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Structure and Development of the Parotoid Gland in Metamorphosed and Neotenic Ambystoma
gracile
Author(s): Lawrence E. Licht and David M. Sever
Source: Copeia, Vol. 1993, No. 1 (Feb. 11, 1993), pp. 116-123
Published by: American Society of Ichthyologists and Herpetologists (ASIH)
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COPEIA, 1993, NO. 1
116
S. H. Weitzman for their critiques of an earlier
draft; D. J. Stewart for contributing literature
and information concerning Neolebias; and G.
G. Teugels and D. A. Hendrickson for the loan
of museum specimens. Funding for field research was furnished by the United States Center for International Exchange of Scholars and
the United States Information Service in the
form of a Fulbright Research Grant to the first
author.
gique de l'Angola. PublicationCulturaisCompanhia de Diamantesde Angola 75:1-381.
ANDJ. P. GOSSE. 1963. Revision des genres
Nannaethiops Giinther, 1871 et Neolebias Steindach-
__---,
ner, 1894 et descriptuionde troisespeciesnouvelles
(Pisces; Citharinidae).Annls. Mus. R. Afr. Centr.
116:5-40.
AND. 1982. Rehabilitation des genres
---,
Congocharax Matthes, 1964 et Dundocharax Poll,
1967 (Pisces, Distichodontidae)mis en synonymie
par R. P. Vari, 1979 avec NeolebiasSteindachner.
Bull. Inst. R. Sci. Nat. Belg. 54:1-8.
TEUGELS,
LITERATURE CITED
BELL-CROSS, G., ANDJ. L. MINSHULL.1988. The fish-
es of Zimbabwe. National Museums and Monuments of Zimbabwe,Harare, Zimbabwe.
HUBBS, C. L., AND K. F. LAGLER. 1958. Fishes of the
Great Lakes region. Cranbrook Institute of Science, Bloomfield, Michigan.
KELLEY, D. W. 1968. Fishery development in the
central Barotseflood plain. FAO Fish. UNDP(TA)
Rep. FRi/UNDP(TA): 1-151.
LEVITON, A. E., R. H. GIBBS, JR., E. HEAL, AND C. E.
DAWSON. 1985. Standardsin herpetology and ichthyology: Part 1. Standardsymbolic codes for institutionalresource collections in herpetology and
ichthyology. Copeia 1985:802-832.
POLL,M. 1967. Contributiona la faune ichthyolo-
G. G., ANDT. R. ROBERTS.1990. Descrip-
tion of a smalldistinctivelycoloured new species of
the characoidgenusNeolebiasfrom the Niger delta,
West Africa(Pisces;Distichodontidae).J.Afr. Zool.
104:61-67.
VARI,R. P. 1979. Anatomy, relationshipsand classificationof the familiesCitharinidaeand Distichodontidae (Pisces,Characoidea).Bull. Br. Mus. Nat.
Hist. (Zool.) 36:261-344.
WINEMILLER, K. 0. 1991. Comparativeecology of
Serranochromis
species (Teleostei: Cichlidae)in the
Upper ZambeziRiver.J. Fish Biol. 39:617-639.
DEPARTMENT
OF WILDLIFE AND FISHERIES SCI-
ENCES, TEXAS
A&M
UNIVERSITY,
STATION, TEXAS 77843-2258.
COLLEGE
Submitted
22
Nov. 1991. Accepted 12 Feb. 1992. Section
editor: R. Winterbottom.
Copeia, 1993(1), pp. 116-123
Structure and Development of the Parotoid Gland in
Metamorphosed and Neotenic Ambystomagracile
LAWRENCEE. LICHT AND DAVID M. SEVER
Structure and development of the parotoid gland were examined in larval,
metamorphosed, and neotenic Ambystoma gracile. Larvae first show gland enlargement when they are near 50 mm SVL, the size at metamorphosis. The gland
is well developed in metamorphs and only partially developed in neotenes. In
neotenes, gland height is reduced but histology and histochemistry do not differ
from that in metamorphosed individuals. Lowering the head and positioning of
the gland as part of defensive behavior first appears at metamorphosis, and such
behavior is readily shown by most metamorphs. Larvae and neotenes do not
show the same defensive behavior as metamorphs.
metamorphosis, larval salamanders undergo numerous morphological changes
involving skin texture, skull shape, tongue appearance, hyobranchial apparatus, and gill slit
closure and loss (Lauder and Shaffer, 1986;
Reilly, 1987; Reilly and Lauder, 1988). Some
salamanders, however, may not metamorphose,
AT
and neoteny, the attainment of sexual maturity
with the retention of larval morphology, occurs
in all families (Dent, 1968; Duellman and Trueb,
1986). Neoteny can be either complete or incomplete (Reilly, 1986, 1987). For example, in
the family Salamandridae, neoteny is incomplete, and many features such as gill structures
@ 1993 by the American Society of Ichthyologists and Herpetologists
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LICHT AND SEVER-PAROTOID
remain larval, but skull morphology becomes
adult. In the Ambystomatidae, there is complete larval morphology in these characters
(Reilly, 1986, 1987), although at sexual maturity, the cloacal glands in neotenes become adult
(Licht and Sever, 1991).
Facultatively neotenic species are those in
which some individuals of a population undergo
metamorphosis and others remain larval (Duellman and Trueb, 1986). The Northwestern Salamander, Ambystomagracile, is unique among
such species in having enlarged, raised parotoid
glands on the head behind the eyes (Bishop,
1943; Stebbins, 1951). The general morphology of the gland, effects of glandular secretions,
and use of the gland in defensive behavior in
metamorphosed A. gracile have been described
by Brodie and Gibson (1969) and Williams
(1983). Neither the ontogeny of gland development in metamorphs nor the extent of gland
development in neotenic individuals has been
studied in detail. Thus, we examined a series of
A. gracile including larval, neotenic, and metamorphosed individuals to describe histological
details of gland structure and to determine the
extent of gland development in all life-history
forms of the species. Moreover, because most
morphological features in neotenic A. gracile remain larval (Reilly, 1987), the comparative development of the parotoid gland in neotenes
and metamorphs represents a useful character
to establish a more complete description of heterochronic characters in this species.
MATERIALS AND METHODS
Ambystomagracile were collected from the Little Campbell River in Langley, British Columbia, and Hollyburn Mountain in West Vancouver, British Columbia. Some specimens,
collected in 1969, 1984, or 1988, were used in
other studies (Licht, 1975; Lowcock and Licht,
1990; Licht and Sever, 1991) and were preserved in 5% formalin. Voucher specimens are
deposited in the Canadian Museum of Nature,
NMC 33903-1 to 33903-7. Three types of individuals were examined: juvenile larvae, metamorphs, and neotenes. Sexual maturity was established by the presence of enlarged testes and
coiled, pigmented vasa deferentia in males and
yolk-filled, pigmented ova in females (Semlitsch, 1985; Lowcock and Licht, 1990).
Parotoid gland dimensions.-The parotoid gland
is visible as a dorsal swelling on either side of
the head and extends from the neck to the eye.
Gland length through its midline was measured
in the metamorphs from the posterior margin
DEVELOPMENT IN AMBYSTOMA
117
of the gland on the neck to the anterior margin
at the eye. In branchiate individuals, because
the gland was not conspicuously enlarged, this
measurement was made at a point from the third
gill arch to the eye. The snout-vent length (SVL
to nearest 1 mm) and length of gland site (nearest 0.5 mm) were measured in larvae, metamorphs, and neotenes. For visibly enlarged
glands, gland height and width (at midpoint of
the gland to nearest 0.5 mm) were measured by
use of a dissecting microscope. All measurements were log transformed, and the relationship between SVL and gland dimensions was
analyzed by least-squares regression. Males and
females (excluding juvenile larvae) were analyzed separately to test for sexual dimorphism
in gland length. Measurements for metamorphs
and neotenes were compared by analysis of covariance (Zar, 1984). All statistical analyses were
run on Statistical Analysis System computer
programs (SAS, 1985).
Histology.-For histological studies, six metamorphs (three females, 71-80 mm, and three
males, 74-79 mm SVL) and seven neotenes
(three females, 63-76 mm, and four males, 6577 mm SVL) were examined. All specimens were
laboratory-reared individuals of the same age
(13 months' posthatching). The parotoid gland
region for three larvae (35-48 mm SVL) was
also examined histologically. Parotoid glands
were excised from specimens preserved in 5%
formalin, rinsed in water, dehydrated in a graded series of alcohol, cleared in toluene, and embedded in paraffin. Sections 10 ,m thick were
cut on a rotary microtome and affixed to albuminized slides. Some slides from each specimen were stained in hematoxylin-eosin whereas
others were treated with the "histochemical
quad stain" (Floyd, 1990), which contains stains
and reactions diagnostic for proteins (naphthol
yellow S), neutral carbohydrates (periodic acid/
base fuchsin Schiff-PAS),
and acidic mucosubstances (Alcian blue at pH 2.0).
The heights of 11 gland tubules selected at
random from the midline of the parotoid gland
cluster were measured to the nearest 0.01 mm
with an ocular micrometer in a microscope at
100 x magnification.
Behavior.-As part of another study (Licht,
1992), a series of larvae of A. gracile were raised
in the laboratory under constant temperature
(19.5 ? 1 C) and food levels. After a number
of months, some larvae metamorphosed, and
other remained branchiate as neotenes. The
neotenes were kept separately in glass bowls with
one liter of dechlorinated water, and meta-
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COPEIA, 1993, NO. 1
118
.'043
CB7
0
Fig. 1. Parotoid gland in Ambystomagracile: (A) Gland enlargement in metamorphic (on left) compared to
neotenic individual, each 65 mm SVL; (B) cut parotoid gland showing depth (2 mm); (C) newly metamorphosed
individual showing defensive behavior and parotoid gland directed toward probe.
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LICHT AND SEVER-PAROTOID
morphs were held individually in plastic boxes
lined with wet paper towels. All were kept at
19.5 (+1) C.
To assess salamander defensive behavior, especially as it related to the use of the head and
parotoid gland area, 23 neotenes and 27 metamorphs were each touched with a glass probe
on the tip of the snout or the gland. Twelve
other individuals were tested as they completed
metamorphosis and still retained gill stubs. All
neotenic and metamorphosed individuals were
over 55 mm SVL. Observations were made on
the immediate response and general behavior
of each individual, but results were not quantified.
RESULTS
Parotoid gland dimensions.-The gland is a kidney-shaped swelling on the head behind the eye
(Fig. 1). Gland length, or the site for gland location, did not differ between metamorphosed
males (n = 26) and females (n = 23) {F = 2.15,
P > 0.20}, nor between neotenic males (n = 22)
and females (n = 31) {F = 1.21, P > 0.20}.
Gland length varied directly with SVL in larvae,
metamorphosed and neotenic individuals [sexes
combined (Table 1)]. The regression lines for
gland length and SVL for metamorphosed and
neotenic groups differ significantly (ANCOVA,
= 60.25, P < 0.001).
F[1,981
The gland is conspicuously raised in metamorphosed individuals but not in either larvae
or neotenes (Fig. 1A-B). Mean gland height for
20 metamorphosed individuals (56-80 mm SVL)
was 1.90 mm (SD = 0.20), and mean width was
6.15 mm (SD = 0.56). Gland width was significantly correlated with SVL (r = 0.868, df = 18,
P < 0.001), but gland height was not (r = 0.222,
df = 18, P > 0.20). Swelling of the gland was
barely perceptible in neotenes, and neither
height nor width was measured.
The gland first became visibly enlarged as
larvae, from 50-55 SVL, began to metamorphose. By the time gills are resorbed, the gland
was fully enlarged in length from neck to eye
and was 1.5-2.0 mm high (n = 12). An individual (55 mm SVL) with gill stubs had enlarged
glands (Fig. 1C).
Histology.-The parotoid gland is composed of
numerous granular glands in the dermis (Williams, 1983). No sexual dimorphism in gland
cytology was observed. In metamorphosed individuals, the granular glands are tightly packed,
columnar, and filled with granules of two sorts
(Fig. 2A). The most numerous granules are eoin diameter. These
sinophilic and about 5
tsm
DEVELOPMENT IN AMBYSTOMA
119
are separatefromsmallgroupsof largergran-
ules, about 20 Ctmin diameter, that vary from
basophilic to eosinophilic. Both types of granules stain positively for proteins and give a
slightly positive PAS reaction for neutral carbohydrates.
Nuclei are flattened and are limited to the
periphery of the gland. Usually a distinction
between cytoplasm and luminal contents was
not resolved, especially in the most full glands.
Where the luminal-epithelial borders are apparent, the epithelial layer is a narrow, simple
band of cells, cuboidal toward the epidermis
and squamous distally. Thus, the bulk of each
gland is composed of granules in the lumen.
At the junction between each gland and the
basal surface of the epidermis is a narrow duct
that leads from the granular gland lumen to the
surface of the epidermis (Fig. 2B). A demilune
containing PAS-positive granules is near the base
of the duct. Superficial to the epithelial cells is
a thick sheath that contains melanin and PAS+
collagen fibers. As noted by Williams (1983), a
superficial collar of smooth muscle surrounds
the base of the excretory duct. Alcian blue-positive mucous glands also occur in the dermis,
and the epidermis is composed of three layers
of cuboidal epithelial cells covered by a superficial squamous keratinized layer.
The main difference between the granular
glands in metamorphosed compared to neotenic individuals is gland height. In metamorphosed individuals, mean gland height is 1.11
mm (SD = 0.19); in neotenes, mean gland height
is 0.54 mm (SD = 0.19). No difference in gland
height exists between sexes of either neotenes
or metamorphs. Otherwise, metamorphosed and
neotenic individuals differ only in skin anatomy
(Fig. 2C). In the neotenes, the outer keratinized
layer of the epidermis is absent, and Leydig
cells, containing PAS-positive and naphthol yellow-positive granules, are numerous in the epidermis. As in metamorphs, mucous glands are
abundant in the dermis of neotenes.
Evidence of dermal gland development was
seen in only one immature larval specimen, 47
mm SVL, that possessed flattened granular
glands 0.10-0.18 mm in diameter (Fig. 2D).
These glands contain small granules that give
slightly positive reactions to PAS and naphthol
yellow and large vacuoles containing a flocculent substance that reacts positively to naphthol
yellow. Mucous glands are absent in this individual. No glands occur in the dermis in the
other immature larvae.
Behavior.-Larvae and neotenes responded to
a touch on their snout or neck from a glass
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COPEIA, 1993, NO. 1
120
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- • ••-.•. "-=l ••':•;•":,
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Fig. 2. Sagittalsections through granularglands in the parotoid region of Ambystoma
gracile.(A) Overview
of several granular glands in a metamorphosed male, 79 mm SVL. (B) Higher magnification of same specimen
used in (A), showing details of the excretory pore and surroundingstructures.(C) Neotenic female, 65 mm
SVL. (D) Immature larva, 47 mm SVL. Scale bar in lower right corner-120
gm for (A), 4 4m for (B), and
6 gm for (C) and (D). Ep = epidermis; Fs = fibrous sheath surroundingeach granular gland; Lc = Leydig
cells in the epidermis; Lg = large granules; Mg = mucous gland in dermis; Nu = nucleus of granular gland
epithelial cell; Po = excretory pore of a granular gland; Sc = stratum compactum of the dermis; Sg = small
granules; Sk = skeletal muscle; Sm = smooth muscle around neck of excretory duct; and Va = vacuoles in
larval granular gland.
probe by quickly swimming away. Flight was
immediate and no individual appeared to twist
its head or neck region nor butt or press against
the probe. In contrast, 23 of 27 metamorphs
bent their heads and arched the parotoid gland
toward the probe (Fig. 1C); 22 individuals lifted
their tails and tilted their bodies toward the
probe when they were touched on the side of
the head. The behavioral response shown by
most postmetamorphic animals was also shown
by all 12 individuals tested that were losing tail
fins and gills and at the beginning of metamorphosis.
DISCUSSION
The parotoid gland is characteristic of the
toad genus Bufo, and, in that taxon, the gland
structure and secretions have been well studied
(e.g., Noble, 1931; Low, 1972; Cannon and
Hostetler, 1976). Among salamanders, the
glands are known from species in several fam-
ilies including Plethodontidae, Salamandridae,
and Ambystomatidae (Anderson, 1961; Steward, 1970; Brodie, 1977).
In A. gracile, the fully enlarged gland is found
only in metamorphosed individuals, and gland
enlargement begins at metamorphosis. Whereas other features, such as gill structure and skull
morphology, remain larval in neotenic A. gracile
(Reilly, 1986, 1987), the parotoid gland can be
considered intermediate in development. In
neotenes, the gland enlarges but not to the full
extent seen in metamorphosed individuals. The
skin of neotenes remains larval, retaining components such as Leydig cells (Dodd and Dodd,
1976), but the slight enlargement of the parotoid represents a tendency toward metamorphosis. Thyroxine influences the development
of skin glands at metamorphosis (Dodd and
Dodd, 1976), and, thus, the partial enlargement
of the parotoid gland in neotenes likely indicates increase in levels of thyroxine or other
hormones. In A. gracile and other neotenic am-
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LICHT AND SEVER-PAROTOID
DEVELOPMENT IN AMBYSTOMA
121
TABLE 1. COEFFICIENTS FOR LEAST-SQUARES REGRESSION OF LOG GLAND LENGTH (G)* AGAINST LOG SVL (S)
FOR LARVAL, NEOTENIC,AND METAMORPHOSED
Ambystomagracile.The straight line is log G = log A + b log
S; log A and b are intercept and slope, respectively,with standarderror (SE).
Form
Larval
Neotenic
Metamorphosed
n
SVL
a (mm)
Gland
length
x (mm)
b (SE)
log A (SE)
r
P
52
54
48
29.8
68.1
69.8
5.93
14.05
12.25
1.8246 (0.0922)
0.9828 (0.0809)
0.9037 (0.1112)
-1.9363 (0.1350)
-0.6553 (0.1482)
-0.5792 (0.2049)
0.942
0.860
0.768
<0.001
<0.001
<0.001
* G is distance from neck to
eye and length of visibly enlarged gland in metamorphosed individuals, undeveloped gland site in larvae, and
partially enlarged gland in neotenes.
bystomatids, neotenes show increased thyroid
activity at the time when metamorphosis would
normally occur, but levels of thyroid hormones
are lower than in metamorphs and not high
enough to induce transformation (Norris and
Platt, 1973; Eagleson and McKeown, 1978). The
parotoid glands begin to enlarge in larvae of
about 50 mm SVL, the size when metamorphosis usually occurs (Licht, 1992). Likely changes
in levels of hormones in neotenes are also reflected by the full development of the cloacal
glands and the synchrony of sexual maturity in
neotenes and metamorphs (Licht and Sever,
1991).
The distance from neck to eye represents actual length of enlarged glands in metamorphs
and site of the partially developed gland in neotenes. Gland length and width vary directly with
SVL, and the significantly shorter gland length
in metamorphs compared to neotenes (Table 1)
is a consequence of the presence of eyelids in
metamorphs only (Fig lA). Height of the gland
is nearly the same for all metamorphosed individuals and does not vary with SVL.
The granular glands of the parotoid gland
are not different in cytology from those elsewhere in the dermis of the skin (Williams, 1983),
except for their relatively larger size in metamorphosed individuals. Granular glands in the
dorsal tail base, however, also may be enlarged,
at least in well-fed individuals, and these caudal
granular glands function in nutrient storage as
well as predator defense (Williams and Larsen,
1986).
Williams (1983) also noted two types of granules in the granular glands of A. gracile. At the
ultrastructural level, the larger granules are
membrane bound and separated from the more
numerous (70% of the secretion) small granules
by unit membrane. As found here, he also reported that both types of granules give positive
reactions to protein stains; presumably both
form part of the excreted product. Viewed with
transmission electron microscopy, the granular
gland epithelia form a syncytium, and myoepi-
thelial cells occur in the sheath surrounding each
gland. These features were not observed in the
parotoid gland cluster, but they may not be resolved by light microscopy.
Brodie and Gibson (1969) reported that the
gland first appeared about one month after
metamorphosis. In contrast, we found the gland
already enlarged by the completion of gill resorption (Fig. 1C). Gland appearance may depend on the size and condition of the individual
at metamorphosis and subsequent growth rate.
For example, in toads, the parotoid gland becomes fully formed 3-4 weeks after metamorphosis in laboratory-raised animals (Licht, 1967),
but larger, wild-caught individuals show enlarged glands 7-10 days earlier (LEL, pers. obs.).
Observed use of the gland in defensive behavior by metamorphosed individuals is the same
as that described by Brodie and Gibson (1969).
A metamorphosed individual does not flee but
rather lifts its body, may raise its tail also containing skin glands, and directs the parotoid
gland toward a potential threat (Fig. IC). In this
way, maximal gland surface area is exposed to
an attacker. This posture and positioning of the
gland are exhibited only by metamorphosing
animals, and such behavior is acquired at the
same time as the gland is structurally enlarged.
The gland may also be useful in intraspecific
interactions. Metamorphosed A. gracile emit
sounds that may function to reduce intraspecific
aggression (Licht, 1973), and vocalization coupled with defensive positioning of the parotoid
glands could function to prevent bites and
wounds during agonistic encounters. Larvae and
neotenes flee rapidly if provoked and show no
tendency to confront a potential threat; neither
makes the sounds emitted by the metamorphs
(LEL, pers. obs.).
The parotoid gland is probably more effective in a terrestrial setting and may not be useful
for larvae or neotenes in the aquatic environment. Gland secretions are water insoluble,
highly viscous and adhesive, and very effective
in adhering to the mouth and eyes of potential
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COPEIA, 1993, NO. 1
122
mammalian predators (Brodie and Gibson,
1969). On land, the metamorphs face a large
variety of vertebrate predators likely sensitive
to the bitter taste and toxic effects of the secretions. In contrast, neotenes face few predators except fishes: for example, large trout are
reported to prey on A. gracile (Efford and Mathias, 1969). Those fishes large enough to capture neotenes may swallow them whole and are
not as prone to chew their prey as do mammalian predators. Thus, the adhesive quality
and distasteful properties of gland secretions
may not be effective on aquatic predators, and
the insolubility of secretions in water may reduce transmission of secretions. Of relevance is
the fact that neotenic populations of A. gracile
predominate in high altitude, permanent, typically fish-free lakes (Sprules, 1974), and, except
as small larvae prone to insect predation, larger
neotenes apparently face little predation.
ACKNOWLEDGMENTS
We thank M. Stasiuk for help with manuscript
preparation. This research was supported by
the Natural Sciences and Engineering Research
Council of Canada Grant 3142 to LEL and National Sciences Foundation Grant BSR 87-15341
to DMS.
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DEVELOPMENT IN AMBYSTOMA
123
DEPARTMENT
OF BIOLOGY,
YORK
(LEL)
UNIVERSITY, 4700 KEELESTREET, TORONTO,
ONTARIO M3J 1P3 CANADA; AND (DMS) DEPARTMENT
OF BIOLOGY,
SAINT
MARY'S
COLLEGE, NOTRE DAME, INDIANA 46556. Sub-
mitted 19 Aug. 1991. Accepted 1 Feb. 1992.
Section editor: D. G. Buth.
Copeia, 1993(1), pp. 123-133
Phylogenetic Relationships of the Sternoptychid
Argyropelecus(Teleostei: Stomiiformes)
ANTONY S. HAROLD
The seven valid species of Argyropelecusare the subject of a phylogenetic
analysis. Selected aspects of osteological and photophore anatomy were surveyed
for these taxa and the outgroups Argyripnus,Polyipnus, Sonoda, and Sternoptyx.
Argyropelecusand Sternoptyxare corroborated as sister groups and Polyipnus as
their sister group. Of Argyropelecusspecies, affinisand gigas form a clade which
is the sister group to the remaining members of the genus. Those five species
are united by eight synapomorphic aspects of pelvic girdle, anal-fin pterygiophore, pleural rib, and parasphenoid form. The three species lychnus, olfersi,
and sladeni form a clade which is sister group to aculeatus and hemigymnus.
These last two species are highly disparate in overall form, but that is inferred
to be due mainly to paedomorphic features, including small adult body size and
body shape little modified from postlarvae, in hemigymnus.Five synapomorphies,
notable among them highly compressed supraneural shafts, support this relationship of aculeatus and hemigymnus.
T
HE Sternoptychidae, according to the phy-
logenetic analysis of Weitzman (1974),
comprises the 10 genera Araiophos, Thorophos,
Maurolicus, Danaphos, Valenciennellus, Argyripnus, Sonoda, Polyipnus, Sternoptyx,and Argyropelecus. Among the many synapomorphies uniting
these taxa are the presence of Type Alpha photophores and their occurrence in glandular
clusters, only three branchiostegal rays associated with the posterior ceratohyal, parietals separated by the supraoccipital bone, lack of a basihyal, and absence of mesopterygoid teeth. The
last three genera, the hatchetfishes, are deepbodied, their name referring to the apomorphic
abdominal keel structure and highly compressed body. To date, there is little understanding of relationships within any sternoptychid genus nor has any explicitly been
demonstrated to be monophyletic. To develop
reliable classifications at higher levels the integrity of recognized genera should be probed.
My research on Polyipnus, to be published separately, indicates that the genus is monophyletic. This conclusion is vital to the interpretation of characters occurring among Argyropelecus
species that bear some resemblance to conditions in derived members of Polyipnus.
ArgyropelecushemigymnusCocco, 1829, was the
first member of the genus to be described, and
it remains the smallest species, not known to
exceed 40 mm standard length. Several similar
nominal species have been described, A. durvilli
Cuvier and Valenciennes, 1849, A. intermedius
Clarke, 1877, and A. heathi Harvey, 1952; but
these were justifiably synonymized by Baird
(1971) with A. hemigymnus.Argyropelecusolfersi
(Cuvier, 1829), originally ascribed to the genus
Sternoptyx,is one of the larger, very deep-bodied
? 1993 by the American Society of Ichthyologists and Herpetologists
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