 |
 |
| Highlights |
| |
|
| Highlights |
Amanda
Cass
Research Training Program, 2003
|
A preliminary survey of the dorsal gill arches of flatfishes (Pleuronectiformes) and an examination of potential phylogenetic consequences. ABSTRACT The
flatfishes (Pleuronectiformes) are a diverse and widespread order of
fishes united by their ocular asymmetry. Due to their reduced skeletal
morphology, much of their classification is based on loss and fusion
characters, especially those of the caudal skeleton. Though these characters
are used in other classifications and may be phylogenetically informative,
no studies of pleuronectiform ontogeny have been conducted to determine
homologies of the remaining elements, thus making them a weak tool for
phylogenetic analysis. Another character system used to determine phylogenies
is the gill arches, specifically the dorsal complex. To date, no systematic
study of the Pleuronectiformes has included a detailed analysis of gill
arch variation. The aim of the current study was to document variation
within this structure across taxa and determine whether it warranted
further study. Representatives from each of 10 families were prepared
for dissection via clearing and counter-staining. The gill arches were
removed from these specimens, observed and documented. Pleuronectiform
gill arches were found to vary widely, with several characters delineating
previously identified taxa and others drawing connections between groups
thought to be only distantly related. Further studies including more
taxa are planned and may elucidate the underlying patterns present in
this important character system. It is hoped that these studies will
shed some light on the obscure interrelationships of these fishes. The flatfishes (Pleuronectiformes) are a diverse order of fishes, comprising over 570 species in 11 families. For many years they were united by only one shared derived character, their unique metamorphosis from pelagic symmetrical larvae to benthic asymmetrical adults. During this metamorphosis many changes occur, the most dramatic of which is the migration of one eye over the middorsal line. As Pleuronectiformes are popular food fishes (including flounder, sole and halibut) and live worldwide, they are of great importance to many economies. Due to their atypical development and adult morphology, they are also of particular interest to science. Yet, much about them is still unknown. Their unusual asymmetry has minimized challenges to their monophyly, but both super- and sub-ordinal relationships are difficult to determine and have been revised many times. Some difficulty in determining their relationships arises from skeletal reduction—many bones, distinct in other fishes, are absent or fused. Furthermore, their tendency to lose rather than gain characters means that more often one finds shared losses, rather than shared structures. Few synapomorphies have been identified that unite families or groups of genera. To date, the caudal skeleton has provided most of the characters that define subgroups.
Posterior
to the mouth and ventral to the braincase lies a complex structure with
a framework of bones and cartilage called the gill arches. This apparatus
serves two important functions; it supports the gill filaments used
in respiration and assists with the ingestion and mastication of food.
This structure is composed of a medial and ventral series of bones,
basibranchials, with bilaterally paired elements extending vertically.
These paired elements are composed of many smaller bones, each described
as lying either in the dorsal (pharyngobranchials and epibranchials)
or ventral (ceratobranchials and hypobranchials) portion of the arch.
Although the ventral complex is phylogenetically informative, more characters
are usually found in the arrangements of tiny bones and cartilages in
the dorsal gill arches, which vary considerably between groups. To date,
pleuronectid gill arch characters have not been studied closely.
The
caudal (tail) fin of fishes is composed of many fan-like bones associated
with the terminal centrum. Bones that form above the notochord of a
developing embryo are referred to as epurals (EP) and those that develop
below are referred to as hypurals (HP). Other, smaller bones referred
to as uroneurals (UN) may be present in the dorsal portion of the fin.
Primitively, fish have many bones in the tail, but these may be lost
or fused. Similarities and differences in loss and fusion patterns are
often used in systematic studies, as there are many possible patterns
of fusion and shared patterns can indicate relationships. However, adult
fins that appear similar do not necessarily result from the same developmental
pathways and thus may not be homologous. There is no way to determine
definitively or refute homology, but careful study of larval series
can identify ontogenetic fusions. Currently a great deal of pleuronectiform
classification rests on caudal skeleton characters; however, ontogeny
has been studied in few taxa. Though a great deal of change has occurred
in these fishes from a caudal skeleton like that of Psettodes (long
presumed to be the basal taxon) to the forms below, because the changes
are reductions, shared ontogenetic trajectories would give us more confidence
when using similar adult endpoints as characters.
The National Science Foundation Research Experiences for Undergraduates program for funding this research (Award: #DBI-0243512 ). The Smithsonian Museum of Natural History Research Training Program for providing the circumstances for it to occur. Dr Lynne Parenti and Dr G. David Johnson for their advice and guidance. Sandra Reardon for her help with digital photography and x-ray. Want to ask a question? Visit the Message Board Virtual
Poster Session Message Board Research
Training Program
|
||
|
Research
from
Systematic Biology |
||
|
ENTOMOLOGY "Databasing
Moth Genitalia Facts" |
||
|
VERTEBRATE
ZOOLOGY |
||
|
Research
from
Paleobiology |
||
|
Research
from
Mineral Sciences |
||
|
Research
from
Anthropology |
||
|
ANTHROPOLOGY "Sexual Dimorphism of the Distal Humerus" |
||
|
ANTHROPOLOGY "Nonmetric Racing of the Skull in Blacks and Whites" |
||
|
More RTP Class of '03 Links |
|
Research
Abstract
|
Information
|
Highlights
|
Alumni Pages
Application
Procedure
|
Requirements
|
Application
Status
Application
Form
|
Cover
Letter Form | Letter
of Recommendation Form
| Advisor
List
| NMNH Home | What's New ? | Calendar of Events | Information Desk | Search |
