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Some New Zealand Parasitic Copepoda of the Family Anthosomidae

Aethon percis (Thomson, 1889)

Aethon percis (Thomson, 1889)

Lernanthropus percis Thomson, 1889, p.336-7, pl. 27, figs. 2a-j
nec              "              "              "             Wilson, 1936, p.340.


From gills of Parapercis colias (blue cod):

One female, collected at the northern end of Kapiti Is., by the author, 4 February 1967; three females from the collection of the Otago Museum in a tube containing three labels "Lernanthropus percis—gills of blue cod", "G.M.T. dep. 1910" and "label on outside of tube—from gills of blue cod 23/10/79" (there is now no label page 13on outside of tube). One of these latter specimens had been dissected and may be the specimen from which Thomson made his original description and drawings.


Female (figs. 37-50).

Overall length (5.15 mm - 5.73 mm).

Cephalothorax, width four-fifths length (1.63 mm - 1.95 mm × 1.18 mm - 1.47 mm), subovate, posterior margin sublinear, raised in a ridge; antennae borne laterally on a short anterior area, one-third carapace width posteriorly, narrowing to two-thirds this width anteriorly, anterior margin straight, marked off from carapace by lateral grooves, extending a little beyond anterior margin of remainder of cephalothorax; lateral margin of carapace expanded as two lobes, each half length of margin, anterior lobe bulging posterolaterally, posterior lobe bulging anterolaterally, posterior lobe slightly bifid distally in some specimens, posterior lobe sometimes unequally developed (see fig. 37 in which right hand lobe is larger than left).

Second and third thoracic segments fused, but distinguished by a groove which runs dorsally and laterally; combined length three-quarters width, second segment four-fifths width of third (combined length 1.05 mm - 1.21 mm, second segment width 1.13 mm - 1.39 mm, third segment width 1.44 mm - 1.67 mm), second and third segments each with most of dorsal surface covered by a poorly developed plate; lateral margin of combined second and third thoracic segments extend laterally as wing-like expansions, which increase the width by up to one-fifth, lateral margins of expansions sublinear, anterior angles rounded, posterior angles extended posterolaterally for a distance equal to two-fifths combined segments length, narrowing and rounded distally, not fused with plate of fourth segment.

Fourth thoracic segment including plates, slightly longer than wide (2.43 mm - 3.32 mm × 2.47 mm - 2.87 mm), lateral margins sublinear and parallel for the anterior four-fifths of their length, then slightly angled, posterior margin an entire curve except for a concave invagination medially which is one-sixth plate width, and one-eighth length of plate; actual segment short, length one-third width (0.72 mm - 0.80 mm × 2.11 mm - 2.53 mm), subrectangular, angles rounded.

Genital segment subrectangular, a little wider than long (0.42 mm × 0.49 mm), widest anteriorly, narrowing to two-thirds this width distally.

Abdomen subovate, as wide as long (0.55 mm × 0.53 mm), fused to genital segment anteriorly, bearing caudal laminae posterolaterally.

Caudal laminae very small, length twice width (0.18 mm × 0.09 mm), subtriangular, lateral margins irregular, with a lateral projection one-quarter distance from base, one-fifth as long as a lamina, a little wider than long, and tipped with a sharp spine; lamina with a further sharp spine at the distal apex, and further spine on medial margin near the apex.

Egg strings long, complexly coiled and hidden in dorsal view by the plate of the fourth thoracic segment, in ventral view by the third pereiopods.

First antenna of seven subrectangular segments, ratio of length from the basal segment outwards being approximately 3:3:2:3:1:1:1; first segment width two-thirds length, with a small spine on outer margin, second segment width four-sevenths length, with three setae on outer margin, third segment as wide as long, with a small seta on outer margin, fourth segment width half length, with one long and one short seta on outer margin (fig. 40 shows fourth segment joined to fifth by a short membrane which could be a thinly chitinised segment), fifth segment as wide page 14

Aethon percis (Thomson, 1889) female: fig. 37: dorsal view; fig. 38: lateral view; fig. 39: ventral view; fig. 40: first antenna; fig. 41: second antenna; fig. 42: mandibular palp; fig. 43: maxilla; fig. 44: tip of maxilla; fig. 45: maxilliped; fig. 46: first pereiopod; fig. 47: second pereiopod; fig. 48: fourth pereiopod; fig. 49: genital segment, abdomen and caudal laminae; fig. 50: caudal lamina.

page 15 as long, with two small setae on outer margin, sixth segment as wide as long, with one small seta on outer distal angle seventh segment rounded distally, width three-quarters length, with six setae on distal margin.

Second antenna of two segments, subchelate; first segment, basal width half length, narrowing to three-quarters this width distally, curved, with a stout spine on inner margin near base; second segment basal width half length, narrowing steadily to a sharp point distally, strongly curved, with a stout spine near inner margin one-quarter of distance from base.

Month tube small (0.5 mm in length), sharply pointed distally.

Mandibular palp, with two rami on a flattened subrectangular base; outer ramus, width two-fifths length, rounded distally, with two setae and one subrectangular process distally; inner ramus half length of outer, width two-thirds length, rounded distally, with two setae distally.

Maxilla of two segments, segments subequal in length; first segment, basal width half length, narrowing slightly distally, distal margin rounded; second segment basal width one-tenth length, narrowing gradually distally except that distal seventh narrows suddenly and bears a longitudinal row of small spines reaching almost to the slightly blunted tip, a small process, one-third length of narrowed distal region, width half length, is situated near inner margin one-quarter distance from tip, and bears two setae distally, the larger setea subequal in length to process, the other a little smaller.

Maxilliped of two segments, subchelate; first segment, basal width two-thirds length, narrowing slightly distally, distal margin rounded; second segments basal width half length, narrowing gradually to a distal point, sharply curved over distal two-thirds, with a small spine on inner margin one-third distance from base.

First pereiopod biramous, each ramus of one segment; basipod swollen, subsemicircular, length two-thirds width, with one small spine medial to endoped and another on a raised boss lateral to exopod; exopod subovate, subequal in length to basipod, width half length, with five flattened spines on distal margin; endopod subtriangular, subequal in length to exopod, basal width half length, distal apex bearing a spine.

Second pereiopod biramous, the rami fused to the basipod, length including rami more than twice length of first pereiopod, as long as basal width, narrowing steadily distally, to end in a sharp point, the terminal section presumed to include the endopod, distal portion curving sharply medially and posteriorly away from the body, inner margin with a well developed subtriangular expansion one-third distance from base, and a further process on outer margin two-thirds distance from base is assumed to be the reduced exopod; exopod one-quarter length of combined basipod and endopod, basal width half length, narrowing gradually for three-quarters its length then more suddenly to one-third basal width, tip rounded.

Third pereiopod very large, two-thirds length of body, biramous, the rami lamellar, directed posteriorly and separated but fused with base which is one-fifth length of subequal rami; exopod a flattened subsemiovate lamella, basal width two-thirds length, narrowing slightly distally, distal margin rounded, hiding the egg strings in lateral view; the endopod is folded almost into a right angle in cross section, the inner portion lying flat against egg strings, the outer portion turned ventrally, involving one-quarter width of endopod distally, less proximally, the ventrally turned portion lying almost parallel to exopod, endopod if flattened would be subrectangular, width one-quarter length, posterior angles rounded.

page 16

Fourth pereiopod biramous, total length half total length of third pereiopod, rami fused with basipod which is one-quarter length of exopod and narrows towards its base so that its lateral margins form entire curves with outer margin of exopod and inner margin of endopod; exopod one-quarter as wide at the base as long, narrowing and rounded distally, outer margin a convex curve, inner margin sublinear, with a very small spine on a raised boss on outer margin near junction with basipod; endopod three-quarters exopod length, width at midpoint half length, narrowed slightly proximally, more narrowed distally, rounded distally, outer margin an entire curve, inner margin sublinear.

Fifth pereiopod borne near midpoint of genital segment lateral margin, almost as long as genital segment, width one-quarter length, rounded distally, bent into a right angle, directed laterally, then posteriorly, with a small seta on a well developed projection near distal margin.


The material on hand agrees well with Thomson's description and figures (1889, p.366-7, pl. 27, figs. 2a-j); this could be expected in the case of the Otago Museum material since it had been identified as L. percis by Thomson himself.

Wilson (1936, p.340) claimed to have found this species on Promicrops itaira at the Dry Tortugas. The only description he gives is that the first antennae "prove to be six-segmented, with no setae except two minute ones at the tip of the end segment". Since in A. percis the antenna is seven segmented with setae on every segment it seems that Wilson's record is of yet another species of Aethon.

A. percis resembles A. quadratus Krøyer in its general form and in its possession of forked, unsegmented, second pereiopods, and in the development of the third pereiopods, but it can be separated from A. quadratus in that the latter has the posterior lateral lobe of the cephalothorax extending out well beyond the anterior (which may even be missing; Krøyer does not make this clear) while in A. percis the posterior lobe is only slightly longer than the anterior. Further, A. quadratus has a well developed neck region, almost entirely missing in A. percis.