The chromosome numbers of Hydroides norvegica, Mercierella enigmática, and Pomatoceros triqueter were determined from squashes of somatic cells in young embryos obtained by artificial fertilization, and stained with iron-alum/aceto-carmine. All had a diploid count of 2n = 26 chromosomes. Mitotic and meiotic divisions in the 5 species of Spirorbis examined, and in Filograna implexa, all revealed a diploid chromosome number of 2n= 20. A diploid chromosome number of 14 is suggested for the ancestral serpulid.
With two plates (figs, 1 and 2)
Investigations dealing with the nuclear cytology and chromosome numbers in the polychaete worms have been neglected in the past. This group has both hermaphrodite and bisexual forms and includes a wealth of species with diverse modes of life. It seemed that investigations of chromosome number and nuclear cytology in Polychaeta, along similar lines to the work on lumbricids (Muldal, 1949), might yield results of taxonomic value and indicate probable lines along which the evolution of the group has taken place.
While chromosome numbers vary considerably in different families of Polychaeta, there may also be diverse counts within one family. Amphitrite sp. has a diploid count of 2n =22 (Scott, 1906), while Lanice conchilega Malgreu, which belongs to the same family, the Terebellidae, has a count of 2n = 6 (Dehorne, 1911). Polyploidy may have played a part in the evolution of the group ; the phenomenon seems widespread in hermaphrodite annelids (White, 1940). On the whole, however, higher counts seem predominant among sedentary polychaetes, e.g. Arida sp. with 2n = 18 (Kostanecki, 1909); Chaetopterus pergamentaceus Cuvier with 2n = 18 (Mead, 1898), and Serpula crater Claparède with 2n =14 (Soulier, 1906).
Mitotic and meiotic divisions were studied from preparations made by an iron-alum/aceto-carmine squash technique (Belling, 1926) involving the use of a separate bath of iron-alum mordant (Godward, 1948; Austin, 1959).
In the sub-family Serpulinae, the preparations were made from developing embryos obtained by means of artificial fertilization. Male and female worms were taken out of their tubes and placed in a dish containing sea-water. The sex cells were seen streaming out of the body and were allowed to mix together for fertilization to take place. Foyn and Gjoen (1954) described artificial fertilization in P. triqueter L. and we found it to be equally successful in H. nor- végien and M. enigmática. At 18 ° to 20 ° C the first cleavage in these 3 species occurred between and h after fertilization. The best preparations were obtained with embryos between I ½ and I ¾ h old, when active cell-division was taking place to form a many-celled blastula. Later stages were less suitable as the rate of mitosis slowed down. Cell-division almost stopped after the trochophore stage.
In the sub-family Spirorbinae both embryos and mature worms were examined. Spirorbis borealis shows a tidal periodicity in liberation of its larvae (Garbarini, 1933). They are liberated during neap tides, so that the ideal time for collection of material both for maturation division in adult worms and for mitotic activity in the embryos occurs a few days before spring tides, i.e. between the last batch of liberated larvae and the next spring tide.
In F. implexa mitotic divisions in somatic cells of young worms were examined. Table 1 (see Appendix, p. 400) gives the sources of material of the species studied.
The cytological observations made on the species under study are sum- marized in table 2 (see appendix). Both in mitosis and in meiosis the prometaphase and metaphase presented the clearest figures from which counts could be made.
The species H. norvegica (Gunnerus), P. triqueter L., and M. enigmatica Fauvel were found to possess a chromosome number of 2n = 26. The centromere occurs in a terminal or sub-median position on the chromosomes of all three species (fig. 1, A, B, D, E). The chromosomes are all of similar size and form. At metaphase the chromosomes arrange themselves around a central spindle element (fig. 1, B) and anaphase separation is normal (fig. 1, c). The only available material of Serpula crater Claparède yielded only a single countable nucleus; this confirmed Soulier’s count of 2n = 14.
In the Spirorbinae, 5 species of Spirorbis were investigated. Both mitotic divisions in somatic cells and meiotic divisions during spermatogenesis were studied. All 5 species showed a uniform count of 2n = 20 (fig. 2, A-F). In general, the chromosomes of this sub-family have terminal and sub-terminal centromeres. At meiotic metaphase I, 10 bivalents are discernible which assume characteristic shapes, there being no central element in the spindle.
A possible hybrid between S. borealis and 5. corallinae has been reported by de Silva (1960).
At metaphase II the chromosomes are highly contracted and dot-like. In 5. borealis (fig. 2, B) and S. corallinae (fig. 2, c) one of the chromosomes (marked with an X) is larger than the rest, while in the other species they are all of about the same size (fig. 2, D-F). The nucleolus could be demonstrated only in S. tridentatus. It disappeared in early prophase (fig. 2, E).
In F. implexa (figs. 1, F; 3) the chromosome number is 20. Ten distinct bivalents can be seen during meiosis in both spermatogenesis and oogenesis. Chromosome size and position of centromere are similar to those found in the Spirorbinae.
The family Serpulidae has three sub-families : the Serpulinae, Spirorbinae, and Filograninae. While the Spirorbinae and Filograninae are hermaphrodites, P. triqueter (Foyn and Gjoen, 1954) and H. norvegica (Ranzoli, 1954) of the Serpulinae are protandrous hermaphrodites. The chromosome numbers in this family are as follows :
H. norvegica (Gunnerus) 2n = 26
M. enigmática Fauvel 2n = 26
P. triqueter L. 2n = 26
Serpula crater Claparède 2n = 14
Spirorbis borealis Daudin 2n = 20
S. corallinae de Silva (1960) 2n = 20
S. tridentatus Levinsen 2n = 20
S. pagenstecheri Quatrefages 2n = 20
S. spirillum L. 2n =20
F. implexa (Berkeley) 2n = 20
The occurrence of 2n =14 in Serpula crater and 2n = 26 in the other three species suggests that polyploidy may have played a part in the evolution of some of the species of the Serpulinae (fig. 4). If in this way the original
number 2n = 14 was doubled to 2n = 28, then, by the loss of a pair of chromosomes in the course of evolution, the count found in Hydroides, Mercierella, and Pomatoceros of 2n = 26 could have been arrived at. If we take a form with 2n = 14 as the ‘ancestral serpulid’, the families Filograninae and Spirorbinae could be considered as 2n- + n—1 = 20, with the loss of a single chromosome taking place in the course of evolution of the group. The assumption that one chromosome from the haploid constituent is lost is supported in the sub-group Serpulinae, where two may have been lost (see fig. 4).
We are grateful to Dr. D. J. Crisp for his ready encouragement, to Dr. C. Burdon-Jones for supplying specimens of F. implexa, and to Dr. T. B. Reynoldson and Dr. D. H. Stone for valuable criticism.