Steluga Huso (Acipenser) transmontanus (Richardson, 1836)
David Stephen
9127 Paseo Grande Way, Elk Grove, California 95758, USA
davidstephen2004@yahoo.com
SUMMARY
This study provides morphological, osteological and biological evidences to show that the White
Sturgeons of the Pacific coast of North America hitherto identified as Acipenser transmontanus
(Richardson, 1836) are composed of two distinct species, belonging to two genera. The White
Sturgeons in the Frazer River (Canada), the Columbia River (USA) and the Sacramento River (USA)
with the characteristic short-snout belong to the Genus Huso and therefore require a taxonomic
revision of the scientific name to Huso transmontanus (Richarson, 1836). The presence of Huso
transmontanus in this region extends the geographic distribution of this Genus from the East Pacific.
Whereas, the sturgeons in the Nechako River (BC, Canada), the Kootenai River (BC, Canada; USA)
and the Snake River (USA) with the characteristic long-snout belong to the Genus Acipenser,
however, they have remained taxonomically unknown and therefore a new species status is proposed
with the scientific name Acipenser columbiaensis sp.nov. and the common name, the Columbia
Sturgeon. The study also discusses some of the implications of this discovery on commercial
farming, current conservation programs and the speciation of sturgeons.
INTRODUCTION
Taxonomic records show only two species of sturgeons in the Pacific coast of North America. These
are the White Sturgeon Acipenser transmontanus (Richardson, 1836), and the Green Sturgeon
Acipenser medirostris (Ayres). Their distribution along this coast overlaps and extends from the
Mexican coast in the south to the Alaskan coast in the north. However, from an examination of several
adult specimens of the so called White Sturgeons from the Snake River in Idaho (USA) and the
Sacramento River in California (USA), it became clear that the sturgeons from the two rivers were
morphologically distinct and therefore warranted further examination of their taxonomic characters.
During these examinations, a serendipitous discovery was made when the Sacramento River sturgeon
hitherto identified as A. transmontanus (Richardson, 1836) showed the unique distinguishing
characters of the Genus Huso. Only two species have been recorded in this Genus, Huso huso
(Linneaus, 1758), the Beluga Sturgeon of the Caspian Sea, the Black Sea and the Adriatic Sea and
Huso dauricus (Georgi, 1775), the Kaluga of the Amur Basin in the East Pacific. Whereas, the Snake
River sturgeon hitherto also identified as A. transmontanus (Richardson, 1836), showed the
distinguishing characters of the Genus Acipenser. Richardson’s original description of A.
transmontanus was based on a single specimen taken off Vancouver Island, Canada, and that
description does not fit the Snake River sturgeon. These two sturgeons show numerous anatomical,
osteological and biological differences that make reproductive isolation self-evident. Therefore, a new
species status is proposed for the sturgeons of the Snake River, with the scientific name Acipenser
columbiaensis sp. nov. and the common name, the Columbia Sturgeon. The discovery of the new
species is discussed in a separate report (see Stephen and Goodman, 2004).
These two discoveries may influence current recovery programs and also change evolutionary and bio-
geographical theories about the Genus Huso. There is reason to believe that because of the taxonomic
muddle, the two species have been cross-fertilized in state and private hatcheries to improve the
genetics of the White Sturgeon A. transmontanus (Richardson 1836). These inter-generic hybrids may
have been introduced into natural waters under recovery programs; such hybrids are found in a
number of sturgeon farms. Therefore, some recent studies on the genetics of the White Sturgeons,
identified as A. transmontanus (Richardson 1836) may not be representative of the true species; since
two species and hybrids are implicated. The main objective of this report is to bring to the attention of
scientists engaged in conservation, genetics and taxonomic research and commercial farm operators
the existence of a new species of the Genus Acipenser and the existence of the Genus Huso in the
Pacific coast of North America.
MATERIALS AND METHODS
For the purpose of this study, wild adult sturgeons from the Sacramento River were captured upriver
near Grimes (Colusa, County), while early fingerlings and yearlings produced from wild parents
caught from the same river were obtained from farms in Sacramento County. Early fingerlings,
yearlings and adults of the Snake River Sturgeon were obtained from farms near Hagerman in Idaho;
these are progeny of wild parents from the Snake River. All specimens were examined fresh. For
osteological purpose, specimens were boiled and bones were separated. Valuable information and
photographs of sturgeons were obtained from the Internet sites of fishing guide services on the Pacific
coast.
RESULTS AND DISCUSSION
The Genus Huso (Brandt and Ratzeberg, 1833) is characterized by two salient features: the
branchiostegal membranes are joined to each other and form a free fold below the isthmus and a
crescentic mouth (Brandt, 1869; Berg 1948). Furthermore, the palatoquadrate articulates with the
symplectic by means of the Meckel’s cartilage, and the stylohyal articulates with the anterior part of
the symplectic (Tatarko 1936). In this respect, the Sacramento River White Sturgeon (California,
USA) shows these distinguishing characters of the genus Huso (Brandt and Ratzeberg) and not those
of the Genus Acipenser (Linneaus, 1758). Plate 1 shows external morphological differences between
the Sacramento River Sturgeon and the Snake River Sturgeon and the morphological similarities
between the Snake River Sturgeon and two other sturgeons, the Nechako Sturgeon and the Kootenai
Sturgeon. Plate 2 shows differences in the snout shape and position of barbels; differences in the
structure of the mouth and differences in the nature of the branchiostegal attachment in the two
sturgeons. Plate 3 shows two specimens of the Sacramento River sturgeon with the distinguishing
characters of the Genus Huso and distinct features of the species. Richardson (1836) may have been
unaware of the creation of the new Genus Huso by Brandt and Ratzeberg (1833), some three years
earlier. The Genus Acipenser (Linneaus, 1758) is characterized by branchiostegal membranes that are
directly joined to the isthmus; a transverse mouth cleft; the palatoquadrate articulates directly with the
symplectic and the stylohyal articulates with the posterior part of the symplectic (Tatarko, 1936).
Therefore, the Sacramento River White Sturgeon A. transmontanus (Richardson, 1836) should be
placed in the Genus Huso (Brandt and Ratzeberg, 1833, Brandt, 1869) and the taxonomic name revised
to Huso transmontanus (Richardson 1836). In addition to its historic common name, the White
Sturgeon, Huso transmontanus (Richardson, 1836) is given a new common name, the Steluga, in
keeping with the names of two related species, the Beluga (Huso huso Linneaus, 1758) and the Kaluga
(Huso dauricus Georgi 1775).
Based on an extensive review of photographic evidences and descriptions of the so called “White
Sturgeons” in the Pacific coast of North America, particularly the Frazer River System (including the
Nechako River, British Columbia, Canada), the Columbia River System (including the Snake River,
USA and the Kootenai River, Canada and USA) and the Sacramento River System (USA), two
morphologically distinguishable groups are evident in this region. Sturgeons with the characteristic
short blunt snout in the lower Columbia River (USA) and lower Fraser River (Canada) hitherto
identified as A. transmontaus (Richardson, 1836) are morphologically similar to the Sacramento River
White Sturgeon and are considered to be conspecifics of Huso transmontanus (Richardson, 1836).
Whereas, sturgeons with the characteristic long pointed snout in the Kootenai River (USA and Canada)
and Nechako River (Canada) hitherto also identified as A. transmonatanus (Richardson 1836) are
morphologically similar to the Snake River Sturgeon (see Plate 1). The Snake River Sturgeon has
branchiostegal membranes directly attached to the isthmus and therefore correctly belongs to the
Genus Acipenser. Hence, a new species status must be accorded to the type found in the Snake River
and conspecifics in the Kootenai and Frazer Rivers. Incidentally, the new species, A. columbaiensis sp.
nov (Stephen and Goodman, 2004) is not found in the Sacramento River in California or further south
of the coast. These river-isolated populations of the new species in the Snake River, Kootenai River
and Nechako River may have once formed a larger interbreeding population because there is evidence
that post-glacial connections once existed between these rivers.
Apart from these generic level differences, there are numerous species level osteological and
anatomical differences between the two sturgeons as well. Furthermore, the two sturgeons have
different spawning periods and habitat preferences, which is indicative of reproductive isolation. The
following comparative analyses of the two sturgeons provide some fresh insights into their
developmental history and ecology, but new questions emerge on their phylogenetic relationship to
each other and sturgeons elsewhere. Cursory osteological comparisons of the ventral rostral bones and
body scutes of the two sturgeons, Huso transmontanus (Richardson, 1836) and Acipenser
columbianesis sp. nov. (Stephen and Goodman, 2004) reveal significant dissimilarities (Plate 4: e).
The Steluga shows a much higher level of ossification than the Columbia sturgeon in the head bones
(Plate 3). Furthermore, based on dissections, it is clear that the Steluga and the Columbia Sturgeon are
not only very different in their gross anatomy, musculature and osteology, but their cartilage, egg,
meat and fat characteristics are also distinct (see Stephen and Goodman, 2004). However, in view of
the typical difference in the snout shape and the nature of the attachment of the branchiostegal
membranes, attention is focused only on the head-snout morphology and associated osteology of the
two sturgeons in this report.
There is a major difference in the shape of endochondral rostrum in the two sturgeons. The rostrum is
short and semi-circular in the Steluga, whereas it is elongated and triangular in the Columbia sturgeon
(Plate 4: a). In the Steluga, the fingerlings have a broad pointed shovel shaped snout that reduces
gradually in the one year olds and this seems to transform into the short blunt snout as the fish grows
to maturity. Whereas, in the Columbia Sturgeon the snout is elongated in the fingerling and yearling
stage and remains as such in the adult (Plate 4: d). Of particular interest is the development of the
ventral rostral bone in the fingerlings and its final form in the adults. In the case of Huso transmontaus
(Richardson), the anterior portion of ventral rostral bone of fingerlings and juveniles (one year) is an
elongated bone with dentition (pitted surface), where the posterior end is a webbed triangular plate
with two basal pointed ends, the dorsal pointed end of the web articulates directly with the anterior end
of the parasphenoid, while the ventral end is free. In the adult, the anterior end of the ventral rostral
bone is reduced and is vestigial, while the posterior webbed process is very prominent. Whereas, in A.
columbianeis sp. nov (Stephen and Goodman, 2004), the posterior end of the ventral rostral in the
fingerling and yearling stage bears a single basal bone to which is attached a bundle of loosely held
elongated bones that form the stem and a cluster of smaller bones at the anterior end. However, the
ventral rostral bone in the adult Columbia Sturgeon is a single long bone, probably formed by the
fusion of these elongated bones; has a trident like anterior end and the posterior end lacks the webbed
process so prominent in the Steluga (Plate: 4: e).
The nature of the attachment of the branchiostegal membranes to the isthumus shows remarkable
changes during the life history of these sturgeons. At the early fingerling stage, the branchiostegal
membranes are joined to each other and form a free fold over the isthmus in both species. In the
Steluga, the ventral rostral bone supporting the shovel shaped snout in the juveniles (one year olds) is
prominent, exposed and armed with dentition and probably serves as a supporting structure for digging
up buried worms and crustaceans. Oddly enough, at this juvenile stage the branchiostegal membranes
of the Steluga (Genus Huso) are joined to the isthmus similar to juveniles and adults of the A.
columbiaensis sp.nov. (Plate 4: b, c). The edges of the branchiostegal membranes where they attach to
the isthmus are black, but they loose the pigmentation as the fish grows older. The juveniles of Steluga
are known to be residents of upper and mid-river sections and are benthic feeders. During the second
and third years, the Steluga is known to move to lower river stretches, open bays and coastal waters
and change to a more pelagic life. Apparently, at this stage the two edges of the branchiostegal
membrane fuse with each other. In the adults the snout is greatly reduced, but the branchiostegal
membranes are joined to each other and form a free fold over the isthmus as seen in Plate 2: d and c.
In order to confirm this change, juveniles of the White Sturgeon in the Sacramento River are being
sought and observations are continuing to be made on yearlings in farms.
Intergeneric hybrids of the two sturgeons, that is, crosses between the Columbia Sturgeon with the
Steluga from either the Sacramento River and or the Columbia River generally show a narrow
attachment of the branchistegal membranes to the isthmus (Plate 5: c). However, one six year old
showed membranes directly attached to each other with a free fold (Plate 5: d, e). These hybrids
generally shown more Steluga like external characters. Further studies are under way to evaluate the
benefits of such hybrids in aquaculture for caviar and meat production.
A side-by-side examination of adults of the Columbia Sturgeon and the Steluga clearly shows
differences in the thickness of the branchiostegal membranes and their coverage of the ventral pectoral
girdle. In the Steluga, the membrane is thicker and barely covers the girdle, while in the Columbia
sturgeon the membrane is thinner and broadly covers the girdle (see Plate 2: a). Therefore, in the
juveniles of the two genus or species, the attachment of the branchiostegal membranes to the isthmus
and the associated extension of these membranes over the ventral pectoral girdle may facilitate a more
efficient suction of food organisms, such as mollusks and crustaceans in the benthic feeding stage.
Whereas, the joined branchiostegal membranes and the free fold over the isthmus in the adult Steluga
may be associated with swimming and feeding in the pelagic zone. In addition, the peculiar jaw joint in
the Genus Huso, where the pelatoquadrate articulates directly with the Meckel’s cartilage may indeed
facilitate pelagic feeding by opening the mouth forward and in grasping of prey. The White Sturgeon is
known to feed aggressively on salmon during their migration run in the Columbia River and on
shoaling pelagic herring and their eggs in the San Francisco Bay in California. This is similar to the
behavior of the Beluga and the Kaluga. In fact, the Steluga is aggressive when hooked on a fishing line
and is not easily handled as compared to the Columbia Sturgeon, which is rather docile. The White
Sturgeon is also known for its huge size and records of over 1500 lbs exist for fish caught in the
Columbia River and the Sacramento River. Clearly, this suggests that the Huso is able to find larger
sources of food to supports its gigantic body mass in comparison to the Acipensers. Nevertheless,
further comparisons of the two sturgeons are beyond the scope of this report and shall be dealt with in
subsequent publications (see also Stephen and Goodman, 2004).
The discovery of the Genus Huso in the Pacific coast of North America extends the distribution of this
Genus to a new geographical region from the East Pacific where Huso dauricus is found. However,
the discontinuous geographical distribution of this Genus in the Caspian Sea and Amur River is
explained by the Holarctic pattern of distribution (see Bemis and Kynard, 1997). The genetic
relationship between the three geographically isolated species of this Genus will provide some
interesting information on speciation and evolution of sturgeons. Nevertheless, it is postulated that
competition for food caused a divergence in the ancestor(s) of extant species of sturgeons grouped
under Acipenseridae and this is reflected in the food and feeding habits, and habitat preferences of the
members of Husinae as compared to members of Acipenserinae. From an anadromous population of
an ancestor species (example, a Huso), some juveniles may have remained back in certain rivers for
longer periods when food was available and continued as benthic feeders; cohorts of these may have
changed their migration and spawning behavior as competition set in, they became the Acipensids and
Scaphyrhychids . This view may be supported by the fact that in the juveniles of the Columbia
sturgeon and the Steluga, the branchiostegal membranes are attached to the isthmus for effectively
feeding on benthic organisms; the change to a joined branchiostegal membrane and free fold is seen
only in the Steluga (a Huso), as their juveniles leave for open waters and live a pelagic life and show
piscivorous feeding behavior. These morphological and behavioral changes are of evolutionary
significance in the genus Huso. However, it may also be argued that from an ancestral Acipenser, the
Huso emerged as an advanced group that ventured out to sea and exploited new food resources in the
pelagic zone and retuned to spawn in the rivers. In fact, the Steluga shows a much higher level of
ossification than the Columbia sturgeon, supporting the notion that the Genus Huso is more advanced
or recent. Be that as it may, the view of Findeis (1997) and others, who consider Huso to be
phylogenetically primitive within Acipenseridae, and the proposed peramorphic evolutionary trend in
this family, must also recognize the underlying problem in sturgeon evolution or speciation and that is,
the effect of polyploidy on gene expression and genome structure.
The occurrence of polyploidy is well established in sturgeons (see Birstein et al., 1997). Evidently,
polyploidy is an aberration in the reproductive process and lower temperatures can promote unreduced
gamete formation, resulting in genome duplication. The glacial periods of the geologic past and
episodic rapid ice melts in the mountains that flow to rivers where sturgeons arrive for spawning have
the potential to cause polyploidy. Such aberrations may have occurred in a common diploid ancestor
of all Acipenseriformes. Birstein et al. (1997) suggest that all species with about 120 chromosomes
are tetraploid, while others consider them to be functional diploids (ex. Fontana 1994). However,
Ludwig et al. (2001) believe that the common diploid ancestor of Acipenseriformes had about 60
chromosomes and a genome duplication event occurred in this ancestor. According to them
subsequent genome duplications also occurred in the genus Acipenser; but once in the Atlantic Clade at
an early stage and three times at a later stage in the Pacific Clade species. However, in the Pacific
Clade, A. columbiaensis sp.nov. in the Kootenai River is known to have about 243 chromosomes
(personal communications, Paul Anders, July 2004), whereas Huso transmontanus (Richardson, 1836)
(= Sacramento River Sturgeon) from the San Francisco Bay, California has 274 chromosomes
(personal communication, A. L. van Eenennaam, August 2004; see also van Eenennaam et al., 1998).
This would make Huso transmontanus (Richardson, 1836) a unique tetraploid because Huso dauricus
has 120 and Huso huso has 118+/-2 chromosomes (see Ludwig et al., 2001). Interestingly, the
Steluga shares some characters (first dorsal scute is the largest and barbels are without foliate
appendages) with the Kaluga, and may indeed be a tetraploid sister species.
Many ontogenetic and phylogenetic characters seem to cross over between genera and species of the
Acipenserid group; this perhaps reflects past gene conflicts that originate in the new polyploidic
condition but has since been resolved by selected developmental pathways through gene silencing,
chromosomal rearrangements and transpositions in some species, while in others gene expression may
still be unresolved. The author considers genomes to be accumulations of ancestral genes as well as
new genes that characterize the species. Under polyploidy, gene expression may not follow the
evolutionary hierarchy (dominant and recessive genes / traits) seen in normal diploid organisms, until
the process of diploidization is completed. In a polyploidic condition, some gene expressions (between
many alleles) may stochastic. So individuals of a species or a species as whole may have a unique
ontogenic pathway that presents an apparent paedomorphic and or peramorphic form of evolution at
certain nodes. In fact, many anamolies were seen in Acipenser columbiaensis sp.nov., for example, in
one adult the parasphenoid had one internal carotid foramen and in another there was none, whereas in
one juvenile there were two internal carotid foramina and in a third specimen there was only one. In
several specimens of the Columbia Sturgeon, the ectopterygoid was fused to the dermopalatine on one
side but was separate from it on the other side Interestingly, in Scaphirhynchus, the ectopterygoid is
fused to the dermopalatine, whereas they are separate elements in all other Acipenseriformes according
to Findeis (1997).
Given the polyploidic condition in sturgeons, conventional cladistic analyses and molecular taxonomy
may lead to erroneous evolutionary relationships between genera and between species of this group.
Contrary to the view that polyplidization leads to speciation, the author believes that the evolutionary
mechanism is independent of mere gene or chromosome numbers and this is self-evident. Instead,
polyploidy may negatively interfere with the evolutionary mechanism until the process of diplodization
is completed and in sturgeons, it appears to affect their osteology, anatomy and general morphology
and may be the reason for the apparent “stagnant evolution” in this group, rendering all of them as
“living fossils.” Be that as it may, the discovery of the Genus Huso in the Pacific coast of North
America fills a lacuna in the understanding of the distribution and evolution of this Genus. Further
research is being conducted on the relationship of Huso transmonatus (Richardson, 1836) to other
species of this Genus. The follow up publications to this report shall contain relevant osteological
descriptions of Huso transmontaus (Richardson, 1836) and further discussions on speciation in the
Genus Huso.
ACKNOWLEDGEMENT
Brief discussions with several sturgeon specialists, especially Serge Doroshov, Joel Van Eenennaam,
Ron Bruch, Eric Findeis, Boyd Kynard and Eric Hilton were encouraging and are gratefully
recognized.
REFERENCE CITED
Bemis W. E. and Kynard B., 1997: Sturgeon rivers: an introduction to acipenseriform biogeography
and life history. Environmental Biology of Fishes. 48, 167-183.
Berg, L. S. 1948: Freshwater Fishes of the U.S.S.R. and Adjacent Countries. Izdatel’styo Akademii
Nauk SSSR,Vol. 1, Fourth Edition.
Birstein, V. J., Hanner, R., and DeSalle, R. 1997: Phylogeny of the Acipenseriformes: cytogenetic and
molecular approaches. Environmental Biology of Fishes. 48, pp127-155.
Brandt, J. F. and Ratzeburg, J. C.T., 1833: Medizinische Zoologie, oder getreue
Darstellung und Beschreibung der Thiere. die in der Arzneimittellehre in Betracht kommen, in
systematischer Folge herausgegeben. Medizinische Zool. i-iv, pp 1-364
Brandt, J. F., 1869: Mel. Bio. Acad. Sc. Petersbourg, VII, p 111 (type: Acipenser huso)
Findeis, K. E., 1997: Osteology and phylogenetic interrelationships of sturgeons ( Acipenseridae).
Environmental Biology of Fishes. 48, 73-126.
Fontana, F., 1994. Chromosomal nucleolar organizer regions in four sturgeon species as markers of
karyotype evolution in Acipenseriformes (Pisces). Genome 37: 888-892.
Georgi, J. G., 1775: Bemerkungen einer Reise im russischen Reich in 1772 St,
Petersburg. 2 vols. Reise Russ. Reich. pp 1-920
Mitchell, S.L., 1815: The Fishes of New York described and arranged. Trans. Lit. Phil. Soc. N.Y., pp
335-492
Linnaeus, C. 1758: Syst. Nt., ed.X, p 237 (Type; A. sturio). Berg, Fauna Rossii, Ryby, I 1911, p. 172.
Ludwig, A., Belfiore N.M.,, Pitra, C., Svirsky, V., and Jenneckens, I. 2001. Genome duplication
events and functional reduction of ploidy levels in sturgeons (A., Huso and Scaphirhyncus. Genetics,
Vol. 158, 1203-1215.
Richardson, J. 1836: The Fish. In: Fauna Boreali-Americana; or the zoology of the
northern parts of British Columbia; containing descriptions of the objects of natural history collected
on the late northern land expeditions under the command of Sir John Franklin. R. N. Fauna Boreali-
Americana I-xv, pp 1-327
Stephen, D and Goodman J., 2004: A new species of sturgeon in the Pacific Coast of North America:
The Columbia Sturgeon, A. columbiaensis sp.nov. pp 1-3.
Tatarko, K. 1936. Der Kiemendeckelapparat und seine Verbindung mit dem hyoid- und Kieferbogen bei
den Acipenseridae. Trav. Inst. Biol. Acad. Sci., Ukraine. 3: 5-67.
van Eenennaam, A. L., Goodman, J.D. and Medrano, J.F. 1998. Mitotic analysis of the North
American white sturgeon, Acipenser transmontanus Richardson (Pisces, Acipenseridae), a fish with a
very high chromosome number. Genome 41; 266-271.
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LIST OF FIGURES
Plate 1: Two species of sturgeons from the Pacific coast of North America.
- Adult White Sturgeon or Steluga Huso (Acipenser) transmontanus (Richardson, 1836) from
the Sacramento River captured near Grimes, Colusa Country, California, USA. - Top: Adult White Sturgeon or Steluga Huso (Acipenser) transmontanus (Richardson, 1836)
from the Sacramento River captured near Grimes, Colusa Country, California, USA.
Bottom: Adult Columbia Sturgeon Acipenser columbiaensis sp.nov. farm raised offspring
of parents from the Snake River, Idaho, USA. Note the morphological differences between the
two sturgeons. - Close up of heads. Note shape of snout and head.
- Adult wild specimen of the Nechako River Sturgeon, British Columbia, Canada, a conspecific
of the Snake River Sturgeon Acipenser columbiaensis sp.nov. - Adult wild specimen of the Kootenai River Sturgeon, Idaho and British Columbia, Canada, a
conspecific of the Snake River Sturgeon Acipenser columbiaensis sp.nov.
Plate 2: Ventral view of the Columbia Sturgeon and the Steluga.
- Left: Adult Acipenser columbiaensis sp.nov.; Right: Adult Huso (Acipenser) transmontanus
(Richardson, 1836). Note – 1. Shape of snout; 2. Position of barbels in relation to mouth and
tip of snout; 3. Shape of mouth and lip characteristics; 4. Coverage of branchiostegal
membranes. - Adult Acipenser columbiaensis sp.nov showing attachment of branchistegal membranes directly
to the isthmus. - Adult Huso (Acipenser) transmontanus (Richardson, 1836) showing branchiostegal membranes
joined to each other and forming a free fold over the isthmus. - Close-up of joined branchistegal membranes and free fold shown above.
Plate 3: Close-up of Adult Huso (Acipenser) transmontanus (Richardson, 1836).
- Note the high profile of head, short snout and position of barbels.
- Heavy ossification and strong bony plates of the head, the large first dorsal scute and the frontal
position of spiracles. - Crescentic shape of mouth, branchiostegal membranes joined to each other and forming a free
fold; position of barbels. - Ventral view showing branchiostegal membranes joined to each other and forming a free fold.
- Head-on view: note the rounded form of the head, location of spiracles and the high profile of
head.
Plate 4: Characteristics of Juveniles.
- Note shape of endochondral rostrum. Left : Huso (Acipenser) transmontanus (Richardson,
1836); Right: Acipenser columbiaensis sp.nov. - Huso (Acipenser) transmontanus (Richardson, 1836): Note - Branchiostegal membranes
attached to the isthmus (arrow points to snout) in one year old. - Acipenser columbiaensis sp.nov.: Note - Branchiostegal membranes attached to the isthmus
(arrow points to snout) in 2 year old. - Top: One year old Huso (Acipenser) transmontanus (Richardson, 1836); Bottom: Two years
old Acipenser columbiaensis sp.nov. Note - relative length of snout. - Top row: Huso (Acipenser) transmontanus (Richardson, 1836): Left to Right: Juvenile - first
dorsal scute, second dorsal scute, dorsal view of Ventral Rostral Bone (VRB); side view of
VBR, note posterior webbed structure; Adult VRB, note enlarged webbed structure, first dorsal
scute (larger), second dorsal scute. Bottom Row: Acipenser columbiaensis sp.nov: Left to
Right: Juvenile - first dorsal scute, second dorsal scute, dorsal view of VRB made up of many
bony elements; Adult VRB, note differences with Steluga above, first dorsal scute (smaller),
second dorsal scute, note differences with Steluga.
Plate 5. Inter-generic Hybrids.
- Two to three year old farm raised hybrids.
- Same hybrids dorsal view, note variations in snout shape.
- Branchiostegal membranes are narrowly attached to the isthmus in this hybrid.
- Six years old female with branchiostegal membranes joined to each other and forming a free
fold over the isthmus in this hybrid. - Close up of free fold, note attachment to the isthmus (cut line).
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