Review of the genus Monotheca (Hydrozoa: Leptolida) from Australia with description of a new species and a note on 
Monothecella Stechow, 1923 
75 
base to posterior of hydrotheca; margin perpendicular to or 
slightly oblique to hydrocladial axis, rim weakly to noticeably 
sinuous, margin slightly everted with thickened perisarc. 
Nematothecae of two different shapes and sizes: one 
cauline similar to laterals in axil of hydrocladial apophysis, a 
small cylindrical hydrostatic pore (mamelon of former authors) 
on apophysis beside base of nematotheca; one hydrocladial 
median inferior on hydrothecate internode between base of 
hydrotheca and node, base robust, cup incipiently bithalamic, 
scoop shaped, sometimes without a line of demarcation 
between cup and base, closely adpressed to wall of hydrotheca, 
a distinct flange connecting abcauline wall with internode; 
twin lateral nematothecae bithalamic, directed outwards or 
downwards in front of hydrotheca, base large, cup excavated 
adcaudally to base. 
One to three gonothecae borne on lower stem, male and 
female often on same stem; gonothecae large, turgid, inserted 
on a short, smooth pedicel below apophysis, standing out 
perpendicularly from stem or recumbent to substrate, 
abcauline (upper) wall inflated, adcauline wall straight to 
weakly convex, body narrowing distally to an upwardly bent 
cylindrical neck with a wide circular thickened aperture with 
broadly everted rim. Immature female gonophore with several 
ova, reduced to one large planula at maturity. 
Perisarc of stems thick, translucent white when young, 
becoming pale-straw coloured with age; gonotheca brown, 
female gonophore cream. 
Monotheca australis , measurements (/<m) 
Hydrorhiza, width 
~ 289 
Stem 
length of internode 
230-260 
width at node 
20-40 
width at level of apophysis 
90-149 
Hydro cl adium 
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. South-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 
