Utricularia gibba

U. gibba is an annual or perennial submerged or free-floating carnivorous aquatic plant. It has been identified as such a specialist invasive species and may outcompete native bladderworts in lowland wetland ecosystems in countries where it is introduced. It was intentionally introduced, as an aquarium plant, to New Zealand in 1980, where it is now fully naturalized.

U. gibba is predominantly dispersed by water fowl. It can also rapidly colonise new water bodies by stem fragmentation and via its seeds. It forms a mat over the water surface which reduces light to plants growing beneath and could be a problem for irrigation and drainage. U. gibba is on the National Pest Plant Accord list of New Zealand and is designated as an Unwanted Organism, and banned from sale, propagation and distribution.

The Lentibulariaceae (bladderworts) are the largest family of carnivorous plants and comprise ca. 325 species in the three genera Genlisea, Pinguicula, and Utricularia, which are clearly differentiated with regard to their trapping system (Fischer et al., 2004). Utricularia is the most derived genus of the carnivorous family Lentibulariceae. So far, 220 species have been described (Barthlott et al., 2004), of which about 25% are aquatic (Taylor, 1994). All species of this genus are characterised by the presence of traps which are borne in large numbers on stems and leaves. These traps, or 'bladders', operate by sucking in prey which is then digested.

Utricularia gibba L., Sp. Pl. 18. 1753 is the accepted name by IPNI (2012). Various subspecies have been proposed for U. gibba because it is one of the several most variable species in the genus due its large range; however, based largely on the work of Taylor (1989), all of them are considered synonymous. Genus Utricularia is from the Latin utricularius, 'a small bag or bladder'; gibba, from the Latin gibbus, 'hunched, humped', a reference to the inflated base of the lower lip of the corolla.

U. gibba is an annual or perennial submerged or free-floating aquatic plant. Rhizoids absent or present, filiform, branched. Stolons filiform, much branched, often mat-forming. Traps lateral on leaf segments, stalked, ovoid, 1-2.5 mm, mouth lateral; appendages 2, dorsal, branched, setiform, with shorter setae. Leaves numerous on stolons, 0.5-1.5 cm; primary segments (1 or) 2, unbranched or sparsely dichotomously branched into 3-8 ultimate segments; ultimate segments capillary, slightly flattened, margin entire or sparsely denticulate, apex and teeth setulose. Inflorescences erect, 2-15 cm, 1-3(-6)-flowered; peduncle terete, 0.3-0.5 mm thick, glabrous; scale 1, similar to bracts; bracts basifixed, semiorbicular, ca. 1 mm, minutely glandular, apex truncate and obscurely dentate. Pedicel erect to spreading, 2-12 mm, filiform; bracteoles absent. Calyx lobes subequal, broadly ovate to orbicular, 1.5-2 mm, apex rounded. Corolla yellow, 4-8 mm; lower lip slightly smaller than upper lip, base with a prominent 2-lobed swelling, apex rounded; spur narrowly conic to cylindric from a conic base, shorter or longer than corolla lower lip, distal part sparsely stipitate glandular, apex obtuse; palate densely pubescent; upper lip broadly ovate to suborbicular, ca. 2 × as long as upper calyx lobe, apex obscurely 3-lobed. Filaments 1-1.5 mm, curved; anther thecae confluent. Ovary globose; style evident; stigma lower lip transversely elliptic, upper lip obsolete. Capsule globose, 2-3 mm in diam., 2-valvate. Seeds lenticular, 0.8-1 mm in diam., margin broadly winged, wing shallowly and irregularly dentate; seed coat with small prominent reticulations (Zhenyu and Cheek, 2011).

U. gibba has a global distribution, with pan-tropical distribution in particular (Taylor, 1994). It occurs naturally in North America and is well distributed in the following areas: along the east coast throughout Florida and Louisiana, to Quebec and eastern North America; from southern British Columbia to California; and the West Indies (USDA-NRCS, 2012). It is also well distributed in Central and South America (GBIF 2012; Tropicos, 2012); the western Mediterranean (Greuter et al., 1989); southern Africa (African Plants Database, 2012); China, India, South West Asia, the Indian Ocean islands, the Pacific islands (Zhenyu and Cheek, 2011); and Australia (CHAH, 2009; Rowe and Brown, 1992).

In its non-native distribution, it is introduced in the Hawaiian Islands (USDA-NRCS, 2012) and Slovakia and Hungary (DAISIE, 2012). In New Zealand (North Island) it is extensively naturalised (Webb and Sykes, 1997; Salmon, 2001). Further information about the distribution of this species can be found in FBIS (2005).

U. gibba was first recorded as U. biflora Lam. for New Zealand in 1980 (Bangerter, 1980), where it was probably introduced from Europe and southern USA (NZPCN, 2010). It has spread throughout gum fields and dune lakes in part of northern New Zealand (DOC, 2005) and is currently spreading across the lake landscape in New Zealand (Compton et al., 2012). From 1990 to 2010 it was recorded in nearly 40 lakes (Champion and Clayton, 2000) and between 2004 to 2008 invaded more than 34 lakes in North Island (New Zealand) alone (Winton et al., 2009). It is widely distributed in the northern Auckland region (Champion et al., 2002; Wells and Champion, 2010) and has probably reached its potential range limit in Northland. It is now spreading in Auckland and Waikato Regions (Wells and Champion, 2010).

U. gibba is commonly cultivated as an ornamental plant (Biosecurity New Zealand, 2008); it may have been introduced originally as an aquarium plant (Webb et al., 1988). It is predominantly dispersed in New Zealand by water fowl and can rapidly colonise new water bodies by stem fragmentation and seed (Compton et al., 2012), and natural spread between catchments is likely. Currently, the areas in which it appears to be spreading by natural means are limited, and the risk of long-distance spread thus reduced (Biosecurity New Zealand, 2008).

It has also failed weed risk assessments for New Zealand, so further introductions, at least in this region, are unlikely (Champion and Clayton, 2000; 2001). In addition, Plantlife (2010) has identified an urgent need for a detailed risk assessment of U. gibba in the UK; the introduction of this plant into the UK is therefore unlikely. U. gibba is included in the list of environmental weeds in New Zealand (Howell, 2008) and in the First Schedule of the National Pest Plant Accord (NPPA). All plants on the list are designated as Unwanted Organisms, and are banned from sale, propagation and distribution throughout New Zealand (Biosecurity New Zealand, 2008).

Utricularia species can be distinguished from all other plants by the bladders on the leaves when these are present. Otherwise, they can be distinguished by the finely dissected compound leaves which are divided in two; this is as opposed to Batrachian Ranunculus species, which are divided in three; or Cabomba caroliniana, which are divided more than three times. Plants are also not rooted in the substrate. U. gibba is difficult to distinguish from C. caroliniana in its vegetative condition, but it could be distinguished from other bladderworts by its bottom-creeping habit and by leaves which radiate from the base of the flower stalk.

In New Zealand, as regards non-native distribution, the only bladderwort resembling U. gibba is the rare U. protrusa, which has many-branched, filamentous leaves, 2-3 mm long bladders, 9 mm diameter flowers (rarely seen). It is found only in still, nutrient-poor water (NZPCN, 2010).

The internal glands of the bladders could be used in the identication of Utricularia species (Yuen-Po et al., 2009).

U. gibba is frequent in canals, reservoirs, pools and is well extended in wetlands. It also occurs along lake edges, in lowland marshes and fens (Taylor, 1989; GISD 2012) and in shallow still or slow-flowing water (Rowe and Brown, 1992). In China it is found in bogs and rice fields (Zhenyu and Cheek, 2011). The invaded habitats in New Zealand include lowland freshwater wetlands (Landcare Research, 2012). It is found in pools and backwaters associated with Scirpus spp. in New Zealand (Landcare Research, 2012).

Rook (2004) reports that in America, U. gibba is occasionally eaten by muskrats, ducks and other waterfowl.

U. gibba forms mats over the water surface; in New Zealand it has been reported that this could be a problem for irrigation and drainage (Champion and Clayton, 2000; Biosecurity New Zealand, 2008).

U. gibba is reported as a weed in botanic gardens throughout the world; it poses no known harm to human health (Biosecurity New Zealand, 2008).

An interactive key for invasive plants in New Zealand has been developed by Dawson et al. (2010).

U. gibba spreads naturally in its non-native range and is too widespread and to enable it to be managed effectively (Wells and Champion, 2010). It is difficult to control due to a lack of effective control methods and its aquatic habitat (Biosecurity New Zealand, 2008), as well as its resistance to sprays (HEAR, 2005).

All aquatic Utricularia species are equipped with highly sophisticated suction traps which produce a negative hydrostatic pressure in a hollow bladder. In most species, the stimulation of sensitive hairs by tiny animals triggers a rapid influx of water which carries the prey into the trap (Sydenham and Findlay, 1973; Juniper et al., 1989; Guisande et al., 2007). Inside, the prey dies from anoxia (Adamec, 1995; 2007) and is dissolved by digestive enzymes produced by glandular hairs that line the inner side of the trap (Vintejoux, 1973; 1974; Vintejoux and Shoar-Ghafari, 2005).

In aquatic species, most of the animal prey consists of crustaceans, although insects, rotifers, nematodes, acari and protozoa are also frequently consumed by aquatic species from northern temperate regions (Mette et al., 2000; Harms, 2002; Seine et al., 2002; Barthlott et al., 2004). In tropical areas the bladder content of U. gibba has shown the following zooplankton groups: rotifers, copepods, annelids, rhizopodeans, insects and cladocerans, as well as the following phytoplankton divisions: Bacillariophyta, Chlorophyta, Cyanophyta and Euglenophyta (Gordon and Pacheco, 2007).

Depending on the aquatic environment, a significant amount of Utricularia prey can consist of algae: in mesotrophic ponds with hard water, only a few algae were found, whereas in oligotrophic peat bogs with soft water, algae formed the great majority of the trap content (Peroutka, 2008). The prey composition and variation is seasonal (Guiral and Rougier, 2007). Mette et al. (2000) showed that Utricularia in different European habitats do not capture their prey selectively but are dependent on the existing prey community.

Further work is needed on reproduction strategies in native and introduced ranges. More research needs to be carried out on control methods, including biocontrol studies, particularly in New Zealand, where U. gibba is already invasive.