Monotheca australis (Kirchenpauer, 1876)

Watson, Jeanette E., 2011, Review of the genus Monotheca (Hydrozoa: Leptolida) from Australia with description of a new species and a note on Monothecella Stechow, 1923, Memoirs of Museum Victoria 68, pp. 71-91 : 75-76

Monotheca australis , measurements (µm)

Hydrorhiza, width ~ 289

Stem length of internode 230–260 width at node 20–40 width at level of apophysis 90–149

Hydrocladium length of athecate internode 50–110 length of thecate internode 140–200

Hydrotheca diagonal length, margin to base 160–172 depth, margin to floor 90–180 diameter of rim 80–140

Nematotheca total length of median inferior 30–60 length of lateral including base 40–60 width of cup 20–50 width of axillar pore 14–20

Gonotheca length, including pedicel 650–1000 maximum width 200–350 diameter of neck 100–220 diameter of aperture 110–250

Remarks. The material in the dry Kirchenpauer collection of Museum Victoria is probably that on which Kirchenpauer (1876) described Plumularia (Monopyxis) obliqua var. australis . The specimen (NMV F58239) comprises many stems, some fertile, on dried leaves of the seagrass Amphibolis antarctica , formerly known as Zostera . Stranks (1993) nominated this material as possible syntype. Because of its unusual hydrocladial median inferior nematotheca, Bale (1884) raised Kirchenpauer’s var. australis to specific rank as Plumularia australis . I designate F58239 as lectotype of Monotheca australis Kirchenpauer, 1876 .

Compared with the height of the stems, the hydrorhizal stolons of M. australis are very wide, the strongly developed flexion joints permitting movement of the hydrorhiza on the Amphibolis leaves in strong water movement. Although the nematothecae are all essentially bithalamic, in some colonies the cup of the hydrocladial median inferior nematotheca is so adcaudally reduced that the line of demarcation between peduncle and cup is either very faint or altogether lost.

The small hydrostatic pore on the apophysis of the stem is obscured by the axillar nematotheca and is only visible in cleared specimens. The pore was described by Mulder and Trebilcock (1916) as similar in shape to a ‘steamship ventilator’; in fresh material, however, it is cylindrical and likely to bend under pressure of a cover slip.

Monotheca australis is an obligate epiphyte of seagrasses, growing on Amphibolis antarctica in cool temperate south-eastern Australia and on Posidonia australis in warm temperate waters of southern and south-western Australia. In the rigorous Amphibolis habitat, the hydroid colonises the inner, sheltered leaves of the seagrass, while in the quieter Posidonia habitat it occurs on all but the outermost leaves of the plants. Colony size and cauline dimensions of M. australis on Amphibolis are usually smaller than those on Posidonia .

Boero (1981a, 1984) reported an increase in cauline length and loss of the intrathecal septum in M. obliqua from Posidonia seagrasses with increasing depth and decreasing water movement in the Mediterranean Sea. Because M. australis also grows on Posidonia in southern Australia, specimens from Posidonia habitat from 0–25 m depth were examined to investigate whether the same relationship exists among Australian Monotheca seagrass epiphytes. No such gradient was found; any tendency to loss of the intrathecal septum probably being lost in the more rigorous Australian Posidonia habitat.

Type locality. Port Phillip, Australia.

Known distribution. S outh-eastern and southern Australia to depth of 25 m.

Status of Monotheca compressa ( Bale, 1882) , Monotheca aurita ( Bale, 1888) and Monotheca obesa ( Blackburn, 1938) . Bale (1882) described Plumularia compressa from fertile material collected by Mr T. B. Smeaton at Robe, South Australia. (The gonotheca was figured in 1884). The collection in Museum Victoria contains three microslides labelled by Bale ‘ Plumularia compressa Bale, 1884 ’. These are presumably from Bale’s 1882 collection and relabelled in 1884. The specimens are probably from the shallow water seagrass Posidonia australis common at Robe. As Bale did not designate a holotype I designate a microslide NMV F59056 of a fertile colony as lectotype of Monotheca compressa .

Bale (1884, 1888, 1894) recognised the close relationship between M. compressa and M. australis , but nevertheless kept the two species separate — the main points of difference were length of stem, height and shape of the hydrotheca, position of the intrathecal septum and minor differences in shape of the neck of the gonotheca. Length of stem is an unreliable character as it may vary considerably between the same or different colonies according to environmental conditions and rate of growth. Height of the hydrotheca is quite variable over the range of material examined in the present study and is actually related to the degree of torsion around the intrathecal septum; shape of the base of the hydrotheca is also quite variable, some australis morphotypes being almost hemispherical in shape. The position of the foramen in the floor of the hydrotheca is also variable and depends on the degree of torsion of the hydrotheca around the intrathecal septum. It is more or less central in 75% of the australis morphotypes examined but is displaced towards the rear in the compressa form. Hydrothecae with a long, backwardly curved septum typical of Kirchenpauer’s australis occur in colonies from the cool temperate Amphibolis antarctica habitat, the septum in these hydrothecae penetrating at least halfway into the hydrotheca, providing a pivotal point for torsion of the hydrotheca. In the warm temperate Posidonia habitat, the hydrothecae are taller and the septum is often directed forward as a rudimentary intrathecal ridge rather than a deep inflexion. Gonothecae vary in size and shape from almost cylindrical to turgid, and no relationship between sexual dimorphism, habitat or geographical distribution could be found to account for these variations.

Bale (1888) described Plumularia aurita from Botany, New South Wales. A microslide (NMV F58776) in the collection of Museum Victoria is the only known material of the species and was nominated syntype by Stranks (1993). As this is the only known specimen, by monotypy it is the holotype of the species. Bale’s description and figure of the hydrotheca of M. aurita shows no intrathecal ridge, yet examination of the microslide clearly shows a backwardly oblique ridge passing almost halfway into hydrotheca. The incipiently monothalamic hydrocladial median inferior nematotheca is the same as that of M. australis , and the single small gonotheca on the slide although somewhat damaged confirms the species as a morphotype of M. australis . Although Bale provided no ecological notes, the species was almost certainly collected from Posidonia seagrass in Botany Bay.

The holotype microslide of Plumularia obesa Blackburn, 1938 , in the collection of Museum Victoria, consists of three small, infertile stems labelled ‘ holotype NMV F70661, Plumularia obesa Blackburn, 1938 , Reevesby Island, South Australia, December, 1936’. No other mounted or preserved material is known to exist. Since the microslide specimen was not cleared of tissue before mounting many critical structures are obscured, making it difficult to accurately measure important features.

In his description of Plumularia obesa, Blackburn surprisingly compared the species with Plumularia spinulosa rather than comparing it with Plumularia australis , which he also recorded from the same locality. The hydrotheca of Monotheca obesa closely resembles the compressa form of M. australis from Posidonia seagrass, the major difference being the shorter and more robust hydrocaulus resembling Bale’s M. aurita . Blackburn described and figured the nematothecae of P. obesa as bithalamic with slender bases; however, examination of the type shows that the hydrocladial median inferior nematothecae have sturdy bases similar to those of P. australis . The bases of the twin lateral nematothecae are not slender as figured by Blackburn, nor are the cauline nematothecae of the holotype similar to the laterals and the bases are not so slender as to be considered truly moveable. Apart from the much smaller and more robust hydrocaulus, and taller hydrothecae, P. obesa is indistinguishable from the extreme compressa form of M. australis . Although the gonosome of P. obesa is unknown, I am confident that the species is a morphological variant of M. australis and, accordingly, I include it in the synonymy of M. australis .

Monotheca australis is a highly variable species, the size of the colony and structural dimensions over the geographic range are closely related to substrate type and environmental conditions.