Monotheca amphibola, Watson, 2011

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 : 72-74

Monotheca amphibola , measurements (µm)

Hydrorhiza, width 160–170

Stem internode length 272–360 width at node 36–40

Apophysis, adcauline length 32–56 diameter at distal node 40–52

Hydrocladium length adcauline athecate segment 32–36 basal length of thecate internode 176–200

Hydrotheca length, base to margin along axis 100–108 diameter at margin 116–128

Nematotheca cauline, length of base 40–50 cauline, depth of cup 28–20 cauline, diameter of cup 28–32 axillar, length of base 30–36 axillar, depth of cup 12–14 axillar, diameter of cup 28–34

median, length of base 34–40 median, depth of cup 12–16 median, diameter of cup 30–34 lateral, length of base 38–42 lateral, depth of cup 10–16 lateral, diameter of cup 36–40 hydrorhizal, length of base 98–100 hydrorhizal, depth of cup 30–32 hydrorhizal, diameter of cup 28–36

Remarks. The Mulder and Trebilcock collection in Museum Victoria contains five microslides ( Canada Balsam–mounted) labelled ‘ Plumularia setaceoides var. crateriformis , 1910’. Two of these slides are labelled ‘type’ by Mulder and Trebilcock (see Mulder and Trebilcock 1910, p. 118). The locality of the specimens is Bream Creek on the central Victorian coast. Unfortunately, the specimens are so badly fragmented that it is impossible to determine whether they were originally Plumularia or Monotheca . These, and a third slide from the same series (NMV F57987, F57988, F57989), were designated syntypes by Stranks (1993). I select the best of these (F57987) as lectotype of Plumularia setaceoides var. crateriformis .

Stechow (1925) raised the var. crateriformis to specific rank, recording the species from brown algae at 12–14 m depth in Warnbro Sound, Western Australia. Watson (2005) also reported P. crateriformis from algae at 16 m depth in the nearby Archipelago of the Recherche, Western Australia. Plumularia nodosa Stechow, 1924 , also from Western Australia, is probably conspecific with P. crateriformis ; however as neither species is referrable to Monotheca as defined here, they are not considered any further.

Two of the three remaining Mulder and Trebilcock microslides are labelled ‘ Plumularia setaceoides var. crateriformis, Bream Creek , January 1914 ’ and are presumably those on which Mulder and Trebilcock (1915) based their augmented description of the var. c raterifiormis. The third slide is labelled ‘ Plumularia setaceoides var crateriformis, Torquay , February 1915 ’. The Bream Creek specimens have only one hydrotheca on the hydrocladium and are referrable to Monotheca , whereas the Torquay specimen has more than one hydrocladial hydrotheca and thus belongs to Plumularia . The 1914 Bream Creek specimens are identical to Monotheca amphibola from Queenscliff.

The conspicuous anterior and posterior hydrothecal flanges of Monotheca amphibola resemble those in some of the smaller and more variable forms of the Plumularia setaceoides species group. These structures, together with the strong internal cauline segmentation and the wide, flat hydrorhiza with flexion joints are adaptations to strengthen the hydrocaulus in the high-energy Amphibolis seagrass habitat. The tall hydrorhizal nematothecae are probably for defence against the many grazing predators in the seagrass habitat. The smooth texture of the gonothecal wall is visible only in fresh material, as the gonothecae tend to crumple and become corrugated in mounting.

Etymology. The species is named for the seagrass Amphibolis antarctica upon which it grows.