| Ophiuroid (Brittlestar) |
 | |  | |  | |  | |  | |  | |  | |  | |  |
General body plan & External features
Five (rarely 6-7) symmetrically placed, long, slender, often spiny jointed arms radiate from a small flattened disk
(which is 10-30 mm diameter). The disk may have a circular, pentagonal or scalloped contour and the arms are
sharply demarcated from it. The arms are long compared to the disc, typically 3-6 times the disk diameter, sometimes
much more. The serpentine appearance and movement of the arms give the ophiuroids their name of serpent stars.
Basket Stars are ophiuroids (F. Gorgonocephalidae) with highly branched arms. The arms can branch multiple times
create a ball-like tangle of snake-like tendrils!
The aboral disk surface (the surface opposite the mouth, which is the top surface) may be smooth and leathery,
granulated, spiny, scaly or the embedded plates may be visible. The arms lack ambulacral grooves (sea asteroids)
and have joints composed of internal ossicles (called vertebral ossicles or vertebrae).
The disks tend to have duller colours than many Asteroids, including: cream, yellow, green, olive, gray, brown,
maroon, purple, black and may have spots or bands. The arms are often a different colour to the disk.
The body wall
An epidermis may be lacking over much of the body surface, and when present it may be syncytial (that is the adjacent
cells fuse to form an undivided multi-nucleate sheet). Cilia (flagella?) are restricted to the areas around the bursal slits
and in some species they also occur on the oral surface (of the disk and arm bases). There are no pedicellariae (see
asteroids) and no papulae (see asteroids). The dermis contains the endoskeletal ossicles.
Endoskeleton
Superficial ossicles form shields. The most prominent are the radial shields at the base of each arm, which may be
spoke-like. Each arm is covered by a series of arm shields: aboral and oral arm shields (often much reduced) and
prominent lateral shields. The aboral shield may be split-up into a mosaic of small plates. The lateral shields are
equivalent to the adambulacral ossicles of asteroids and may have spines (0-15 spines per shield) which may be
glandular, and are possibly poisonous.
Deeper ossicles form the vertebral ossicles (vertebrae) in the arms. These are disc-like with lateral wings for the
attachment of muscles. They form elaborate joints with each other, either of the peg and pit variety or of opposing
'hourglass articulations’. Two abutting hourglass-shaped surfaces at right angles forming an articulation.
The nature of the vertebral joints and the range of movement that they allow form the basis for the classification of the
ophiuroids into two orders:
Order 1. Ophiurae
• Arms unbranched ;
• Arms move horizontally;
• Pit/projection joints between vertebral ossicles;
• Arms cannot twine around objects.
Order 2. Euryalae
• Arms may be branched;
• Hourglass articulations between vertebral ossicles;
• Arms can move vertically;
• Arms can entwine around objects.
The vertebral ossicles are thought to have derived from fused ambulacral plates, which became internalised with the
closing over the ambulacral grooves, which became an internal canal (with its roof formed by a notch in the oral edge
of each vertebra). This canal caries the radial nerve, water canal, and the haemal/perihaemal radial sinuses.
The mouth frame is comprised of five wedge-shaped interradial jaws (modified plates) bearing teeth (modified spines).
The jaws are formed by the fusion of two sets of plates, and each such ‘half-jaw’ has two podial pores for the buccal
least in part, by fusion of ambulacral plates.
Podia
The podia (tube-feet) which are characteristic of echinoderms are reduced to small papillae (tentacles) and there is
one pair per arm joint on the oral surface. These papillae are often protected by immovable tentacle scales. The
papillae have adhesive gland cells.
Ophiuroids are, like asteroids (starfish) echinoderms
('spiny-skinned'). They may look like starfish, but they
typically move not y slow crawling of the tube-feet, but by
horizontal or side-to-side serpentine undulations of the
arms. Note the central disc which has a central plate and
prominent radiating radial plates or radial shields. Above:
top-view or aboral view. Note the rows of spines along the
sides of each arm.
('spiny-skinned'). They may look like starfish, but they
typically move not y slow crawling of the tube-feet, but by
horizontal or side-to-side serpentine undulations of the
arms. Note the central disc which has a central plate and
prominent radiating radial plates or radial shields. Above:
top-view or aboral view. Note the rows of spines along the
sides of each arm.
Above: the underside or oral view. Note the star-shaped mouth
formed by the plate-like jaws and the bursal slits, one pair of
which flank each arm. Also, running down the underside of each
arm are two rows or protuberances, these are the tube-feet.
Tech-level: 2/3
Prerequisites: asteroids
formed by the plate-like jaws and the bursal slits, one pair of
which flank each arm. Also, running down the underside of each
arm are two rows or protuberances, these are the tube-feet.
Tech-level: 2/3
Prerequisites: asteroids
Transport systems
The water-vascular, haemal and perihaemal systems are similar to those of asteroids, except that the madreporite is
on the oral surface.
Excretion
The bursae may be the main centre for waste removal, including waste-laden coelomocytes (cells that float free in the
coelomic fluid of the coelom body cavities).
Respiration
The bursae are 10 sac-like invaginations in the oral disk wall alongside the arm bases, occupying the spaces
between the stomach pouches. They open by the prominent elongated bursal slits on the oral side of the disk.
(Except in Ophioderma, which has two pores in place of each slit). They are flanked by the genital shields. The
bursae may fuse into a single chamber in some species, and are absent in some species. Respiratory water currents
circulate through the bursae, and in some species movements of the aboral disk may pump the system. Gametes are
also shed through the bursal slits, and the bursae may act as brooding chambers.
Nervous system
As in asteroids, the nervous system consists of a circumoral nerve ring (i.e. a ring encircling the oral cavity) and
radial nerves. These systems are double: the outer thick ectoneural system is sensory and motor, and the inner
thin hyponeural system is motor only.
Sensory systems
Some ophiuroids show strong reactions to light and most ophiuroids are negatively phototropic. Ophiocoma wendtii
is one such species. The photoreceptors were for a long time thought to be diffuse epidermal photoreceptors, but
recent research suggests the possible presence of compound eyes with calcite microlenses. The dorsal (aboral) arm
plates (and the dorsal regions of the lateral arm plates) have arrays of hemispheres (each 40-50 mm) on their
external surface. These transparent hemispheres resemble lenses in section and can focus light. Within the mesh of
the ossicle (stereom) nerve bundles are found at the correct depth and it has been hypothesised that these nerves
are the photoreceptors. Pigment is also found in the stereom in chromatophores. It is thought that these
chromatophores regulate the intensity of light reaching the photoreceptors by extending their pigment-filled processes
to cover the lens during the day and retracting them at night. This corresponds to the diurnal colour changes in
Ophiocoma wendtii - from homogeneous dark brown during the day to banded grey and black at night. Ophiocoma
pumila shows little reaction to light, no diurnal colour change, and lacks the calcite lens-like structures.
Podia and spines also have sensory functions. Like sea stars, ophiuroids can detect food at a distance, presumably
via chemical cues. Although each arm has a terminal podium at its tip, this podium is not photoreceptive as it is in
starfish (asteroids).
Luminescence
Several ophiuroid species are known to be luminescent. The luminescence is limited to the arms, being strongest in
the spines and spine bases, and is absent from the disc. Luminescence is restricted to the arm tips or the lateral
shields or oral shields in various species. Podia are not luminescent. The luminescence is yellow or greenish-yellow. It
is triggered by mechanical (and electrical and chemical) stimuli and spreads via the radial and ring nerves. Some
species luminesce in the light, but others will do so only after a length of time in the dark. The process requires
oxygen. The luminescence is thought to originate in specialised gland cells.
Locomotion
Ophiuroids are the most mobile echinoderms. When they move, the disk is raised above the substrate and with two
arms pointing forwards and one or two arms trailing, the two lateral arms perform rapid ‘rowing’ movements against the
substratum, propelling the animal in a gliding motion. The spines on the arms provide traction. There is no preferred
dominant arm (as there is in some asteroids). A few species creep slowly on their podia, as do asteroids. Many
ophiuroids can only move their arms horizontally, from side to side. Some species, however, can also move them
vertically and these can entwine their arms around objects.
Each arm is divided into segments or joints, each bearing one pair of tube-feet (podia or tentacles) on the oral side
(underside). Each arm segment has a superficial skeleton of 4 plates or shields: 2 lateral, one aboral (uppermost) and
one oral (lowermost). We tend to think of this as an exoskeleton, though the shields are embedded in the body wall
and covered with epidermis, at least initially, and so technically constitute an endoskeleton (in insects, for example, the
epidermis is beneath the cuticle which is therefore a true exoskeleton). Each arm segment also has a deeper
endoskeleton of vertebral ossicles or vertebrae. Each vertebra has a thicker central region and thinner side wings
to which intervertebral muscles attach. Medially (towards the midline) each vertebra bears a notch on the oral side,
forming a tunnel to house the radial nerve. A proximal (towards the arm base) pair of canals open from the inside walls
of this tunnel in each vertebra: these house a pair of nerve that innervate the intervertebral muscles. A further more
distal (towards the arm tip) pair of canals house a pair of branches of the water-vascular system which supply the pair
of podia. Each face (proximal and distal) of each vertebra bears a series of projections and depressions which fit into
complementary depressions and projections in the vertebra opposite, forming a pair of articulating surfaces which
generally allow only sideways movements. In some, however, one articulating face bears a horizontal hourglass-
shaped projection which fits into a vertical hourglass projection on the opposite face, allowing the two vertebrae to
rotate relative to one-another. This latter arrangement allows vertical movements, enabling the arms to entwine
around objects.
Each arm segment has a pair of podia which emerge between the lateral and oral arm shields. Each podium is closely
associated with one or more modified spines, called tentacle scales. These tentacle scales are usually immovable,
but in some species some of the scales may be equipped with a muscle and is flagellated (or ciliated?) allowing the
arm to generate water currents used in feeding. Burrowing species excavate mucus-lined tubular burrows by
undulating arm movements, accompanied by lateral digging movements of the podia. Arm undulation ventilates the
burrow. Most brittlestars have no body wall muscles, but some burrowing forms do and these may enable the central
disc to assist pumping.
In some species the podia have minute wart-like papillae which produce an adhesive secretion, allowing the brittlestar
to climb vertical surfaces. Some brittlestars have been observed to swim by undulating movements of the arms and
some have feather-shaped spines to facilitate: during the power stroke the spines erect to increase resistance against
the water and during the recovery stroke they lie flat, the whole movement being accompanied in waves of activity
spreading along each arm from the base to the tip.
Up to about 15 spines may occur on each lateral shield of each arm segment. Spines vary considerably in form and
texture from species to species and may be glandular or modified into hooks. In some species, spines also occur on
the central disc. Each spine has an internal calcareous skeleton enclosing a core of connective tissue and covered by
epidermis and dermis. The spines are often movable.
Nutrition
Ophiuroids are carnivores, scavengers, deposit feeders or filter feeders. Most use several of these feeding modes.
Ophiocomina nigra uses all four, but is predominantly a suspension feeder.
In deposit and suspension feeding, plankton and detritus adhere to mucous strands strung between the arm
spines. The captured food particles are transported to the tentacle scales, by ciliary currents and/or by the tube
feet. In some brittlestars the tentacle scales and the oral surface of the arms are ciliated (or flagellated?) and the
beating of these minute motile hairs generates water currents which can sweep particles towards the central disc;
further tracts of cilia on the oral face of the disc carry particles to the mouth. In burrowing forms the tips of the arms
may remain above the surface, sweeping the sand to collect detritus which the podia pass to the mouth. Podia around
the mouth sort the particles.
In filter feeding arms are elevated with the oral surface facing the water current. The podia extend beyond the
spines to form combs. Food particles stick to the podia and are periodically wiped onto the spines and collected by
other tube feet. In either case, these tube feet compact the particles into a bolus and transport this growing bolus to
the mouth in a wave-like fashion along the midoral line (the midline on the oral surface of each arm) of the arm
towards the mouth.
In scavenging the looping motion of an arm sweeps food into the mouth. The teeth or oral tube feet are used to
browse on algae and carcasses. The alimentary canal is simple; there is no anus or intestine and no diverticula
extending into the arms.
Gorgonocephalids or basket stars have a mass of branched tentacles, armed with microscopic hooks. These form a
net which may trap planktonic animals. Most brittlestars are thought to supplement their diet of detritus with small
items Live animals may be caught by the arms looping like snares.
The mouth has five corners (actually the opening of a preoral cavity which leads to the true mouth) and is armed with
five triangular jaws which may be edged by tiny teeth. Each jaw is comprised of two half-jaw pieces. A series of shields
or plates partially covers the jaws from view.
Reproduction
Most serpent stars are dioecious (with separate sexes), and the sexes are indistinguishable, unless the more intense
colour of the female gonads shows through (this may also account for the slight colour differences seen between the
sexes of some asteroids). Four species are exceptions in exhibiting sexual dimorphism. In these species dwarf males
will cling to the larger females, often mouth to mouth or on the aboral surface.
Many species are hermaphroditic. This may involve changes of sex from male to female (protandry, also occurs in
some asteroids). All hermaphroditic and some dioecious species brood their young. Usually gametes are shed into the
sea, through the bursal slits, but in brooding species the eggs may be shed into the bursal sacs where they hatch
whilst inside the female (oviparity) and then the young live in the bursal sacs, which become brooding chambers.
The young may attach to the bursal wall by a stalk (which is part of the bursa). The wall of the bursa and the egg yolk
provide nourishment. Many species are truly viviparous, the eggs being retained in the ovaries until they hatch. The
young may then reside in the brooding chamber of the bursa until quite large. Usually only 1-2 young occupy each
bursa, but as many as 200 embryos have been seen in a single bursa. At least one species of serpent star is known
to attach 20-2000 yolky eggs underneath stones or seaweed, etc.
The gonads are sacs attached to the coelomic walls of the bursae, near to the bursal slits. There are typically 1-2
gonads per bursa, but sometimes there may be clusters or rows of several thousand per bursa.
Asexual reproduction may occur by fission in six-armed species, especially in young or small ophiuroids (< 3 mm
disk diameter). There is no preformed fissure plane. Serpent stars are able to regenerate lost arms, and actually cast-
off arms if handled, or if the arm is trapped. This gives rise to their common name of ‘brittle stars’. The disk needs at
least one arm to survive and regenerate.
The water-vascular, haemal and perihaemal systems are similar to those of asteroids, except that the madreporite is
on the oral surface.
Excretion
The bursae may be the main centre for waste removal, including waste-laden coelomocytes (cells that float free in the
coelomic fluid of the coelom body cavities).
Respiration
The bursae are 10 sac-like invaginations in the oral disk wall alongside the arm bases, occupying the spaces
between the stomach pouches. They open by the prominent elongated bursal slits on the oral side of the disk.
(Except in Ophioderma, which has two pores in place of each slit). They are flanked by the genital shields. The
bursae may fuse into a single chamber in some species, and are absent in some species. Respiratory water currents
circulate through the bursae, and in some species movements of the aboral disk may pump the system. Gametes are
also shed through the bursal slits, and the bursae may act as brooding chambers.
Nervous system
As in asteroids, the nervous system consists of a circumoral nerve ring (i.e. a ring encircling the oral cavity) and
radial nerves. These systems are double: the outer thick ectoneural system is sensory and motor, and the inner
thin hyponeural system is motor only.
Sensory systems
Some ophiuroids show strong reactions to light and most ophiuroids are negatively phototropic. Ophiocoma wendtii
is one such species. The photoreceptors were for a long time thought to be diffuse epidermal photoreceptors, but
recent research suggests the possible presence of compound eyes with calcite microlenses. The dorsal (aboral) arm
plates (and the dorsal regions of the lateral arm plates) have arrays of hemispheres (each 40-50 mm) on their
external surface. These transparent hemispheres resemble lenses in section and can focus light. Within the mesh of
the ossicle (stereom) nerve bundles are found at the correct depth and it has been hypothesised that these nerves
are the photoreceptors. Pigment is also found in the stereom in chromatophores. It is thought that these
chromatophores regulate the intensity of light reaching the photoreceptors by extending their pigment-filled processes
to cover the lens during the day and retracting them at night. This corresponds to the diurnal colour changes in
Ophiocoma wendtii - from homogeneous dark brown during the day to banded grey and black at night. Ophiocoma
pumila shows little reaction to light, no diurnal colour change, and lacks the calcite lens-like structures.
Podia and spines also have sensory functions. Like sea stars, ophiuroids can detect food at a distance, presumably
via chemical cues. Although each arm has a terminal podium at its tip, this podium is not photoreceptive as it is in
starfish (asteroids).
Luminescence
Several ophiuroid species are known to be luminescent. The luminescence is limited to the arms, being strongest in
the spines and spine bases, and is absent from the disc. Luminescence is restricted to the arm tips or the lateral
shields or oral shields in various species. Podia are not luminescent. The luminescence is yellow or greenish-yellow. It
is triggered by mechanical (and electrical and chemical) stimuli and spreads via the radial and ring nerves. Some
species luminesce in the light, but others will do so only after a length of time in the dark. The process requires
oxygen. The luminescence is thought to originate in specialised gland cells.
Locomotion
Ophiuroids are the most mobile echinoderms. When they move, the disk is raised above the substrate and with two
arms pointing forwards and one or two arms trailing, the two lateral arms perform rapid ‘rowing’ movements against the
substratum, propelling the animal in a gliding motion. The spines on the arms provide traction. There is no preferred
dominant arm (as there is in some asteroids). A few species creep slowly on their podia, as do asteroids. Many
ophiuroids can only move their arms horizontally, from side to side. Some species, however, can also move them
vertically and these can entwine their arms around objects.
Each arm is divided into segments or joints, each bearing one pair of tube-feet (podia or tentacles) on the oral side
(underside). Each arm segment has a superficial skeleton of 4 plates or shields: 2 lateral, one aboral (uppermost) and
one oral (lowermost). We tend to think of this as an exoskeleton, though the shields are embedded in the body wall
and covered with epidermis, at least initially, and so technically constitute an endoskeleton (in insects, for example, the
epidermis is beneath the cuticle which is therefore a true exoskeleton). Each arm segment also has a deeper
endoskeleton of vertebral ossicles or vertebrae. Each vertebra has a thicker central region and thinner side wings
to which intervertebral muscles attach. Medially (towards the midline) each vertebra bears a notch on the oral side,
forming a tunnel to house the radial nerve. A proximal (towards the arm base) pair of canals open from the inside walls
of this tunnel in each vertebra: these house a pair of nerve that innervate the intervertebral muscles. A further more
distal (towards the arm tip) pair of canals house a pair of branches of the water-vascular system which supply the pair
of podia. Each face (proximal and distal) of each vertebra bears a series of projections and depressions which fit into
complementary depressions and projections in the vertebra opposite, forming a pair of articulating surfaces which
generally allow only sideways movements. In some, however, one articulating face bears a horizontal hourglass-
shaped projection which fits into a vertical hourglass projection on the opposite face, allowing the two vertebrae to
rotate relative to one-another. This latter arrangement allows vertical movements, enabling the arms to entwine
around objects.
Each arm segment has a pair of podia which emerge between the lateral and oral arm shields. Each podium is closely
associated with one or more modified spines, called tentacle scales. These tentacle scales are usually immovable,
but in some species some of the scales may be equipped with a muscle and is flagellated (or ciliated?) allowing the
arm to generate water currents used in feeding. Burrowing species excavate mucus-lined tubular burrows by
undulating arm movements, accompanied by lateral digging movements of the podia. Arm undulation ventilates the
burrow. Most brittlestars have no body wall muscles, but some burrowing forms do and these may enable the central
disc to assist pumping.
In some species the podia have minute wart-like papillae which produce an adhesive secretion, allowing the brittlestar
to climb vertical surfaces. Some brittlestars have been observed to swim by undulating movements of the arms and
some have feather-shaped spines to facilitate: during the power stroke the spines erect to increase resistance against
the water and during the recovery stroke they lie flat, the whole movement being accompanied in waves of activity
spreading along each arm from the base to the tip.
Up to about 15 spines may occur on each lateral shield of each arm segment. Spines vary considerably in form and
texture from species to species and may be glandular or modified into hooks. In some species, spines also occur on
the central disc. Each spine has an internal calcareous skeleton enclosing a core of connective tissue and covered by
epidermis and dermis. The spines are often movable.
Nutrition
Ophiuroids are carnivores, scavengers, deposit feeders or filter feeders. Most use several of these feeding modes.
Ophiocomina nigra uses all four, but is predominantly a suspension feeder.
In deposit and suspension feeding, plankton and detritus adhere to mucous strands strung between the arm
spines. The captured food particles are transported to the tentacle scales, by ciliary currents and/or by the tube
feet. In some brittlestars the tentacle scales and the oral surface of the arms are ciliated (or flagellated?) and the
beating of these minute motile hairs generates water currents which can sweep particles towards the central disc;
further tracts of cilia on the oral face of the disc carry particles to the mouth. In burrowing forms the tips of the arms
may remain above the surface, sweeping the sand to collect detritus which the podia pass to the mouth. Podia around
the mouth sort the particles.
In filter feeding arms are elevated with the oral surface facing the water current. The podia extend beyond the
spines to form combs. Food particles stick to the podia and are periodically wiped onto the spines and collected by
other tube feet. In either case, these tube feet compact the particles into a bolus and transport this growing bolus to
the mouth in a wave-like fashion along the midoral line (the midline on the oral surface of each arm) of the arm
towards the mouth.
In scavenging the looping motion of an arm sweeps food into the mouth. The teeth or oral tube feet are used to
browse on algae and carcasses. The alimentary canal is simple; there is no anus or intestine and no diverticula
extending into the arms.
Gorgonocephalids or basket stars have a mass of branched tentacles, armed with microscopic hooks. These form a
net which may trap planktonic animals. Most brittlestars are thought to supplement their diet of detritus with small
items Live animals may be caught by the arms looping like snares.
The mouth has five corners (actually the opening of a preoral cavity which leads to the true mouth) and is armed with
five triangular jaws which may be edged by tiny teeth. Each jaw is comprised of two half-jaw pieces. A series of shields
or plates partially covers the jaws from view.
Reproduction
Most serpent stars are dioecious (with separate sexes), and the sexes are indistinguishable, unless the more intense
colour of the female gonads shows through (this may also account for the slight colour differences seen between the
sexes of some asteroids). Four species are exceptions in exhibiting sexual dimorphism. In these species dwarf males
will cling to the larger females, often mouth to mouth or on the aboral surface.
Many species are hermaphroditic. This may involve changes of sex from male to female (protandry, also occurs in
some asteroids). All hermaphroditic and some dioecious species brood their young. Usually gametes are shed into the
sea, through the bursal slits, but in brooding species the eggs may be shed into the bursal sacs where they hatch
whilst inside the female (oviparity) and then the young live in the bursal sacs, which become brooding chambers.
The young may attach to the bursal wall by a stalk (which is part of the bursa). The wall of the bursa and the egg yolk
provide nourishment. Many species are truly viviparous, the eggs being retained in the ovaries until they hatch. The
young may then reside in the brooding chamber of the bursa until quite large. Usually only 1-2 young occupy each
bursa, but as many as 200 embryos have been seen in a single bursa. At least one species of serpent star is known
to attach 20-2000 yolky eggs underneath stones or seaweed, etc.
The gonads are sacs attached to the coelomic walls of the bursae, near to the bursal slits. There are typically 1-2
gonads per bursa, but sometimes there may be clusters or rows of several thousand per bursa.
Asexual reproduction may occur by fission in six-armed species, especially in young or small ophiuroids (< 3 mm
disk diameter). There is no preformed fissure plane. Serpent stars are able to regenerate lost arms, and actually cast-
off arms if handled, or if the arm is trapped. This gives rise to their common name of ‘brittle stars’. The disk needs at
least one arm to survive and regenerate.
A transverse section through the arm of an ophiuroid. Click images
to enlarge.
to enlarge.
Embryology
The first fully developed larval stage is the pluteus, which has four arms with ciliated bands and a mouth. After about
18 days, this develops into an ophiopluteus, which has 8 arms, with ciliated bands, supported by skeletal rods (and
one pair of epaulettes). The arms continue to lengthen, until after about three weeks the late ophiopluteus resorbs or
discards its arms and sinks to the bottom as its heavy skeletal system develops, and develops into a young brittlestar.
Deviations on this theme occur. Species with large yolky eggs may hatch a cylindroid larva with 4 ciliated circles and
no arms (similar to a doliolaria larva).
Ecology
Ophiuroids are found in all seas, at all latitudes and on all types of substrate, from intertidal to Abyssal depths (6000
m).
Serpent stars are very numerous on tropical reefs. They may form large aggregations of 1000-2000 serpent stars per
square metre. Some serpent stars are epizoic (living on the surface of other animals). The only commensal echinoids
are species of ophiuroid. Some of these live inside the water-canals of sponges, others are commensal on corals,
while others live on the oral surface of feather stars, and some live on the under-surfaces of sand dollars. (A
commensal shares the food of its host, but otherwise causes little harm).
The first fully developed larval stage is the pluteus, which has four arms with ciliated bands and a mouth. After about
18 days, this develops into an ophiopluteus, which has 8 arms, with ciliated bands, supported by skeletal rods (and
one pair of epaulettes). The arms continue to lengthen, until after about three weeks the late ophiopluteus resorbs or
discards its arms and sinks to the bottom as its heavy skeletal system develops, and develops into a young brittlestar.
Deviations on this theme occur. Species with large yolky eggs may hatch a cylindroid larva with 4 ciliated circles and
no arms (similar to a doliolaria larva).
Ecology
Ophiuroids are found in all seas, at all latitudes and on all types of substrate, from intertidal to Abyssal depths (6000
m).
Serpent stars are very numerous on tropical reefs. They may form large aggregations of 1000-2000 serpent stars per
square metre. Some serpent stars are epizoic (living on the surface of other animals). The only commensal echinoids
are species of ophiuroid. Some of these live inside the water-canals of sponges, others are commensal on corals,
while others live on the oral surface of feather stars, and some live on the under-surfaces of sand dollars. (A
commensal shares the food of its host, but otherwise causes little harm).
Article updated:
13 Feb 2017
14 Feb 2017
13 Feb 2017
14 Feb 2017
Above: a transverse (cross) section through the arm of the brittlestar Ophiura. Note the pair
of podia: the pore opening for the right one is almost visible in this section. The radial water
vessel gives off a pair of lateral water canals (mostly not visible in the plane of this section)
which connect to the podia. Note that there is a sinus above the radial nerve cord
(hyponeural sinus) and a sinus beneath it (epineural sinus).
of podia: the pore opening for the right one is almost visible in this section. The radial water
vessel gives off a pair of lateral water canals (mostly not visible in the plane of this section)
which connect to the podia. Note that there is a sinus above the radial nerve cord
(hyponeural sinus) and a sinus beneath it (epineural sinus).
Above: a longitudinal section through an arm of Ophiura, showing the paired intervertebral
muscles and podia and traces of radial structures (probably the radial nerve cord).
muscles and podia and traces of radial structures (probably the radial nerve cord).
Above: a longitudinal section through an arm of Ophiura, close to the plane where the podia
emerge through pores and lateral spines.
emerge through pores and lateral spines.
Above: a cross-section through the arm of the brittlestar Ophiocomina nigra zoomed-in to
show the articulation of a single spine with its tubercle. The lateral plate or shield (LP) has
protuberances called tubercles, each tubercle (T) forming the socket for a spine (S). A
double spine muscle (SM) attaches via tendons (deeply penetrating fibres) to both the base
of the spine and the lateral plate. The nerve (N) supplying the lateral plate sends out a
branch which passes through a canal in each and branches as it enters the spine. This
nerve is a branch of the radial nerve cord running along the arm. A smaller branch from this
spinal nerve enters a ganglion called the juxtaligamental node (JN) (juxta = next to). Cells in
the JN send out a single process each which innervates the spine ligament (SL) which is
connective tissue containing bundles of collagen fibrils. Presumably, the JN responds to
flexion of the spine in a sensory capacity. The ligament is elastic and helps return the spine
to its default position after it has been flexed by the SM. The innervation of the SM was not
visible but may possibly come direct from the spine nerve (N) or from the JN. IM =
intervertebral muscle; L = ligament connecting adjacent lateral arm plates. (After Wilkie, I.C.
2016. Functional morphology of the arm spine joint and adjacent structures of the brittlestar
Ophiocmina nigra (Echinodermata: Ophiuroidea). PLoS One 11: e0167533. (External link:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0167533).
show the articulation of a single spine with its tubercle. The lateral plate or shield (LP) has
protuberances called tubercles, each tubercle (T) forming the socket for a spine (S). A
double spine muscle (SM) attaches via tendons (deeply penetrating fibres) to both the base
of the spine and the lateral plate. The nerve (N) supplying the lateral plate sends out a
branch which passes through a canal in each and branches as it enters the spine. This
nerve is a branch of the radial nerve cord running along the arm. A smaller branch from this
spinal nerve enters a ganglion called the juxtaligamental node (JN) (juxta = next to). Cells in
the JN send out a single process each which innervates the spine ligament (SL) which is
connective tissue containing bundles of collagen fibrils. Presumably, the JN responds to
flexion of the spine in a sensory capacity. The ligament is elastic and helps return the spine
to its default position after it has been flexed by the SM. The innervation of the SM was not
visible but may possibly come direct from the spine nerve (N) or from the JN. IM =
intervertebral muscle; L = ligament connecting adjacent lateral arm plates. (After Wilkie, I.C.
2016. Functional morphology of the arm spine joint and adjacent structures of the brittlestar
Ophiocmina nigra (Echinodermata: Ophiuroidea). PLoS One 11: e0167533. (External link:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0167533).
