Phylum: Chordata
Class: Actinopteri
Order: Thoracostei
Family: Syngnathidae
Subfamily: Phyllopteryginae
Genus: Phycodurus
Species: Phycodurus eques
San Francisco State University
Geography 316: Biogeography
The Biogeography of the Leafy Seadragon
(Phycodurus eques)
by Gregor Fenton Gomory, student in Geography 316, Fall 1999
Kingdom: Animalia
Phylum: Chordata
Class: Actinopteri
Order: Thoracostei
Family: Syngnathidae
Subfamily: Phyllopteryginae
Genus: Phycodurus
Species: Phycodurus eques
Description of Species:
The leafy seadragon, P. eques, is a spectacularly camouflaged
fish found exclusively in the brown kelp beds of Southern Australia. Seadragons
belong to the family Syngnathidae, which also includes the seahorses, pipefish and
pipehorses. Leafy seadragons reach a length of approximately
35 centimeters and are covered with jointed, armor-like plates instead of scales (Shedd
Aquarium 1998). Unlike its seahorse cousins, the leafy seadragon has a laterally
flattened body and swims horizontally through the water column. "Leafies" are
best known for the incredible skin filaments that hang off of their head, body and tail
like so many blades of brown seaweed. The body is protected by multiple long, sharp
spines that run from the dorsal surface and enclose the body.
Habitat:
Phycodurus eques is endemic to the temperate waters along the Southern
coast of Australia living in and among the kelp-covered rocky reefs which fringe
Australia’s southern shoreline. The temperate coastal waters of Southern Australia
contain an astounding variety of marine life with diverse ecosystems ranging from large
metropolitan seagrass beds to saltmarsh and mangrove areas. The leafy seadragon is
superbly camouflaged as a floating piece of seaweed and spends the entirety of its life
swimming among the three genera of brown algae, Macrocystis, Ecklonia, and Lessonia, that
comprise the mid-littoral kelp beds of the Southern Australian coast. The fronds of
the tall kelp forests considerably reduce both cross-shore water movement and along shore
currents; an important asset to the weak-swimming seadragon (Lobban & Harrison
1994). The region serves as the ecotone between the temperate Pacific Ocean and the
tropical Indian Ocean and water temperatures range from 14-19 degrees Celsius throughout
the year. Seadragons are comfortable throughout the water column of the mid-littoral
zone, ranging in depth from roughly 5-35 meters, but are most commonly found at between
5-15 meters feeding on mysid shrimp and other small invertebrates. The habitat of
the Leafy Sea Dragon is restricted to kelp forests primarily because its fantastic
camouflage would serve as a garish advertisement in an environment devoid of the
vegetation.
Natural History:
Although a relatively common addition to the popular scientific press,
ie National Geographic and American Scientist, there have been very few biological studies
on seadragons published. Much of the information available about seadragons has come
from studies of preserved species or recorded observations by recreational divers (Mackay
1998). A great deal of information regarding the lifecycle and natural history of
the seadragon is based on characteristics found in seahorses and remains to be
confirmed. Seadragons are called leafy or weedy depending on the number and
appearance of their distinctly plant-like appendages and are in the family Syngnathidae,
sharing characteristics such as long tube-snouted mouths and bodies enclosed in bony
plates with their more common cousins the seahorses (Hippocampinae) and pipefish
(Syngnathine). The Leafy Sea Dragon is one of the natural world’s undisputed
masters of camouflage, blending perfectly with the surrounding kelp beds.
The combination of armored body, protective spines and little actual
musculature make the sea dragon relatively undesirable as a food source but they are
believed to be preyed upon by skates, some shark species, and some pelagic fishes (Lourie
1999). Researchers in Southern Australia have witnessed the seadragon "curling
up" and presenting the spines to attacking fish (Flaherty 1997) as a defense
mechanism. Although especially susceptible to the seasonal storms that wreak havoc
on kelp communities, the mortality rates for seadragons are believed to be relatively low
due to the amount of eggs produced per season and the general lack of predators who rely
on them as a primary food source (Whitley & Allan 1958).
Almost every physical characteristic of the Syngnathids differs in most
respects from other fish species. While sea horses swim vertically through the water
column, seadragons swim horizontally aided by two sets of very small dorsal fins; one at
the junction of the slender neck and the body and one at the base of the tail.
Balance is maintained by two pectoral fins that end as leafy appendages.
The distinct lack of swimming apparatus make the seadragon a very poor
swimmer but the leafy projections that hang from the head, body and tail aid in balance
and bouancy. It was once believed that the seadragon, like the seahorse, had
prehensile use of its tail and curled the end around thin kelp branches to focus feeding
upon a localized area (Whitley and Allan 1958). Contemporary observations have shown
that the seadragon does not use its tail in a prehensile fashion and, in fact, spends its
life in motion (Mackay 1998). Another unique feature common to Sygnathids are
small,circular gill openings covering tufted gills, very unlike the crescent shaped gill
openings and ridged gills of most fish species (Lourie 1999).
It is believed that like seahorses, seadragons breed once per year,
usually in August and September, and produce approximately 250 eggs (Whitley and
Allan). Sygnathids are unique in that the female deposits unfertilized eggs either
in a specialized brood pouch in the case of seahorses or on a external brood patch at the
base of the tail in seadragons. The brood patch on male seadragons consists of a
spongy area located on the ventral surface of the tail equipped with small cup-like
indentations each designed to house an individual fertilized egg (Mackay 1998). The
eggs are fertilized during the transfer from female to male and remain attached until they
hatch. It is speculated that as in seahorses female seadragons will approach only one male
during breeding but multiple females will use the egg storage capacity of a single male
until his brood patch is completely covered (Whitley and Allan 1958).
Evolution:
Paleontological evidence suggests that there have been living
representatives of the four sub-classes of present day fish - Sharks, Chimaeroids,
Dipnoans and Teleostomes, since the early Devonian Era, more than 400 million years ago
(Carroll 1988). Confined by the liquid medium in which they live, fish have evolved
and diverged structurally, primarily through differences in gills, skin defenses, fins,
and sense organs (Dean 1895). The order Teleost, containing all bonyfish and
appearing roughly 420 million years ago, began to split into the myriad different forms
early in fish evolutionary history (Carroll 1988).
The family Syngnathidae, characterized by fused jaws, is believed to
have evolved from a common ancestor with the perches, bass, sticklebacks and
chiclids. Syngnathids, which include the Seahorses, Pipefish and Seadragons, began
to emerge as a distinct family during the lower Tertiary Period, roughly forty-five
million years ago (Carroll 1988). Unlike many of their cousins, the Syngnathids
evolved highly specialized and unique adaptations to their environment including vertical
symmetry and prehensalism. The Syngnathidae family consists of over two
hundred and fifteen species in fifty-two genera and although primarily marine may also be
found in fresh and brackish water (Lourie, Vincent and Hall 1999). The pipefish,
slender and eel-like, are believed to be the oldest of the Syngnathids and the first to
evolve prehensile use of the tail, an extremely uncommon adaptation (Carroll 1988).
Extinct pipefish from the Eocene genera Urosphen, Solenorhyncus and Calamostoma appear to
have prehensile ability and may be the oldest forms of present day Syngnathids (Whitley
1958). The evolution of the prehensile tail is finely interwoven with the evolution
of the near shore seagrass and kelp communities and the appearance of the seahorse as a
distinct genus (Hippocampus) is a further adaptation to clinging to aquatic vegetation in
search of prey.
Hippocampus took another huge evolutionary step with the adaptation of
vertical symmetry, another extremely unusual adaptation in Teleosts. Although the
date of Sygnathid evolutionary radiation into distinct genera is not well known it is
believed to have occurred at some point near the end of the Eocene epoch roughly
thirty-five million years ago (Carroll 1988). There is some debate among
ichthyologists as to the evolutionary appearance of the horizontally-swimming seadragon,
either as a stepping stone between the horizontal pipefish and the vertical seahorse or as
a divergent cousin of the seahorse. The two distinct and separate genera of
seadragons, Phycodurus and Phyllopteryx, are believed to have divergently evolved from two
separate pipefish genii; ancestral forms of Leptonotus and Haliichtys respectively
(Whitley and Allan 1958). There is no fossil record of the seadragon available for
dating so the exact evolutionary history can only be speculated. At the end of a
long line of extreme evolutionary adaptations, although in no way less dramatic than its
predecessors, are the leafy appendages that serve to blend the seadragon into its
surroundings.
Although the elongated and camouflaged appendages are also found less
dramatically in its close cousins Phyllopteryx taeniolatus (weedy seadragon) and
Haliicthys taeniophora (ribboned seadragon), the leafy seadragon is by far one of the
greatest examples of adaptive camouflage in the animal kingdom.
Distribution
The leafy seadragon is endemic to the brown kelp forest communities
fringing the Southern coastline of Australia. P. eques was initially believed
to be restricted to an area ranging from Kangaroo Island in South Australia to Lancelim in
West Australia (Hutchins 1986). Recent observations have expanded this range roughly
800 kilometers east of Kangaroo Island and 100 kilometers north of Lancelim (Mackay 1998).
Map of Distribution:
Seadragon Conservation:
Both leafy and weedy seadragons are legally protected under the
state legislatures of both South and Western Australia. Regardless of protection
status, seadragons face pressure from poaching for aquaria enthusiasts, curio seekers and
use in the ever-expanding traditional Chinese medicine market (Flaherty 1997). All
genera and species of Sygnathid are extremely vulnerable to human-enduced exploitation due
to their unique ecology and distribution. Two main factors that affect seadragon
recovery rates in exploited areas are the pregnancy of the male, which removes young from
the wild when the father is caught, and the low rates of natural mortality which means
that human poaching is a new and unevolved for pressure (Lourie, Vincent and Hall 1999).
As is the case with many non-commercial species worldwide, there is
very limited baseline information available regarding seadragon population size and
reproduction rates. This lack of information makes the creation of sensible
management plans an impossibility (Flaherty 1997). A community-based marine
monitoring and conservation project called Dragon Search has been successfully implemented
in coastal communities throughout Southern Australia. The goal of Dragon Search is
to integrate local communities, recreational divers and researchers in an effort to both
gather baseline ecological information and to turn the seadragon into a popular symbol for
the diversity and uniqueness of Southern Australia's marine environment (Flaherty 1997).
The initial success of Dragon Search is hoped to prompt similar community-based projects
throughout the world.
Bibliography
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