Zoology Publications from Victoria University of Wellington—Nos. 49 to 51
The genus Nemesis presents some systematic problems since it is capable of considerable contraction (see plate A, page 28) so that relative lengths of segments cannot safely be used as a systematic, character. However the females can be separated into two groups:
|(a)||small specimens (ca 3 mm-5 mm) in which the fifth thoracic segment (fourth freely articulated segment) is significantly narrower (by 25%-50%) than the fourth thoracic segment, and the abdomen is three-segmented.|
|(b)||relatively large specimens (ca 7 mm-12 mm)* in which the fifth thoracic segment is not significantly narrower than the fourth, and the abdomen is two segmented.|
N. robusta, discussed below, belongs to the first group. All the other specimens of Nemesis available to me belong to the second group.
There has been considerable discussion (Parker and Mathews, 1951, p. 571; Delamare Deboutteville and Nunes-Ruivo, 1953, p. 215) about whether this group should properly be divided into more than one species. In an effort to resolve this issue I have compared the widths of segments in figures of these animals by several authors, and in my material.
|1.||In the following tables segment widths are expressed as segment width divided by cephalothorax width in order to allow easier comparison.|
|2.||The host is given only where authors leave no doubt as to which species was host for their figured specimen.|
|Author||Year||Host||Seg. 2||Seg. 3||Seg. 4||Seg. 5||Gen. Seg.||Abd. 1||Total Length (mm)|
|Brian||1906||Either Carcharodon carcharias or Isurus oxyrinchus||1.49||1.59||1.56||1.45||0.45||0.32||9|
|Cressey||1967||Isurus oxyrinchus Isurus sp. Carcharodon carcharias||1.54||1.60||1.59||1.39||0.64||0.28||5.85-7.95|
(seg. = thoracic segment; gen. seg. = genital segment; abd. 1 = anterior abdominal segment)page 13
|Host||Area||Seg. 2||Seg. 3||Seg. 4||Seg. 5||Gen. Seg.||Abd. 1||Total Length (mm)|
These figures have not been treated statistically since samples from several host fishes would be required to make such analysis meaningful. However a general trend is clearly evident.
These figures suggest a strong host influence and their demonstration of continuous variation indicates that all these specimens can be identified as Risso's Nemesis lamna. This identification is further borne out by the similarity of appendages, variations in which, as recorded above and by previous authors, can reasonably be accepted as individual variation.
However, female specimens from Cetorhinus maximums can readily be distinguished from those recorded from other hosts, not only because they are narrower, but also by the shape of the first three free thoracic segments which are subcylindrical and narrowed anteriorly with shallow sinuses between them. In specimens from other hosts the third and fourth thoracic segments have their posterolateral angles swollen posteriorly and the sinuses are deeper. Further, the Cetorhinus females differ more in these characters from specimens from Carcharodon than from Isurus, although in ratios of width they are much closer to these specimens than those from Isurus.
The males from Cetorhinus maximums can similarly be separated from the males from other host species by their shallow intersegmental sinuses, and by the absence of the significant differences in width between the second and third thoracic segments and between the fourth and fifth thoracic segments which are found in specimens from other host species.
I consider that these differences are sufficient to retain Scott's name vermi as a subspecies of N. lamna.page 14
Valle's N. l. sinuata, as recorded by Brian (1906, p. 72), has wider dimensions than all other recorded specimens of N. lamna; further information on it may show it to be a third subspecies.
On the available evidence it seems likely that specimens previously recorded as Nemesis lamna from Cetorhinus maximus are N. l. vermi and those from other species, N. l. lamna. However, in the absence of description or figures by many authors recording this species, no attempt has been made to separate the two subspecies in the following list of previous records.
Northeast Atlantic—on Cetorhinus maximus: Firth of Clyde, Scotland (Scott, 1929, p. 98); Hebrides (Mathews and Parker, 1950, p. 571); Concarneau (Legendre, 1923, p. 278 and Fage, 1923, p. 280).
Mediterranean—no host specified: (Heller. 1868, p. 221). on Cetorhinus maximus: Banyuls-sur-Mer (Delamare Deboutteville 1948, p. 447); Narbonne (Delamare Deboutteville and Euzet, 1952, p. 217).
on Isurus oxyrinchus: (Richiardi, 1880, p. 150 fide Brian, 1906); Palavas (Delamare Deboutteville and Nunes-Ruivo, 1953, p. 213); Genova (Brian, 1898, p. 213); Genova (Brian, 1898, p. 213); Portoferraio (Brian 1906, p. 72); Adriatic (Valle, 1880, p. 66).
on Carcharodon carcharias: Porto ferraio (Brian, 1906, p. 72); Adriatic (Valle 1880, p. 66).
on Lamna nasus: Nice (Risso, 1826, p. 136; Roux, 1828, both fide Brian, 1906).
on Alopias vulpinus: Nice (Roux, 1828 fide Brian 1906); Genova (Brian, 1898, p. 213).
on Odontaspis ferox: Genova (Brian, 1898, p. 213).
on Lichia amia: (Richiardi, 1880, p. 150 fide Brian, 1906). Note: this fish is a teleost and this record must be considered doubtful in the absence of further confirmation.
Northwest Atlantic—on Carcharodon carcharias: Marthas Vineyard, Mass. (Wilson, 1932, p. 461).
Southwest Atlantic—on Lamna nasus: Mar del Plata (Brian, 1944, p. 197).
Northeast Pacific—on L. nasus: Coast of California (Wilson, 1932, p. 461). on Isurus oxyrinchus: La Jolla, Calif. (Cressey, 1968, p. 11).
Southeast Pacific—on Isurus oxyrinchus: off Chile and Peru (Cressey, 1968, p. 11).
Northwest Pacific—on Carcharodon carcharias: Simizu, Siznoka Prefecture, Japan (Yamaguti, 1939, p. 455).
Southwest Pacific—on unnamed shark: Port Jackson, N.S.W., Australia (Heegaard, 1962, p. 184).
Indian Ocean—on Isurus sp., I. oxyrinchus and Carcharodon carcharias (Cressey, 1967, p. 6).
* N. lamna, Heegaard, 1962, fig. 210 which, according to the scale given, is 3.14 mm in length, seems an exception to this size range but agrees with the other two criteria.