Hydroids of Australia

Hydrorhiza tubular.

Stems 0.83-1.33 mm long, 0.06-0.09 mm diameter. Perisarc thick, a spherule between stem and hydrotheca.

Hydrothecae long and tubular. Perisarc thickening distally, a distinct diaphragm near base and a flexure almost two thirds the distance up the hydrothecal wall from the base. Margin with 8-10 teeth. Diameter of hydrotheca at margin 0.32-0.41 mm, depth to diaphragm (including teeth) 0.77-0.92 mm. less ↑

Branched colonies up to 150 mm high and 80 mm wide.

Simple pinnate and branched monosiphonic stems, arising from a tangled hydrorhiza of tough stolons. Branching irregular in one or two orders in one plane, branches directed obliquely upward; pro segments of stem occupying lower third to one half of stem. On branched stems each branch originating from a claw-like knot of stolons ("pseudostolon") tightly wrapped around stem beside a hydrocladium. Pseudostolon without coenosarcal connection to stem or branch; pseudostolon flattened adcaudally, upper side inflated; at point where it becomes free of branch, pseudostolon proximally constricted into a false apophysis, distally with two to four pairs of opposite abcaudal perforations, pseudostolon then becoming a new branch. Stem and branch internodes short, wide, nodes distinct, transverse to slightly oblique, two subopposite hydrocladia per internode on stem and proximal region of branches, gradually becoming alternate and reducing to one per internode distally.

Hydrocladia to 10 mm long, on a short apophysis with transverse distal node, hydrocladium directed slightly upward, internodes fairly short; nodes distinct, transverse to weakly oblique, each with a small distal shoulder; no internal septa.

Hydrotheca seated on top of internode at an angle of ca. 40° to internode; semi-ovoid, widening from base to margin, basal connection with internode surrounded by ragged denticles; margin (lateral view) with a long blade-shaped anterior cusp and three pairs of evenly spaced cusps, anterior pair tall, upright, pointed, frequently obscured by anterior cusp, second and third pair low, rounded, embayments between shallow; in anterior view, first cusp bifid with scoop-shaped central depression, posterior pair rounded.

Median nematotheca tubular, variable in length, often shorter than hydrotheca in proximally, becoming longer distally along hydrocladium; nematotheca where long, erect, curving away from hydrotheca to above margin; nematotheca narrowing distally, terminal aperture narrow, circular, a secondary aperture above hydrotheca. On proximal hydrocladial internode nematotheca truncated below hydrothecal margin and with larger secondary aperture than succeeding nematothecae. Lateral nematotheca flask-shaped, base wide, directed obliquely downwards from hydrotheca, terminal aperture small, circular, a wide secondary aperture connecting with hydrotheca. Three cauline nematothecae, one close below and one on front of internode above apophysis and one behind at base of apophysis; frontal nematothecae with two or three apertures.

Gonothecae crowded along stem, conical, apex truncate, closed by a sheet of tissue.

Perisarc of stem and branches very thick, thinning a little on hydrocladium and hydrotheca, perisarc of gonotheca thin.

Colour: Stems and hydrocladia of simple colonies dark shining brown, branched colonies honey brown; pseudostolons paler yellow-brown. less ↑

The ringed tubularia looks somewhat like a small bouquet of pink flowers. A colony consists of a bushy bunch of stems, each ending with a polyp with tentacles. Ringed tubularia reproduce in two ways. Small jellyfish-like organisms reproduce sexually while attached to the ends of the stems. New polyps hatch immediately out of the eggs. The second manner is asexual: loose pieces of the colony are carried by the sea current and can 'take root' at a new location. less ↑

Branching among the colonies is due to the settling of actinulae on adjacent stems of the parent colony, the annulated stem of the young hydranth forming the base of the new branch. There is a tendency for all hydranths on the one colony to be the same sex; however, both male and female may occur on the one colony.

Both the hydranth and blastostyles are very active, the pendulous blastostyles performing rhythmic lashing movements at intervals. less ↑

Colonies up to 20 mm in height.

Hydrorhiza tubular, wandering over and through the substrate, becoming erect at intervals as single stems.

Stems thick and smooth, unfascicled, sparingly and irregularly branched with up to 10 branches, rebranching common. Stems with 6-12 distinct proximal annulations, ringed at intervals throughout length, branches with 6-10 annulations just above origin.

Hydranths terminal on branches, pedicels indistinctly annulated at intervals, widening distally, sometimes terminating obliquely below hydranth. Hydranths large, with 24-30 tentacles (very long in life), a distinct proximal groove in the lower body of some, but not all hydranths.

Hypostome large, annular to clavate in preserved material.

Sexes on separate colonies, the gonophores scattered on all but the most distal branches. Male gonophores in all stages of maturity borne in a single crowded verticil below a hydranth with a fully-developed hypostome and a full number of partially-atrophied tentacles. Mature gonophore 3 or 4-chambered, immature gonophore with a rod-shaped spadix and a small apical knob without nematocyst pad. Mature gonophore globular or bun shaped, apical tubercule lost at this stage. Female gonophores borne low on stem, clustered in all stages of development below a hydranth with hypostome and a full number of partially-atrophied tentacles. Young gonophores on a long, thin pedicel, disk-shaped with unbranched spadix, mature gonophores retained below hydranth, oval, containing a single egg encased in a tough gelatinous pellicle.

Nematocysts, microbasic euryteles of two sizes present:

— Small microbasic euryteles, capsule 8-9 x 3-4 µm, shaft 6-8 µm long, with very long thread. Abundant in tentacles.

— Large microbasic euryteles, capsule 38-43 x 15-18 µm, bean-shaped, shaft discharging sideways, 30-36 µm long, distal end inflated and spirally annulated, with numerous bristles arising from the annulations, thread very long with distinct spines. Abundant around hypostome and scattered in groups over spadix of immature female gonophore.

Colour: Stems golden brown, body of living hydranth yellow or orange pink, tentacles pale yellow to pink, spadix of female gonophore purplish pink, mature eggs orange, male gonophores pearly pink, spadix orange pink. less ↑

Colonies growing luxuriantly on algal holdfast.

Stems up to 1 cm in height and 0.13-0.18mm in width, unfascicled, irregularly branched, arising from a smooth reticulating hydrorhiza. Stems completely and closely annulated throughout, perisarc very thick.

Hydranths terminal on branches, with 20- 24 tentacles.

Male and female gonophores borne on different but closely associated colonies on algal substrate. Immature male gonophore with an apical tubercule, mature gonophore 1-2 chambered, distal 5 chamber 0.14-0.2mm in diameter, borne on lower stems of colony in a tight cluster of20-30 on a blastostyle devoid of tentacles at all stages. Young female gonophore with a strongly bifurcated spadix; gonophores borne in clusters of 4-6 in various stages of development below a hydranth with a reduced number of tentacles. At maturity, the tentacles are completely absent, with 2-5 mature gonophores scattered along the blastostyle. Length of mature female gonophore 0.3mm, width 0.25mm.

Nematocysts of two kinds present:

(i) Microbasic euryteles, capsule 6-7 x 2.5 µm, shaft 5 µm long, abundant in tentacles.

(ii) Larger microbasic euryteles, capsule bean-shaped, 16.5-18 x 7-8 µm, shaft thick, at least 37 µm long, spinous, the thread very long and covered with bristles. Abundant on hydranth, spadix of female, and on apical tubercule of male gonophore.

Colour: Hydranths and gonophores cream coloured, perisarc dark shining brown. less ↑

Hydrorhiza very wide, flat and ribbon-like with very strongly marked, dark coloured flexion joints extending towards middle of the hydrorhiza.

Stems short, perisarc thick, beginning with 1 - 2 ahydrocladiate internodes with deeply incised nodes. Internodes throughout the stem short, with strong opposed V-shaped joints and a secondary constriction in the internode just above the hydrocladial apophysis. Hydrocladial apophysis short, ending in a slightly oblique distal joint.

Hydrocladia alternate, given off near middle of the cauline internode, each bearing 2 - 3 hydrothecae. Hydrocladium beginning with a very short athecate internode, internodes thereafter alternately thecate and athecate but sometimes two short athecate internodes inserted between the thecate internodes. Hydrocladial nodes deeply incised, thecate internode with a transverse to slightly oblique proximal and strongly oblique distal node. Thecate internode completely occupied by hydrotheca. Abcauline hydrothecal wall strongly sinuate, adcauline wall set well down into the internode, base flat. Margin with two broad, backwardly reflexed lobes.

Nematothecae all of same shape and size, bases robust, cups slightly excavated on the adcauline side, one proximal on each stem internode opposite hydrocladium, another on the apophysis of the stem, but not axillar; one median on athecate internode; thecate internode with one median, halfway along abcauline wall of the hydrotheca and set on a broad shelf, and two laterals in the concavity below hydrothecal margin, facing upwards to margin.

Gonotheca replacing a hydrocladium near base of the stem; very large, flattened, elongate oval, narrowing to base, no definite pedicel, maximum width from about the distal third to top, distal end rounded, no definite orifice. Male gonophore oval, surmounted by a cap. less ↑

Hydrorhiza consisting of several finger-like processes firmly attached to the algal substrate and invested by a wrinkled perisarc.

Hydranth solitary, small, distal (tentacular) region about three times the length of the proximal (gonophore-bearing) region. Distal region of hydranth cylindrical, of same diameter throughout, thickly covered by short, capitate tentacles borne on peduncles and extending from the mouth into proximal region, the tentacles fewer and smaller amongst the gonophores. Hypostome obscured by tentacles.

Reproductive region swollen, gonophores in crowded clusters of one to three borne on a thin peduncle, the younger ones cylindrical in shape, becoming subspherical at maturity, apically flattened with four very small equidistant processes surrounding a low, domed summit. Ectoderm of gonophores very thick and transparent.

Colour: Tentacular region is white, gonophores pink and the hydrorhiza translucent light brown (preserved material). less ↑

Branching frequent and irregular, branches long and flexuous.

Pseudohydrothecae* alternately arranged in one plane, each pseudohydrotheca inserting approximately halfway along adcauline side of preceding one, cylindrical but strongly curved outwards, forming two longitudinal rows. Each pseudohydrotheca in contact with two others; aperture circular; rim even. Branches originating on distal half of free adcauline pseudohydrothecal wall.

Nematothecae finger shaped. Hydranths with c. 10 tentacles. One ovoid gonophore per pseudohydrotheca.

Cnidome composed of microbasic mastigophores, microbasic euryteles, atrichous haplonemes and desmonemes.

*The hydrothecae in Clathrozoellidae have a structure differing from those of Leptothecatae and are here described as ‘pseudohydrothecae’ less ↑