Hydractinia van Beneden, 1841,

Stampar, Sérgio N., Tronolone, Valquiria B. & Morandini, André C., 2006, Description and life cycle of the hydrozoan Hydractinia uniformis, sp. nov. (Cnidaria: Hydrozoa: Hydractiniidae), from the coast of southeastern Brazil, Zootaxa 1200, pp. 43-59: 45-53

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http://doi.org/ 10.5281/zenodo.2646450

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scientific name

Hydractinia van Beneden, 1841


Genus Hydractinia van Beneden, 1841 

Hydractinia uniformis  sp. nov.

( Figures 1–11View FIGURE 1–4View FIGURE 5–10View FIGURE 11)

Type material

Holotype specimen: MZUSP 932View Materials, male medusa, 2.1 mm high, 3.5 mm width, preserved in 4% formaldehyde solution in sea water in 8 October 2003 (from polyp cultures, reared in laboratory for 3.5 months). 

Paratype specimens: MZUSP 934View Materials, one male medusa, preserved in 4% formaldehyde solution in sea water in 20 November 2003 (from polyp cultures, reared in laboratory for 2 months). — MZUSP 935View Materials, five recently released medusae, preserved in 70% ethanol in 5 January 2005 (from polyp cultures). — MZUSP 933View Materials, two polyps, preserved in 4% formaldehyde solution in sea water in 5 January 2005 (from polyp cultures, reared in laboratory), originally collected in 25 July 2002 in Parcel da Praia Grande (23°51.06’S 45°24.77’W); 10 m depth; São Sebastião Channel, Ilhabela county, São Paulo state, BrazilGoogleMaps  MHNG INVE 37181, one male medusa, preserved in 4% formaldehyde solution in sea water 20 November 2003 (from polyp cultures, reared in laboratory for 2 months) and one polyp, preserved in 4% formaldehyde solution in sea water in 5 January 2005 (from polyp cultures, reared in laboratory), originally collected in 25 July 2002 in Parcel da Praia Grande (23°51.06’S 45°24.77’W); 10 m depth; São Sebastião Channel, Ilhabela county, São Paulo state, BrazilGoogleMaps  .

Additional specimens: MZUSP 936, half of a male medusa, preserved in 4% formaldehyde solution in sea water in 20 November 2003 (from polyp cultures, reared in laboratory for 2 months).

Etymology The specific name uniformis  , derived from the Latin unus (one) and forma (form), refers to the monomorphic polyps.


Hydractinia  species with monomorphic hydranths; tentacles in one whorl, extended much longer than polyp height; stolons flat and plate­like; vegetative frustules produced from hydranth. Gonophores produced on hydranths. Gonophore a free medusa; bell top flat; manubrium cross­shaped in section; with peduncle; mouth with four branched lips, each ending in up to four bulbous nematocyst clusters; marginal tentacles up to 24, each with an adaxial ocellus.


Polyps sessile (figs 1 and 2), arising directly from a hydrorhiza consisting of adhering, ramified, flat, plate­like stolons, perisarc thin, without spines. Polyps without notable polymorphism, variable in shape but generally spindle­shaped, 1.30–1.58 mm high and 0.14–0.25 mm wide, hypostome prominent, extensible, nipple shaped. Tentacles in one whorl, filiform, 7–9 in number, in life up to 4 mm long and thus more than two times as long as body height, width rather uniform. Medusa buds developing on lower 2/3 of hydranth, up to two per hydranth. Vegetative frustules about 100 µm long given off from hydranth body. Color: white to yellow­brownish. Nematocysts distributed spirally along tentacles: heterotrichous microbasic euryteles (5.8–6.8 µm x 2.9–3.9 µm) and desmonemes (2.9–3.9 µm x 1.9 µm).

Newly­released medusa round with slightly flattened top (figs 3, 4 and 5), 0.44 mm high, maximal diameter 0.52 mm, mesoglea thin. Radial canals four; ring canal present. Velar opening ca 2/3 of umbrella margin diameter. Four perradial marginal bulbs, each with a short tentacle and an adaxial red ocellus. Manubrium opaque, conical, ca. half the height of subumbrellar cavity, with four perradial simple mouth lips with nematocyst knobs. Gonads not developed. Nematocysts: exumbrella with heterotrichous microbasic euryteles (5.8–6.8 µm x 2.9–3.9 µm); mouth lips with microbasic mastigophores (7.8–8.8 µm x 2.9–3.9 µm) and desmonemes (2.9–3.9 µm x 1.9 µm); tentacles with heterotrichous microbasic euryteles (5.8–6.8 µm x 2.9–3.9 µm) and desmonemes (3.9 µm x 1.9–2.9 µm).

Mature medusa bell­shaped with flattened top (figs 6, 7, 8, and 9), sometimes with slight apical depression, and thus almost cylindrical, bell 0.7–2.1 mm high, 0.9–3.5 mm wide, diameter of umbrella margin 1.8 mm. Mesoglea thicker at top of umbrella. Velar opening ca. 4/5 of umbrella margin diameter. Four radial canals and ring canal present. Tentacular bulbs up to 24, with four perradial ones slightly larger; each bulb bearing one marginal tentacle and one red adaxial ocellus. Tentacles not very contractile, provided with evenly­distributed nematocysts. Manubrial peduncle about 1/3 of subumbrellar height. Manubrium cruciform in cross­section, length about 1/2 of the subumbrellar cavity; mouth lips four, perradial, folded and branched, each ending in four nematocyst knobs (fig. 10). Gonads consisting of four interradial pads, large, covering nearly whole manubrium, adnate perradially, orange to red in living animals. All observed animals were male. Nematocysts: exumbrella with heterotrichous microbasic euryteles (5.8–7.8 µm x 3.9–5.8 µm); mouth lips with microbasic mastigophores (7.8–8.8 µm x 1.9–2.9 µm) and desmonemes (2.9–3.9 µm x 1.9–2.9 µm); tentacles with heterotrichous microbasic euryteles (5.8–6.8 µm x 2.9–4.9 µm) and desmonemes (4.9–5.8 µm x 3.9–4.9 µm).

Life cycle

The monomorphic polyps of Hydractinia uniformis  , sp. nov., release both frustules (up to 8) (fig. 11) and medusae (up to 2) (fig. 2, 11) at the same time. Frustules attach to the substratum on the first or second day of liberation, and three days after settlement a new polyp arises. Over the two­year period of this study we observed continuous production of medusae and frustules. The polyp tentacles are generally bent down, usually all reaching the same length and always longer than the body. The hydrorhiza grows slowly, never forming a dense stolonal network or a closed mat. Notably, the polyps agglutinate around their column several different materials (sponge spicules, foraminiferan tests, etc.). The frustules bud off from all parts of hydroid column and they do not move far, generally attaching themselves close to the parental polyp.

Newly released medusae have four marginal tentacles (figs 4 and 5) and four simple mouth arms with nematocyst knobs. After 15 days, medusae have 8 marginal tentacles (fig. 6) and the gonads start to develop. During this time, the number of branches on the mouth lips increase, reaching up to four branches each; from this stage on the mouth arms grow only in size (fig. 10). Forty days after release the medusae have 16 tentacles (fig. 7), and the gonads increase in size; the umbrella apex begins to flatten and the manubrium reaches almost 1/2 of the subumbrellar cavity. In mature medusae (fig. 8), the outline of the adaxial ocellus is not especially clear. During subsequent days, the medusae grow in size and between each pair of tentacles a new one arises, reaching a maximum of 24. We considered the medusae at this stage as adults, because thereafter they do not change in form, tentacle number (24) does not increase(figs 8 and 9), and during the following months they released gametes (sperm) into the water.

Curiously, the medusae swam only in the presence of light and with air bubbling into the water (which provided some current). Without these factors the medusae remained on the bottom and the marginal tentacles were kept contracted. During swimming, food ( Artemia  nauplii) was held on the umbrella margin and was ingested by contraction of the umbrella and extension of the manubrium and mouth arms.


The most recent published compilation of the nominal species belonging to the genus Hydractinia  was presented by Bouillon & Boero (2000). However, the list included only those species with a medusa stage and no diagnosis or synonymy was included. Bouillon et al. (1997) provided a more detailed list (with some systematic comments), but only for members of the genus Stylactaria  . We thus conducted an extensive search in the literature and found about 97 potentially valid nominal species in the genus (see details in Appendix).

Hydractinia uniformis  , sp. nov., has some unique features that make it distinct from all other congeners: the extraordinarily long tentacles of the polyp; the unusual plate­like stolons; and the production of frustules from the hydranth body. The medusa has some additional features that are not very common in Hydractiniidae  , namely the ocelli and the branched mouth lips. The monomorphic colonies are also unusual among Hydractiniidae  , although colonies of H. sarsii  are likewise monomorphic (see Tab. 1).

Of the species of Hydractinia  having medusae with more than 8 tentacles (see Tab. 2), the one most resembling H. uniformis  , sp. nov., is H. ocellata (Agassiz & Mayer, 1902)  from South Pacific Ocean. However, H. ocellata  has vacuolated cells at the base of the stomach (at the insertion of the radial canals) — a trait otherwise typical for the genus Turritopsis McCrady, 1857  , to which it might perhaps belong (their polyps are unknown so far). Moreover, it possesses up to 50 marginal tentacles. Two other species that also have branched oral lips are Hydractinia borealis (Mayer, 1900)  and H. polystyla ( Haeckel, 1879)  (see Haeckel, 1879; Mayer, 1910; Edwards, 1972). However, H. borealis  does not have ocelli, while H. polystyla  has more tentacles and differs in the shape and size of the umbrella.

The medusa species Hydractinia dubia (Mayer, 1900)  , described from the Tortugas, Florida ( USA), is probably a young stage of another species. It resembles juveniles of Hydractinia uniformis  , sp. nov., but differs in having black ocelli and swellings on the radial canals ( Mayer, 1910: 141).

TABLE 2. Comparison of species of Hydractinia  whose adult medusae have more than 8 tentacles (after Haeckel, 1879; Mayer, 1910; Kramp, 1961; Edwards, 1972, Mills, 1976).

Until now, only two species of Hydractinia  medusae have been recorded from the entire coast of Brazil: H. minima (Trinci, 1903)  and H. minuta (Mayer, 1900) ( Migotto et  al., 2002). Medusae of H. uniformis  , sp. nov., differ from them in tentacle number and in having an ocellus on each tentacle bulb. Even more notably, we never observed production of medusa buds on the manubrium of H. uniformis  , a type of asexual reproduction found in the other two species.

The two dissimilar ways of producing a dispersive phase — by frustules and by medusae — in Hydractinia uniformis  , sp. nov., represent different dispersal strategies of the species. The free swimming medusae increase genetic variability and the distributional range of the species. Meanwhile, the frustules increase the number of individuals in a small area and can act as resting stages.

Many other species of the genus Hydractinia  are epizoic on other invertebrates, particularly encrusting mollusk shells (of living snails or inhabited by hermit crabs) (e.g., Cerrano et al., 2000; 2001). Polyps of H. uniformis  , sp. nov., were collected on dead coral fragments, which were sometimes buried on the sediment.

Based on the available published literature (e.g. Schuchert, 1996; Bouillon & Boero, 2000; Bouillon et al., 2004), several species of Hydractinia  present characters which are not listed or differ from the ones that have been incorporated in the diagnosis of the family Hydractiniidae  . Among these is the occurrence of monomorphic colonies, observed in certain valid species of the genus (see above). Also, the presence/absence of ocelli is a feature that can be misinterpreted depending on the condition of the observed specimen (e.g., ocelli tend to lose their coloration in badly­preserved or long­preserved specimens).

Finally, the family Hydractiniidae  unquestionably needs morphological revision. In particular, the family diagnosis does not include all the variations observed in the genus Hydractinia  . Moreover, complete life cycles are known for few of the species. Finally, when Podocoryna  and Stylactaria  are considered synonymous with Hydractinia  , some secondary homonyms are created (see appendix).

TABLE 1. Comparison between the polyp stages of Hydractinia uniformis, sp. nov., and some other hydractiniids which produce free­swimming medusae, and Hydractinia sarsii Steenstrup, 1850 which also have monomorphic polyps (mostly after Mayer, 1910; Edwards, 1972; Mills, 1976; Hirohito, 1988; Schuchert, 2001).

Polyp character   H. uniformis  sp. nov. H. americana  H. areolata
Frustules   yes no   no
Gonophores   free medusa free medusa free medusa
Gonophores with tentacles   no yes   yes
Gonophores with marginal bulbs no yes   yes
Distinct gonozooid   no yes   yes
Number of gonophores   1–2 2–10 1–2
Spines (mm)   absent 0.2–0.5 up to 0.9
Relative gonozooid/gastrozooid size 1/1 1/2   0.5–0.8
Hypostome shape   nipple dome dome
H. borealis  H. carnea  H. hayamaensis H. minoi H. sarsii H. selena
no no no   no no no
free medusa free medusa free medusa free medusa sporosacs free medusa
yes yes yes   yes no yes
yes yes no   no no yes
yes yes yes   yes yes yes
2–8 2–10 more than 10 4–7 2–4 2–10
0.2 0.2–0.5 0.5   absent 0.1–1 up to 1.2
1/2 1/2 1/2   1/2–1/4 0.8–1 1/2
dome dome dome dome nipple, capitate dome

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