagic larvae of the examined species are compared. 20. According to the type of reproduction the species of the area of investigation may be divided into two main groups: I. Species with entirely pelagic development, II. Species where part of the development takes place in formed structures. To the former group belong the genera: Nerinides, Nerine, Aonides, Laonice, Spiophanes, Prionospio, Scolelepis, Poecilochaetus, and Disoma, to the latter the genera Spio, Microspio, Pygospio, and Polydora. 21. In regard to the manner of nourishment of the larvae the entirely pelagic larvae may be divided into lecithotrophic and planktotrophic. Only the larvae of the Aonides species are lecithotrophic. 22. Among the species which lay their eggs in formed structures the vast majority exhibit brood protection. In Spio filicornis and Spio martinensis, on the contrary, the formed structures are deserted by the mother. 23. The pelagic stage of species with brood protection may be extremely curtailed or totally Jacking. 24. It has been ascertained that eggs with a thick egg-membrane and characteristic vesicles penetrating into the plasm, which are caused by specialization in the membrane, are characteristic of the genera Nerinides, Nerine, Aonides, Laonice, Spiophanes, Prionospio, Scolelepis, Poecilochaetus, and Disoma. It is possible that similar eggs occcur· also in the polychaetc families Euphrosynidae and Amphinomidae. A theory is advanced of the function of the mentioned vesicles in fertilization as volume-regulating organs. Eggs of a simpler type occur in the genera Spio, Microspio, Pygospio, and Polydora. 25. Also in regard to the appearance of the sperms the generic groups enumerated in the foregoing differ from each other by the specialization of the sperms for an internal fertilization in the latter group. 26. It is shown that the larva of Disoma multisetosum has a prostomium which differs from that of the larvae of the other genera by its higher degree of coalescence with the peristomium. 27. In Poecilochaetus the prostomium of young larvae agrees with that of the Spionids. In older stages, however, the larva, develops a nuchal crest with a divergent form. 28. Two pairs of simple eye spots (red with one exception) are found in species belonging to the genera Nerine, Nerinides, Aonides, Laonice, Spiophanes, Prionospio, Poecilochaetus, and Disoma. In the other genera there are three pairs of eye spots, all of which are black. The most distal pair consists of so-called double eye spots. 29. The lateral parts of the peristomium in the genera Nerinides, Nerine, Laonice, Scolelepis, and to some extent Aonides and Spiophanes are well developed and clearly demarcated. Disoma deviates from the others in having a far advanced fusion of prostomium and peristomium. In the others the lateral parts of the peristomium are relatively weakly developed and not well demarcated from the prostomium. It is shown that during the later development the lateral parts of the peristomium pass over into the base of the palpi. The organisation of the vestibule discussed and it is shown that a ridge in the vestibular tegmen in Nerine, Nerinides, and Poecilochaetus is more strongly developed than in the other genera and that it grows into anteriorly directed, robust processes. 30. In all the treated species palpi appear already in the pelagic stage. In Nerine, Nerinides, Laonice, and Aonides they remain relatively insignificant during this period. In these genera as well as Scolelepis and Spiophanes they are attached far out on the lateral parts of the peristomium. In the others more medially. In Disoma and Poecilochaetus they are of different lengths. 31. The segments are formed gradually. The differentiation of the parapodia takes place earlier in certain species belonging to the genera Spio, Microspio, Pygospio, and Polydora than in the others. Branchiae are never formed in a segment which is not definitely branchial. The first-developed pair can however, be formed one segment posterior to the one which will become the first definitely branchial segment. 32. Two kinds of ciliation occur. One is bound to prolonged cells and forms distinct rows, the other is scattered over larger surfaces. Moreover, there are so-called "sensory cilia", which may appear separately or interspersed among cilia of the other types. 33. The prototroch is shaped as two rings of ciliated cells of which the anterior is reduced. In Nerinides tridentata and Disoma multisetosum the prototroch remains biserial during the entire pelagic life. 34. Nototrochs are as a rule found in Spionidae (exceptions: certain species of Nerine and Nerinides), but are lacking in Disoma and Poecilochaetus. They become best developed in Spio, Microspio, Pygospio, and Polydora. In some of these they are composed already in the larva of two rows of ciliated cells. They persist in the adult stage in the branchial segments. 35. Gastrotrochs occur in all species. In all genera except Scolelepis, Microspio, Pygospio, and Polydora they occur in all segments from the third or fourth onwards. In the enumerated genera they occur either regularly in alternate segments from the third onwards (Spio, Microspio) or irregularly. 36. The telotroch is uniserial in all pelagic stages, and occurs in all genera like the prototroch. 37. Certain cells of the nototrochs are differentiated to so-called "grasping cilia" for the purpose of holding together the larval setae while swimming. They appear to be best developed in the genera enumerated in paragraph 35. 38. The neurotroch, which in type differs from the other trochs, is a short ventromedial ciliation which never extends more caudad than through segment 2. It may be lacking (e.g., certain Polydora species). 39. The neurotroch ends as a rule in a ciliated cavity ("ciliated pit"). This structure is situated in segments 1, 2, or 3. It is unique for, and characteristic of the treated species. On either side of this structure there occurs in certain species (Nerine, Scolelepis, Laonice) a pair of ciliated elevations, which are reduced in conjunction with the metamorphosis like the rest of the "ciliated pit" complex. The "ciliated pit" usually constitutes the common outlet for two systems of mucous glands. The anterior system consists of long epithelial tubes, which through a common median, ciliated passage open into the anterior part of the "ciliated pit". Unicellular mucous glands open into the posterior part. In Disoma presumably glands are totally lacking. In the other species the number of anterior glandular tubes may be reduced and/or the posterior unicellular mucous glands may be totally lacking. In Prionospio malmgreni the "ciliated pit" is formed relatively late in the 2-segment stage. A theory is advanced that the "ciliated pit" is a palingenetic organ without, greater biological significance for the larva. The similarities between the "ciliated pit" complex and the foot glands of the prosobranchs have been pointed out. 40. Nuchal organs are found in all the examined larvae, with two exceptions (Disoma multisetosum and Laonice cirrata). They are formed on either side of the nuchal crest and are grooved-shape (longitudinal or transversal) or oval. In Poecilochaetus serpens, which has groove-shaped nuchal organs as a younger larva, they undergo a very marked development before the larva passes over into benthonic life. In the genera Spio, Microspio and Scolelepis the nuchal organs, after at first having been parallel, acquire after the metamorphosis a very characteristic horseshoe shape. 41. The dorsal organs are developed relatively late only. Such have, however, been observed in one pelagic larva (Spio martinensis Type II). 42. The pigmentation in the described larvae is of two main types, namely (I) structured, and (II) structureless. Of the former occur above all melanin and a yellowish white pigment. The latter may comprize a series of different colours. The structureless pigment, and to a certain extent the melanin are most markedly developed in prostomium, pharynx, and anus. 43. The larval setae are formed earliest, and attain their greatest length in the first setiger. All larval setae of the examined species are beset with serrations of varying coarseness. The form and coarseness of the bristles varies greatly. The appearance of the larval setae seems to have little systematic significance. 44. Bristles of the adult type are formed already in larva. They have, with a few exceptions, the same appearance as the definitely permanent bristles. Ventral crochets are found most often more anteriorly in the body of the larva than of the adult. These anterior crochets are successively lost. There is a tendency that the larger the species becomes as adult the greater the number of anterior segments which will shed their crochets. 45. So-called 'bacillary glands" occur in the epithelium in many of the described larvae. 46. On the basis of the morphologic character of the larva three divisions can be distinguished: 1. specific larval characteristics. 2. Larval characteristics which are transformed (metaplastic growth) into adult ones. 3. Adult characteristics (growth without transformation). In a system based on the larval morphology groups 1 and 2 are of the greatest interest. 47. The different groups of characteristics are not wholly independent of each other. 48. The type of development has been declared by a series of investigators to have significance for the larval morphology. This seems to be the case to a lesser degree in the treated larvae. Development through brood protection is attended by reduction of the thickness of the egg-membrane and elimination of the "membrane vesicles". 49. The development of the three families treated agrees in essential points. 1. The egg, with some exceptions (see the foregoing) is a characteristic type with a thick egg-membrane and membrane vesicles. The larvae have: 2. Long, serrated, larval setae, which are longest in the first setiger. 3. Noto- and gastrotrochs inserted between the proto- and telotroch. 4. A vestibule of characteristic type. 5. A short neurotroch, which in segments 1-3 passes over into a "ciliated pit". In the above characteristics these larvae differ from all other known larvae. It is possible that additional families will be placed in close connection to this group. The "Rostraria" larva shows much in common with the described larvae. Probably also Streblospio should be classified here, provided that a larva, described by Höfker (1930) is correctly identified. 50. Jn view of the similarities in development Spionidae, Disomidae and Poecilochaetidae ought to be regarded as constituting a well-defined group among the spiomorphic polychaeles. On account of the differences which are encountered within this group it is, however, not appropriate that they should be united in one family. The larvae of Disoma and Poecilochaetus diverge from that of Spionidae, each in its own way. Since the differences between the larvae of Disoma and Poecilochaetus are relatively great (above all in regard to the development of the head region), Poecilochaetus should be removed from the family Disomidae in order to form the new family Poecilochaetidae. The great differences between the adult animals also support this measure. 51. In regard to the adult morphology, Spionidae may be divided into three groups: l. Genera Nerine, Nerinides, Aonides, Laonice, Spiophanes, and Prionospio. 2. Genus Scolelepis. 3. Genera Spio, Micropio, Pygospio, and Polydora. Group 2 occupies an interesting intermediate position by having characteristics in common with both groups I and 3. Within those groups no division on the basis of larval morphology can be made corresponding to Söderstroms's (1920) classification into subfamilies. 52. A compilation of the occurrence of the different species in plankton is presented. They are all seasonal, and occur during periods of varying length. Some species are encountered during two seasons every year. Regarding the number of species appearing simultaneously there are two maxima and two minima in a year. A spring maximum occurs during March-April, and an autumn maximum during July- October. The spring-spawning species are generally found during a shorter period than the autumn -spawning. 53. Since the larvae have, in most cases, a long pelagic life, conclusions cannot be drawn regarding the spawning of a particular species in the area of investigation simply on the basis of its occurrence in plankton, as the larvae may have beon carried by currents from distant regions. Nor can for the same reasons any conclusions be drawn regarding the occurrence of a species in the area from the appearance of its larva in plankton.

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