51813_Sturgeon biodioversity an.PDF

evolution of respiration in Acipenseroidei. Living
stem actinopterygians and sarcopterygians are not
likely to help in a broader analysis of the evolution
of ram ventilation because none of them exhibits
constant, sustained swimming.

Potamodromy, anadromy and demersal spawning

All living species of sturgeons and paddlefishes mi-
grate upstream to spawning sites, and many species
of Acipenseridae are anadromous. Spawning runs of
some species of sturgeons are legendary, exceeding
2500 km forHuso husoin the Danube and Volga ba-
sins (Balon 1967, &Hensel Holcík 1997 this volume,∨
Khodorevskaya et al. 1997 this volume). Typically.
living acipenseriforms spawn very large numbers of
eggs onto shallow, gravely sites (e.g.,Ryder 1888,
Disler 1949, Soin1951,Dragomirov 1953,1957, Buck-
ley & Kynard 1985). Adults depart the site soon after
egg deposition, anddonot provide any parental care. Figure 25,Method for analyzing ‘less direct’ evidence of paedo-
The phylogenetic distribution of spawning modes
in the other living lion-teleostean actinopteryg ians
offers little information about the evolution of these
reproductive features (see Balon 1975, 1985). None

terygians typically inhabits salt water, and none ex-

hibits such extensive upstream spawning migrations

(for general review, see Breder & Rosen 1966).

SpawningPolypterusdeposit small numbers of indi-

vidual eggs on leaves, rocks or other substrates. At

least some species of Lepisosteusmigrate upstream

to spawn, but spawn far fewer eggs than do sturgeons

and paddlefishes and typically deposit them on

structures in the water, such as Ieaves.Amiaspawn

in nests defended by the male (Ballard 1986).

Extant stem sarcopterygians are similarly unin-

formative on the question: coelacanths are com-

bined livebearers (Wourms et al. 1991, Balon 1985,

1990, 1991), and lungfishes either spawn individual

eggs onto aquatic vcgctation (as inNeoceratodus,

Kemp 1987) or the males guard the eggs in a nest (as

inProtopterusandLepidosiren,Greenwood 1987,

Kerr 1919).

Evidence for anadromy or spawning migrations

in fossil Acipenseriformes or outgroups is obvious-

ly uncertain, but this has not prevented speculation.

morphosis. The cladogram for taxa U through Z is based on char-

acter information other than the development characters

shown. Development is studied in taxa U through Z. It is found

that state’a’ always gives rise to state ’b’ during ontogeny and that

in most clades, state ’b’ goes on to form state ‘c’. This information

has been truncated at state ‘b’, providing indirect evidence that

paedomorphosis has occurred. Modified from Bemis (1984).

Zhou (1992) suggested that †Peipiaosteus may have

been anadromous based on the geographic distri-

bution of fossils. Nielsen (1949) noted that speci-

mens of †Birgeria greonlandica occurred in small

and large sizes only, and regarded the absence of

intermediate sizes as evidence of migration. Yakov-

lev (1977) also speculated on the evolution of repro-

ductive mode in acipenseriforms. At present, how-

ever, we cannot provide a compelling explanation

for the evolution of characteristic spawning pat-

terns of Acipenseriformes.

Of the other living species of non-teleostean actinop- is then ‘laid out’ on the cladogram; in taxa X and Z, devevplment

Paedomorphosis

Paedomorphosis has long been invoked in explain-

ing many anatomical features of living Acipenseri-

formes, chiefly the largely cartilaginous endoskele-