Chromosomes of Some Opisthobranchiate Mollusks from Eniwetok Atoll, 
Western Pacific 1 
J. B. Burch 2 and R. Natarajan 3 
ABSTRACT: Chromosome numbers are reported for nine species of opistho- 
branchiate mollusks from Eniwetok Atoll, Marshall Islands, western Pacific. In 
the Nudibranchia, both Dendrodoris nigra (Dendrodorididae) and Herviella 
mietta (Favorinidae) were found to have 13 bivalents during male meiosis. In 
the Anaspidea, Dolabrifera dolabrifera and Stylocheilus longicauda (Aplysiidae) 
both had 17 bivalents during male meiosis. In the Cephalaspidea, Haminoea linda 
and H. musetta (Atyidae) each had 17 pairs of chromosomes during male meiosis 
and Lathophthalmus smaragdinus and Smaragdinella calyculata (Smaragdinellidae) 
had 18 pairs. In the Soleolifera, Onchidella evelinae had 18 bivalents during male 
meiosis. 
The extreme conservativeness of chromosome numbers in opisthobranchiate mol- 
lusks is demonstrated by that fact that all 18 nudibranchs from 10 families studied 
so far have the single haploid chromosome number 13, and that 18 of the 21 spe- 
cies of the orders Entomotaeniata, Anaspidea, Cephalaspidea, and Sacoglossa have 
17 pairs of chromosomes. The haploid number 18 is here reported for the first 
time for nonsoleoliferan opisthobranchiate mollusks. The more advanced, mostly 
fresh-water, order Basommatophora, in which the haploid number 18 is the basic 
number, may well have been derived from a taxon within or related to this cepha- 
laspid superfamily (Philinacea) . 
In recent years, detailed investigations have 
been made on the chromosomes of many ba- 
sommatophoran and stylommatophoran snails 
(Burch, 1965), but relatively few species of 
the opisthobranchiate 4 orders have been studied, 
1 The field work for this investigation was sup- 
ported by the Division of Biology and Medicine, 
U. S. Atomic Energy Commission. The research was 
supported (in part) by grants from the National 
Science Foundation, Washington, D. C. (GB 787) 
and the National Institute of Allergy and Infectious 
Diseases, U. S. Public Health Service (5 Tl AI 
41-08). Manuscript received March 14, 1966. 
2 Museum and Department of Zoology, University 
of Michigan, Ann Arbor. Supported by a Public 
Health Service research career program award (No. 
5-K3-AI-19, 451-03). 
3 Museum of Zoology, University of Michigan, 
Ann Arbor. 
4 Boettger (1955) considers the orders Opistho- 
branchiata and Pulmonata (together equivalent to the 
subclass Euthyneura) to be unnatural ones, and in- 
stead divides the Euthyneura into seven orders. While 
we do not disagree with this, it is still convenient 
to speak of his five lower (mainly marine) euthy- 
neuran orders (Nudibranchia, Soleolifera, Cephala- 
spidea, Sacoglossa, and Anaspidea) as "opistho- 
mainly because of the difficulties they present 
in collection and identification. Relying on the 
studies of various authors during the early part 
of the present century, Makino (1951) listed 
the chromosome numbers of 16 opisthobran- 
chiate species, but recent investigations by Inaba 
and our present studies indicate that the earlier 
records are not dependable and so are obsolete. 
Previous reliable reports on the chromosomes of 
opisthobranchiate gastropods are those of Inaba 
and Hirota (1954, 1958), Inaba (1959*, 1959£, 
1961), Natarajan (1959, I960), Mancino and 
Sordi (1964^ and b), and Burch (1965). These 
authors give the chromosome numbers of 36 
species belonging to 21 families and 7 orders 
(Tables 1 and 2). This is a very small number 
when compared with the great multitude of 
species currently recognized in the opisthobran- 
chiate orders. The present paper presents the 
chromosome numbers of 9 opisthobranchiate 
branchs” as distinguished from the more advanced 
Basommatophora (mainly freshwater) and Stylomma- 
tophora (land inhabitants). 
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