51813_Sturgeon biodioversity an.PDF

of extensive adult scalation and peduncle scalation

inScaphirhynchus is unclear, but correlates well

with their benthic life history and persistent contact

with the substrate.

tween the ventral scute rows. Among acipenserids,
only scaphirhynchines make extensive contact with
the substrate, and their highly flattened belly serves
as an ideal resting platform as Scaphirhynchusis es-
pecially benthic and mobile on the substrate (see
below). Locomotion
Initially, flattening of the head and trunk seems to
support an expanded suite of ampullary organs as
well as conform the skull to ventral displacement of
the jaws. Trunk compression only occurs in ad-
vanced sturgeons, asAcipensergenerally remains a
genus of mobile predators while scaphirhynchines
become obligate benthivores.

Scalation
Extensive scalation correlates with benthic habits
in many groups of fishes, and may serve for protec-
tion against the substrate or to counter predation.
While acipenserid outgroups lack any extensive
scalation, the most distinctive hallmark of acipense-
rids is their scutes (Character 1). A precise role for
scutes is unknown, but they are prominent and
sharp in juveniles as a tight assemblage covering
well over 50% of the trunk, suggesting that those
ages are most reliant on scalation. In fact, juvenile

Benthic cruising implies mobile interaction with the

substrate. Acipenserids are strong swimmers with

several features facilitating this behavior. Morphol-

ogy of the caudal fin is not a character here due to

variation within Acipenseridae and among acipen-

seriforms, but the hypochordal lobe is often re-

duced to allow sweeping of the tail while close to the

substrate. More pelagic taxa (e.g.,Huso, Acipenser

oxyrinchus) possess deep caudal fins similar to

those of †Chondrosteusand polyodontids, while

more benthic species possess either abbreviated

(e.g.,A. brevirostrum) or obliquely folded hypo-

chordal lobes (Scaphirhynchus).

Pectoral fin spines of acipenserids (see Charac-

ters 2,12,16) are neither sharp, nor possess serrated

edges exposed for protection. Instead, they are per-

manent lateral processes erected during swimming

as diving planes. Acipenserids can excellently main-

tain position over variable bottoms by precisely

acipenserids prefer benthic habitats (Richmond &

Kynard 1995) and even juvenileHuso husofocuses

on benthic prey (Pirogovskii et al. 1989) when

scutes are most developed. Scutes become allomet-

rically reduced in adults in size, roughness, and co-

hesiveness (they become well separated within the

rows) and actually regress significantly and are pro-

gressively covered by skin in some species (e.g.,H.

huso, Acipenser fulvescens, A. nudiventris). As same precision during locomotion.

large fishes, adult sturgeons do not face serious pre-

dation and tend to be more active above the sub-

strate, and we might regard scutes as an adaptation

for juveniles rather than adults in Acipenseridae.

Scaphirhynchines retain well developed scutes in

adults that are broader and more tightly overlap-

ping than other acipenserids. Also, scales on the pe-

duncle are large and organized as multiple, discrete

groups inScaphirhynchus(Character 65) with the

posterior trunk completely armored by overlapping

scales. Peduncle scales are present in all other aci-

penserids, but vary in number and size and never

form an extensive cover (see Character 1). Purpose

regulating inclination of the spines to modulate

depth during swimming. Pectoral fin spines of Sca-

phirhynchusare uniquely curved and are used as

‘legs’ to shuffle along the substrate during feeding

and exploration, something not done by other aci-

penserids. In this scenario, fin spines are locomoto-

ry stabilizers for depth control. Polyodontids lack

fin spines, but are midwater fishes not requiring the

In all cases, acipenserids show phylogenetic

trends suggesting increasing abilities as benthic

fishes. Huso is morphologically and behaviorally

pelagic as adults, but possess underlying characters

indicating an original shift to benthic behaviors in

Acipenseridae. This shift is likely associated with

evolutionary success of juvenile interval in Huso

when benthic prey (notable other acipenserids) and

predator avoidance is most important. Husothen

rises into pelagic environments when large enough

to exploit larger prey. Regardless of the initial rea-

son to exploit benthic habitats in primitive acipen-

serids, original opportunities afforded by the rich