First report about freshwater Bryozoa in Florida (Lake Apopka)

Abstract

This study reports the first information on Phylactolaemata from Florida. Bryozoan floatoblasts were collected from Lake Apopka in February 2007 with a large mesh plankton net. Submerged surfaces of docks and buoys were inspected, as well as stems of Phragmites. The following bryozoan species were found: Urnatella gracilis Leidy 1851 (phylum Entoprocta) and Plumatella bushnelli Wood 2001, Plumatella vaihiriae (Hastings 1929), Plumatella casmiana (Oka 1907) and Plumatella sp. (phylum Ectoprocta, Phylactolaemata).

Until now, U. gracilis is the only species already reported in Florida: it is widely distributed in eastern North America. Due to its presence in Europe, South America, Central Africa, India and Japan, it is considered cosmopolite. P. bushnelli is known just from one site in the USA and one site in New Zealand. Few data are available for P. vaihiriae, but where this species is present it is very abundant. P. casmiana is a common and widely distributed species and can be considered cosmopolite. Future research will address the distribution and role of Bryozoa in shallow subtropical systems in Florida.

Keywords:

• Freshwater Bryozoa
• Urnatella gracilis
• Plumatella bushnelli
• Plumatella casmiana
• Plumatella vaihiriae
• Florida

Introduction

Bryozoans in the ecosystems of shallow lakes may have great importance because they collect suspended particles of food from the water and transfer them to the bottom as faecal pellets. In this role they cooperate with zooplankton especially during the water blooms periods, and at times they may compete with the zooplankton for limiting food resources. Knowledge regarding bryozoan diversity and distribution in Florida is poor and it concerns exclusively the finding of Urnatella gracilis (Hull et al. 1980), whereas studies on bryozoan distribution in the northeastern USA are numerous.

Materials and methods

Sampling sites

Lake Apopka is located at 29°N 82°W in south‐central Florida, with a surface of 125 km² and an average depth of 1.6 m. The lake is hypereutrophic with few submerged vascular plants and frequent algal blooms (Microcystis and Botryococcus), although in recent years nutrient and chlorophyll concentrations have declined and transparency has increased.

In February 2007, floatoblasts were collected from the water surface of SE near‐shore region with a rectangular net (30 meshes/cm) towed from a moving boat. Freshwater weed stems, buoys, leaves and other floating surfaces also were inspected. From the collected material the floatoblasts were picked out by means of stereomicroscopy, treated with a saturated solution of KOH for 30 s at room temperature, and washed in deionized water. Some were cut in the middle with a micro‐dissection needle using a modified method of Wiebach (1964). These samples were freeze‐dried, fixed to aluminium stubs, sputter‐coated with gold‐palladium and viewed in a Philips XL 30 SEM (Taticchi & Pieroni 2005).

Results

With low winter temperatures, only a few colonies were found adhering to a tire hanging in the water on a boat dock; they were very young as was all of the benthic community. In the collected material were found Urnatella gracilis (Phylum Entoprocta) and three Phylactolaemata (Phylum Ectoprocta).

Entoprocta is a small group distinct in almost every way from the Ectoprocta, but historically included together under the name Bryozoa. The two groups have in common only their sessile form, ciliated tentacles and an incomplete separation of budded zooids (Wood 2001a).

Urnatella gracilis Leidy, 1851

Very small and young colonies were found of this species, the only known freshwater species of Entoprocta.

One or two segmented stalks arise from a basal plate. The calyx has a whorl of 14 ciliated tentacles, which surrounds both the mouth and the anus.

Plumatella bushnelli Wood, 2001

We cannot describe in detail the colony of this species because the specimen collected was very young. The tubules are colourless and transparent and contain mostly immature floatoblasts, which, on the contrary, were found to be numerous and mature in the net collected material. The elongate floatoblast has parallel or slightly incurved sides. The dorsal valve (Figure 1A) is most rectangular, sometimes with pointed poles (Figure 1B); the convex ventral valve (Figure 1C) always has pointed poles. Polar grooves are straight on the pointed floatoblast or C‐shaped in the rectangular form (Figure 1D). Both dorsal and ventral valves are covered by numerous tiny nodules, more abundant on the annular surface. On the dorsal fenestra there are equidistant and large tubercles. Ventral fenestral tubercles are similar in size to dorsal, becoming markedly dense towards the fenestra centre; they mount the annulus beyond the outer fenestral margin and blend into the annulus itself. The reticulation on dorsal and ventral fenestra is sometimes evident (Figure 2A) The suture (Figure 2B) bulges out of the annulus and it is like a cord where we can see many nodules and large not alternate tubercles that give a corrugated shape to the floatoblast margin. The annular gas chamber pores are notched as in P. repens (Figure 2C). The sessoblast was found only in net collected material, adhering to a leaf. It is oval in shape; the frontal valve (Figure 2D) is tuberculated with a pointed shape at the centre.

Figure 1 Plumatella bushnelli. A, rectangular dorsal valve. B, pointed dorsal valve. C, convex ventral valve. D, C‐shaped polar groove.

Figure 2 Plumatella bushnelli. A, fenestral reticulation. B, suture. C, annular gas chamber pores. D, sessoblast.

The floatoblasts collected in Lake Apopka generally correspond to those of North Carolina (Wood 2001b) and New Zealand (Wood et al. 1998). We can only add that the dorsal valve has a rectangular shape but sometimes it has pointed poles; the tiny nodules extend on both dorsal and ventral fenestra, where sometimes a faint reticulation can be seen.

Plumatella vaihiriae (Hasting, 1929)

This is the most common species collected in Lake Apopka. We found small colonies, containing statoblasts, on a tire and floatoblasts on the surface of the lake. In the same colony together with sessoblasts there are both elliptical (Figure 3A) and round floatoblasts (Figure 3B). For the first time this peculiarity has been noted. On the elliptical floatoblast the dorsal annulus at the poles is wider than half the fenestra length. On the round form the annulus is as large as the fenestra length. The dorsal valve is reticulated; the reticulum meshes (Figure 3C) wrap around both the annulus and the round fenestra, where, in the elliptical form, they have more evident crests. The polar groove, also reticulated, is flattened. Interstitial tubercles are completely lacking both on the annulus and fenestra. On the ventral valve the annulus is narrower than on the dorsal valve; in fact the fenestra fills three‐quarters of the ventral valve. The reticulum meshes in the central fenestra zone are smaller. The ventral valve is larger than the dorsal. At high magnification we can see small nodules on the reticulum crests. The suture is a thin cord surmounted by alternate tubercles where the reticulum meshes end (Figure 3D). The sessoblast (Figure 4A) is ornamented on the frontal lamella with holes of different size (Figure 4B): these on the external part result from crests of a strong reticulum; in the central part they are small holes. The elliptical floatoblasts of Lake Apopka are shaped like those collected by Wood & Marsh (1999) in a fish hatchery in Williamson County, Illinois, but different from those found in a waste water treatment plant by the same author. The round floatoblasts are more like the last. Moreover, the Florida floatoblasts lack a furrow between the ventral flattened fenestra and the annulus, according to Hasting's original description; while in respect to this description they lack, as found by Wood & Marsh (1999), a button‐like protuberance in the middle of the ventral fenestra. The sessoblast too is similar to those described by Wood & Marsh (1999). With regard to P. vaihiriae recently found in Italy, probably imported with exotic fish and by immigrations of aquatic birds, we can say that the American species has flatter and narrower floatoblasts and on the sessoblast frontal valve the holes are denser and less deep (Taticchi et al. 2008).

Figure 3 Plumatella vaihiriae. A, elliptical floatoblast. B, round floatoblast. C, reticulum meshes. D, suture.

Figure 4 Plumatella vaihiriae. A, sessoblast. B, sessoblast particolar. C, leptoblast. D, dorsal valve.

Plumatella casmiana (Oka, 1907)

In the net collected material we found a half valve of a leptoblast (Figure 4C). This type of thin‐walled floatoblast lacks a capsule and is characteristic of this species.

Plumatella sp 1

A dorsal valve (Figure 4D) of an unnamed plumatellid was found. In this specimen the annulus at the poles is mostly as large as laterally. The dorsal groove (Figure 5A) is small and does not extend on the fenestra. It is not reticulated; the small tubercles are regular spaced and flattened in the central area.

Figure 5 Plumatella sp 1: A, particular of dorsal valve. Plumatella sp2: B, floatoblast; C, dorsal annulus; D, dorsal groove.

Plumatella sp 2

It is impossible to assign the only floatoblast we found to a particular species. Here we can only describe the dorsal valve characteristics which discriminate this floatoblast from the other species of genus Plumatella. The floatoblast (Figure 5B) is small and oval‐round; the annulus width at the poles is twice the lateral width and one half of the fenestra length. The annulus is paved (Figure 5C) and lacks nodules. The dorsal groove is large and it extends across the polar region (Figure 5D). On the non‐reticulated fenestra there are uniform sized large tubercles.

Conclusions

This paper presents the first documentation of Phylactolaemata from Florida, USA. Previously the Entoproct Urnatella gracilis was the only known species for this State. It is present in every continent except in Antarctica and Australia. In North America the reported distribution of this species ranges from the east to the west coast and from Florida, Louisiana, and Texas to as far north as Michigan (Wood 2001a). Moreover, Lake Apopka is the third global site with a report of Plumatella bushnelli, following the findings in North Carolina (Wood 2001b) and New Zealand (Wood et al. 1998). Few data are available for P. vaihiriae; this species is not common and it is recorded from USA (Rogick & Brown 1942; Baily‐Brock & Hayward 1984; Wood & Marsh 1999); South America (Cazzaniga 1988); Tahiti (Hastings 1929); Europe (Taticchi et al. 2008). Plumatella casmiana was described from Japan (Oka 1907) and has now been reported from every continent except South America and Australia (Wood & Okamura, 2005). Further sampling is needed to have a more complete accounting of Bryozoa taxa in Florida lakes, and there is also a need for research to determine their role in the shallow lake ecosystems where the public and staff of water management agencies report that they periodically become abundant.

Acknowledgments

This study was in part supported by the Universities of Perugia (Italy), and of Florida (FL) ‘Cultural Framework Agreement between University of Perugia and University of Florida, 2007’ for the study of ecology of shallow lakes.