A computer model of an Obelia hydroid
colony. The colours have been added for
clarity in distinguishing the parts. In hydroid
colonies, the tentacled feeding zooids or
hydranths are usually white, pink or violet
(white in Obelia) and in Obelia the chitinous
outer covering or perisarc often gives the
colonies a yellowish colour. Obelia is marine.
Branching stolons creep along the
substrate, forming the hydrorhiza, and
upright stalks (hydrocauli, sing,
hydrocaulus) shoot-up from the hydrorhiza
at intervals, forming branching structures -
two feeding polyps can be seen on the left,
a developing polyp as a bud in the centre,
and a reproductive polyp on the right. The
reproductive polyps (gonangia or
blastostyles) generally grow from axils or
angles in the colony, typically in the angle
between a hydranth and its parent stalk.
The colony form depends upon
species, and deviations from the
usual patterns are apparent,
however, the typical pattern is
sympodial growth for the vertical
shoots with each branch
terminating in a hydranth and
giving rise to another branch.
The hydrorhiza is a prostrate
mass of branching tubes
(stolons) from which vertical
shoots arise at intervals, each
forming a vertical stem or
hydrocaulus, reaching 3-5 cm in
height. Gonangia develop in the
axils of branches. Shown in blue
are the developing medusae
buds on the gonangia. Each
hydranth sits on a shelf of
perisarc. The individuals making
up the colony are called zooids
Like hydra, the body wall consists of an outer cell layer or ectodermis and an inner cell layer or endodermis
(or gastrodermis) with mesogloea separating the two. The endodermis encloses a central fluid-filled cavity, the
gastrodermal cavity or enteron. The tubes that make up the hydrorhiza and hydrocauli contain an inner cylinder
of living tissue, the coenosarc (shown as orange in the coloured picture) surrounded by a hollow outer cylinder
or perisarc, which is a chitinous sheath that acts as an exoskeleton supporting the colony. At branches and
beneath the polyps, the perisarc forms a series of rings or annuli which act as skeletal joints and give the colony
more flexibility. The coenosarc consists of an outer layer of ectodermal cells and an inner layer of endodermal
cells enclosing the enteron, and mesogloea between the two cell layers. The enteron forms a continuous
fluid-filled cavity throughout the whole colony, connected the polyps. In the hydranths the enteron is enlarged to
form a stomach cavity. The coenosarc is separated by a fluid-filled space from the perisarc (which is porous to
sea-water) and the two only make contact at intervals.
The tentacled feeding polyps are called hydranths and the perisarc enclosing them forms a supporting cup called
a hydrotheca. Each hydranth terminates in a cone (hypostome) bearing the mouth and surrounded by a ring
of about 24 tentacles. Each hydranth is a feeding polyp that resembles a hydra. However, the tentacles are
solid (they are hollow in hydra) and each is filled with a cord of endodermal cells with secreted matrix material
separating adjacent cells. The tentacles and the hypostome are the only parts of the colony that generally bear
nematocysts (stinging structures located in cells called nematoblasts, nematocytes, cnidoblasts or cnidocytes).
There is only one nematocyst type in Obelia - a barbed penetrant, which fires a barbed harpoon-like thread. The
nematoblasts are derived from interstitial cells. The hypostome has the highest density of sensory cells. The
tentacles catch any prey that triggers the nematocysts and which is not large enough to escape. The tentacles
pass the ensnared prey to the mouth and hence into the enteron where the first-phase of digestion, which is
extracellular, takes place.
The enteron of the hydranth connects to the enteron of the coenosarc and the endodermis lining the enteron
contains flagellated cells. The beating of the flagella generate currents that carry the fluid containing the partially
digested food around the colony. Endodermal cells throughout the colony phagocytose the food particles and
complete digestion intracellularly. Soluble products of digestion are then passed to the ectodermal cells. Thus,
even the gonangia, which cannot feed themselves, derive nourishment via the hydranths and the coenosarc
Asexual reproduction has been shown to occur in some species. Shoots of the coenosarc that begin
development as gonangia, instead enlarge and detach and slip out from the perisarc and float away, attach to a
substrate and develop into a new colony.
Sexual reproduction occurs through the medusae. The medusa is the sexual stage and medusae develop as
buds on the blastostyle. Initially the enteron of the medusa is continuous with that of the gonangium and hence
the rest of the colony, but eventually constricts and the medusa is pinched off. The perisarc exoskeleton
enclosing each gonangium is called the gonotheca and has a terminal pore through which the medusa escapes.
Each gonangium has no tentacles and no mouth and cannot feed itself. The alternation between asexual hydroid
colonies and sexual medusae is an alternation of generations. The existence of different types of individual
zooids or polyps that perform different tasks (a division of labour) is called polymorphism.
The medusae are released in Spring and Summer and are about 3 mm in diameter. They resemble tiny jellyfish,
but in hydrozoa the medusae only have a thin layer of mesogloea rather than the thick jelly-like mass
characteristic of medusae of the Scyphozoa (true jellyfish). Initially the medusa has 24 tentacles, but the number
increases with age. The body of the medusa is a dome-shaped umbrella. The body consists of an outer cell layer
or ectodermis, the upper (convex) surface of the dome being called the exumbrella and the lower (concave)
surface the subumbrella. The umbrella is quite flexible, flexing in and out as the medusa pulses and swims by
jet=propulsion. Just inside the rim of the umbrella is a circular shelf of tissue, called the velum (a structure not
present in scyphozoan jellyfish). In hydroid medusae the velum acts to constrict the opening to the subumbrella
when it contracts, funneling the water into a more focused jet, however, in Obelia the velum is less important and
Hanging beneath the umbrella is a tube, called the manubrium, bearing the 4-lobed mouth at its apex.
A typical Obelia colony - The structure is sympodial, meaning that rather
than a single growing axis, the main axis is made up of several units added
on top of one-another. Each unit or module is shown as an arching stalk
terminating in a hydranth and branching off the next module above it. The
hydranths alternate. Gonangia are shown developing in the axils. The
terminal bud (apical bud) is a developing hydranth. Many plants also grow
in a sympodial manner.
Obelia colonies vary in colour from cream to light-brown and grow
attached to seaweeds, shells, rocks and wooden piles. The
example above has somewhat anomalous growth (abnormal form).
The tentacles hang vertically downward from the margin of the umbrella. The mouth leads into a small gullet or
pharynx which crosses the manubrium and opens into the gastric cavity (stomach cavity or stomach) from
which four radial canals radiate out to the circular canal in the umbrella margin. Each quadrant between the
radial canals contains a flat sheet of endodermal cells (endodermal lamella) above which the mesogloea is
thickened. The tentacles are solid and may be swollen at their bases which contain masses of interstitial cells
which replace discharged nematoblasts. The ectodermis at the base of the tentacles may contain pigment
spots which may be photoreceptors.
The four tentacles opposite the radial canals are called per-radial tentacles and mid-way between these are
the inter-radial tentacles. Between the inter-radial and per-radial tentacles are ad-radial tentacles containing
organs/sensors of balance and orientation, called statocysts, in swellings at their bases (the statocysts are
shown in red in the computer model above).
The four gonads hang down beneath the radial canals, half-way along their length, and are ectodermal sacs
into which folds of the radial canals project (the gonads are shown in yellow in the model). The reproductive or
germ cells originate in the ectodermis of the manubrium and pass through the endoderm to the gonads where
they mature. When ripe, the gonads burst and release their gametes near the surface of the sea where
external fertilisation takes place. The sexes are separate, with male and female medusae.
Development and Growth
The zygote (the diploid cell formed by fertilisation of the egg) divides to produce a hollow ball of cells or
blastula, which is comprised of a single layer of cells. A gastrula, or ball of cells several cell layers thick,
forms. In this case the gastrula forms by the division of cells at one pole, with the cells migrating into the
blastula cavity. Coleneterates / Cnidarians are diploblastic acoelomates. Diploblastic means that two
principle cell layers or masses in the embryo contribute to the formation of adult tissues. The gastrula,
possessing cilia (or flagella ?) on its outer surface by which it swims, becomes the planula larva. This larva
contains a single layer of ciliated ectodermal cells surrounding a solid core of endodermal cells. A cavity
forms inside the endodermal mass, the cavity becoming the enteron (gastrovascular cavity) of the adult.
The planula has one broader end which attaches to the substrate and puts out a horizontal creeping branch
or stolon (which may form a branching and anastomosing horizontal network) from which vertical stalks rise up.
The vertical branches bear other individuals which bud from the stalk by asexual reproduction. The other end
of the planula develops a mouth and becomes the first founding hydranth (polyp or zooid) of the hydroid
The hydroid colony possesses a double nerve net similar to that in hydra. Sensory cells are concentrated on
the upper portion of each hydranth, especially on the hypostome. The medusa also possesses a similar
double network, but also has two nerve rings, one either side of the radial canal - an outer nerve ring and
an inner nerve ring. The outer nerve ring processes signals from the statocysts, the inner nerve ring
regulates the activity of the muscles of the subumbrella. The statocysts sense tilting of the medusa and are
fluid-filled sacs, each lined with ectoderm with a single large cell hanging from the roof. This hanging cell
contains a granule of calcium carbonate called a statolith. On the outer side of this statolith-containing
process are sensory cell processes. The statolith weighs the process down and when the umbrella is upright,
the statolith hangs vertically down without touching a sensory cell process, but if the medusa tilts, then the
statolith, still hanging straight down will touch the outer sensory processes in those statocysts that are on the
down-tilted side of the margin. These stimulated statocysts send nervous impulses to the outer nerve ring,
which then sends signals to the inner nerve ring which sends signals to the appropriate bell muscles, to right
Stimulation of the colonies of at least some Obelia species (e.g. Obelia geniculata) results in the colony
emitting flashes of bioluminescent light from points within the colony. These same spots flash repeatedly. The
light-producing cells, called photocytes, are situated in the endoderm and are more frequent in the pedicels
and nodes and are more frequent in the tip of the upright parts of the colony. In photocytes, light is released
from photoprotein molecules, when these are activated by an increase in calcium ions inside the photocyte (in
response to colony stimulation) and some of this energy is transferred to a secondary flourescent protein
which then contributes light of a different colour (usually green) by secondary flourescence. The locations of
the primary photoprotein and the secondary fluorescent protein appear to coincide, so that these systems
work together. Gonangia have a few photocytes in the blastostyle and medusa have 16 green fluorescent
spots located in a ring near the ring canal. These spots increase in intensity with age of the medusa, but
medusae do not appear to luminesce in response to stimulation of the colony to which they attach. In free
medusae, the tentacle bulbs luminesce.
Computer model of a hydrozoan medusa of the Obelia type. The four gonads are shown in yellow, the
statocysts in red. The medusa is almost transparent, making it almost invisible in the water.
References / Bibliography
1. A.E. Vines and N. Rees, 1972. Plant and Animal Biology, Volume 1. 4th ed. Pitman (pub).
2. S. Crowell, 1953. The Regression-Replacement Cycle of Hydranths of Obelia and Campanularia.
Physiological Zool. 26: 319-327.
3. T. Hunter, 1989. Suspension Feeding in Oscillating Flow: The Effect of Colony Morphology and Flow
Regime on Plankton Capture by the Hydroid Obelia longissima. Biological Bulletin 176: 41-49.
4. J.G. Morin and G.T. Reynolds, 1974. The Cellular Origin of Bioluminescence in the Colonial Hydroid
Obelia. Biological Bulletin 147: 397-410.