Red Algae (Rhodophyta) in Rivers

Abstract

Freshwater red algae are presently classified into five classes based on recent molecular phylogenetic and supporting morphological analyses: Bangiophyceae, Compsopogonophyceae, Florideophyceae, Porphyridiophyceae, and Stylonematophyceae. Red algae are well represented in river ecosystems and many freshwater members occur exclusively in lotic habitats. These algae are usually important constituents of stream floras, either in terms of abundance or distribution from local scale to biomes. This chapter treats all the genera from river habitats, with an emphasis on the benthic forms and macroscopic taxa. Basic descriptions of 26 genera are provided, along with illustrations and information on the habitat.

Introduction

Phylogenetic Relationships and Features of the Red Algae

The higher taxonomic categories of red algae were recently revised by Yoon et al. (2006) using an analysis of the PSI P700 chl a apoprotein A1 (psaA) and rbcL coding regions river-inhabiting rhodophytes are now placed in five classes based on this phylogeny and related morphological features: the Bangiophyceae, Compsopogonophyceae, Florideophyceae, Porphyridiophyceae, and Stylonematophyceae. Each of these classes is briefly described below.

Bangiophyceae

These organisms typically have a heteromorphic life history with an alternating macroscopic gametophyte and microscopic sporophyte, the “Conchocelis” stage (Müller et al. 2010). The Bangiophyceae almost exclusively consist of species attached to substrata, such as rough boulders in intertidal and upper subtidal zones in both temperate and tropical oceans. One order, the Bangiales, extends into rivers.

Compsopogonophyceae

The class is differentiated from other bangiophyte groupings by having the combination of monosporangia and spermatangia cleaved by curved walls from vegetative cells, Golgi body–ER associations, encircling outer thylakoids in the chloroplast and a biphasic life history where known (Saunders and Hommersand 2004). All the members of the Compsopogonophyceae are benthic, either being epilithic or epiphytic. Most are intertidal to upper subtidal, but some genera can extend into freshwater streams, such as Boldia and Compsopogon of the order Compsopogonales.

Porphyridiophyceae

This class consists of unicellular red algae with a single branched or stellate plastid with or without pyrenoid; Golgi association with mitochondria and ER; cells with floridoside as a low molecular weight carbohydrate; reproduction by cell division. These algae are found on moist soils but the genus Kyliniella of the order Porphyridiales has been collected in streams from Europe and North America (Kumano 2002; Sheath and Vis 2015).

Stylonematophyceae

This group has unicellular, pseudofilamentous, or filamentous red algae; various plastid morphologies with or without pyrenoid; Golgi association with mitochondria and ER; reproduction by cell division or monospores (Yoon et al. 2006). Taxa within the Stylonematophyceae are generally rare and often overlooked in floras, as they typically occur as epiphytes on larger algae or macrophytes or as small colonies in members of the order Stylonematales (Sheath and Vis 2015).

Florideophyceae

The florideophytes are characterized as a separate group of red algae by molecular data and two reproductive features, the tetrasporophyte and the carposporophyte (Graham et al. 2009). There are more than 20 orders distinguished by molecular, reproductive, and structural features, including pit plugs. In river environments there are seven orders: Acrochaetiales, Balbianales, Batrachopermales, Ceramiales, Gigartinales, Hildenbrandiales, and Thoreales.

Sample Collection and Preservation

Information on procedures, equipment, and tools for collection and preservation of freshwater red algae can be found in Entwisle et al. (1997), Lowe and Laliberte (2007), Eloranta and Kwandrans (2012), and Sheath and Vis (2015). We will describe hereafter the most relevant aspects for a nonexperienced collector. The vast majority of red algae representatives are macroalgae and thus we will concentrate on macroscopic forms. Considering that freshwater red algae are usually not very frequent and abundant in drainage basins (Sheath and Vis 2015), it is often necessary thoroughly search stream segments, as well as to take into account the best season for collecting. One should be keep in mind to avoid rainy season in tropical regions (due to high flow and turbidity) and summer in temperate areas (due to heavy canopy cover by surrounding vegetation). Usually stream segments of 20–50 m length should be examined, using a view box with glass bottom, for a representative sampling. Tools as long forceps, razor blades, spatulas, and vacuum pipettes are useful for successfully removing the specimens from the variable substrata (rock beds, boulders, stones, logs, tree roots, macrophytes, and other macroalgae). Equipment for measuring selected stream environmental variables usually include, as portable single meters or multiple probes: thermometer, pH meter, conductivity meter, turbidity meter, and flowmeter.

Specimens are best observed shortly after collection in a living state. Brief examination during collection with a portable field microscope are useful in some cases (e.g., more than one taxon present or rough identification). For morphological purposes, they should be fixed in 2.5 % CaCO₃-buffered, histological-grade glutaraldehyde, and kept in a dark and cool environment. Other fixatives include 4 % CaCO₃-formaldehyde , Transeau, FAA, or Lugol’s, but might cause more distortions in morphology and pigmentation. For molecular purposes, specimens can be previously cleaned, if necessary, with a dissecting microscope, and then frozen or dried in silica gel desiccant. Drying of herbarium specimens are desirable for usual archival procedures but result in considerable morphological damage, in some cases not allowing observation of essential features. The correct preservation is essential for observation of the vegetative and reproductive diagnostic characters required for a proper identification, as indicated in the key and descriptions below.

Distribution of Red Algae in Rivers

Red algae are primarily marine, with less than 3 % of the over 6500 species occurring in freshwater habitats (Guiry and Guiry 2015; Sheath and Vis 2015). Most of the inland rhodophytes are restricted to streams and rivers, though some species can be collected in lakes, on moist soil, in sloth hair, and on cave walls. In major surveys, 18–65 % of river reaches contained red algae (Sheath and Hambrook 1990; Necchi et al. 1999 and references therein). The maximum number of species collected per river segment ranges from three to six (Sheath and Hambrook 1990; Necchi et al. 1999).

Most riverine red algae occur in moderate flow regimes (mean 29–57 cm s⁻¹) (Sheath and Hambrook 1990; Necchi et al. 1999). Moderate flow enhances primary productivity, pigment levels, respiration rates, and nutrient uptake rates as well as causing washout of loosely attached competitors. Flowing waters also replenish nutrients and gases and reduce boundary layers and the zone of depletion around the thallus. Some freshwater red algae, such as Hildenbrandia, Lemanea, and Paralemanea, are common in stream reaches with high current velocities (>1 m s⁻¹).

The light regime, which includes changes in intensity, quality, and photoperiod, is one of the major factors affecting the distribution and seasonality of freshwater red algae (Sheath and Hambrook 1990). Radiation affects algal growth by photosynthesis, processes indirectly related to photosynthesis, and those processes unrelated to photosynthesis. In deciduous forested regions, the surrounding tree canopy can reduce light penetration in a shaded reach by 90–99 %, and freshwater rhodophytes tend to disappear during periods of peak shading, such as Sheathia americana in a headwater stream (Sheath and Vis 2015). In addition, stream inhabiting rhodophytes exhibit photosynthetic characteristics of shade-adapted algae : occurrence of photoinhibition, low values of the saturation (<225 μmol m⁻² s⁻¹) and compensation irradiances (<20 μmol m⁻² s⁻¹), and relatively high values of the effective quantum yield of photosystem II (ΔF = F′_m 0.45) (Necchi 2005). Localized distribution in a drainage basin will also be affected by the light regime, and red algae tend to be most abundant in mid-order reaches (Sheath and Hambrook 1990).

Temperature influences the latitude, elevation, and drainage basin distribution patterns of freshwater red algae (Sheath and Hambrook 1990). In North America, the greatest species diversity has been observed in temperate and tropical latitudes. Certain taxa are most commonly distributed in tropical and subtropical regions, such as Compsopogon and Kumanoa. In contrast, the genus Lemanea is widespread in boreal and alpine habitats. In temperate regions, most red algae are restricted to elevations less than 900 m above sea level, and they exhibit their maximum biomass, growth, and reproduction from late fall to early spring. This seasonality is related to temperature but probably more so to light and nutrient availability.

In general, red algae occur in river parts that have low to moderate nutrient levels and are good indicators of relatively little disturbed sites. Flow replenishment and reduction of the boundary layer are factors accounting for this distribution, and many freshwater red algae can produce colorless hair cells that can increase the surface area for nutrient uptake (Sheath and Hambrook 1990). A number of taxa are considered to be indicators of good water quality (Eloranta and Kwandrans 2004). The interaction between pH and the form of inorganic carbon greatly influences the distribution of freshwater red algae. Rates of photosynthesis in response to pH showed that several species of freshwater red algae are able to use inorganic carbon as carbon dioxide or bicarbonate (Necchi and Zucchi 2001). Many species most frequently occur between pH 6.0 and 7.5, a range in which both forms are reasonably plentiful. The majority of these species is restricted to freshwater habitats and can be classified as specialists in terms of habitat (Sheath and Vis 2015). Competition for suitable substrata can occur among species of freshwater red algae or with other benthic algae in streams and rivers. Forms growing within and near the boundary layer, such as the “Chantransia” stages of the Batrachospermales, Thoreales, and Balbianiales, Audouinella and Hildenbrandia, are components of the stream epilithic community and compete with a complex periphyton association usually dominated by diatoms. The semierect forms compete with macrophytes , such as bryophytes, as well as other species of macroalgae.

Taxonomic key to the genera of red algae  in rivers

1a	Thalli unicellular, colonial, or pseudofilamentous	2
1b	Thalli filamentous, filliform, sac like, or pseudoparenchimatous	4
2a	Cells solitary or joined	Chroothece
2a	Cells forming pseudofilaments	3
3a	Pseudofilaments with subspherical to elliptical cells	Chroodactylon
3a	Pseudofilaments with discoid or cubic cells	Kyliniella
4a	Thalli consisting of a monostromatic sac	Boldia
4b	Thalli filamentous, filiform, or pseudoparenchymatous	5
5a	Thalli filiform, unbranched, consisting of unbranched cylinders	Bangia
5a	Thalli filamentous or pseudoparenchymatous	6
6a	Thalli uniseriate or multiseriate	7
6a	Thalli entirely multiseriate	13
7a	Thalli entirely uniseriate	8
7b	Thalli uniseriate in young parts and corticated when mature	10
8a	Spermatangia unstalked, arising on branch tips	Audouinella
8b	Spermatangia stalked, arising on specialized colorless branches	9
9a	Erect thalli consisting of elongate cylindrical cells (10–15 times longer than wide)	Balbiania
9b	Erect thalli consisting of large barrel-shaped axial and determinate lateral branches with smaller cells	Rhododraparnaldia
10a	Cortex formed by polygonal and irregularly arranged cells	Compsopogon
10a	Cortex formed by cylindrical and regularly arranged cells	11
11a	Thalli consisting of flat blade with a distinctive midrib	Caloglossa
11b	Thalli with cylindrical axes, polysiphonous	12
12a	Trichoblasts (hair cells) present, stichidia (inflated structure with tetrasporangia) lacking	Polysiphonia
12b	Trichoblasts lacking, stichidia on terminal branches	Bostrychia
13a	Thalli multiaxial, cord like (Thoreales)	14
13b	Thalli uniaxial, compact, or hollow	15
14a	Outer region formed by densely arranged filaments; sporangia located at the thallus surface	Nemalionopsis
14b	Outer region formed by loosely arranged filaments; sporangia located in the inner part of the thallus	Thorea
15a	Axial cell surrounded by tightly aggregated layers of medullary and outer cortical cells	Sterrocladia
15b	Axial cells surrounded by loosely aggregated layers of medullary and outer cortical cells	16
16a	Thalli cartilaginous, rarely mucilaginous, pseudoparenchymatous	17
16b	Thalli mucilaginous, not pseudoparenchymatous	22
17a	Thalli with beaded appearance (with nodes and internodes), sometimes only when young	18
17b	Thalli lacking beaded appearance	21
18a	Thalli hollow, with central axis not connected to the outer cortex	19
18b	Thalli compact, with central axis connected to the outer filaments	20
19a	Central axis surrounded by rhizoidal filaments; spermatangia formed in rings at nodal regions	Paralemanea
19b	Central axis lacking rhizoidal filaments; spermatangia formed in patches at nodal regions	Lemanea
20a	Outer portion formed by densely arranged filaments; secondary fascicles abundant, covering the internode	Nothocladus
20b	Outer cortex formed by loosely arranged filaments, forming an outer cortex; secondary fascicles lacking	Tuomeya
21a	Thalli stalked, hollow, with axial cells surrounded by rhizoidal filaments not connected to the outer cortex	Petrohua
21b	Thalli unstalked, compact, with axial cells surrounded by medullary filaments, connected to the outer cortex	Psilosiphon
22a	Carposporophytes indefinite, diffuse	Sirodotia
22b	Carposporophytes definite, spherical, or semispherical	23
23a	Carpogonial branches helically twisted (more than one turn) or less often curved, carpogonia asymmetric	Kumanoa
23b	Carpogonial branches straight or less often curved (less than one turn), carpogonia symmetric	24
24a	Axis heterocorticated (rhizoidal filaments with bulbous cells)	Sheathia
24b	Axis homocorticated (rhizoidal filaments without bulbous cells)	Batrachospermum

Descriptions of Genera of Red Algae in Rivers

Phylum Rhodophyta

Class Stylonematophyceae—Order Stylonematales: Genera Chroodactylon, Chroothece

Chroodactylon Hansgirg (Fig. 4.1a)

Fig. 4.1

(a) Chroodactylon . (b and c) Chroothece : (b) general view of the gelatinous matrix; (c) detail of cells. (d) Kyliniella. (e and f) Boldia: (e) general view of a dried thallus; (f) cells in surface view. (g–i) Compsopogon : (g) uniseriate filament; (h) corticated filament with axial cells (ax) and cortical cells (cc); (i) surface view with cortical cells (cc), monosporangia (mo) and microsporangia (mi). (j and k) Bangia: (j) basal part with uniseriate filaments; (k) multiseriate filament. (l) Audouinella: tuft epiphytic on Lemanea. Scale bars: (e) = 10 mm; (l) = 1 mm; (b) = 250 μm; (h, i) = 50 μm; (a, c, d, f, g, k) = 25 μm; (j) = 10 μm. Image authors: (a, i) C. Carter; (b, d, k) M. Aboal; (c) I. Chapuis; (f) Sheath and Vis (2015, Elsevier); (j) I. Bárbara

Pseudofilaments with false branching or unbranched, composed of subspherical to elliptical cells, encircled in a broad, gelatinous sheath with an irregular and uniseriate arrangement. Cells are 6–28 μm in length and 3–17 μm in diameter. Each cell contains one blue-green, stellate, axial chloroplast, with a prominent pyrenoid. Reproduction by monosporangia and fragmentation. Sexual reproduction unknown.

Remarks: two species were described from freshwater (Kumano 2002; Sheath and Vis 2015) and occur usually as epiphyte on macroalgae (e.g., Cladophora), largely in hardwaters with specific conductivity of 170–540 and pH of 7.8–8.5.

Chroothece Hansgirg (Fig. 4.1b, c)

Cells solitary or joined pole to pole, immersed in a firm and stratified gelatinous matrix, the basal pole extending into a lamelated stalk; cells elipsoidal to cylindrical, 30–46 μm in length and 20–30 μm in diameter. Each cell contains one blue-green, stellate, axial chloroplast, with a prominent pyrenoid. Reproduction by cell division and possibly by colony fragmentation. Sexual reproduction unknown.

Remarks: few species have been described from freshwater and are usually rare components of streams in North America and Europe (Sheath and Sherwood 2002; Sheath and Vis 2015).

Class Porphyridiophyceae—Order Porphyridiales: Genus Kyliniella

Kyliniella Skuja (Fig. 4.1d)

Pseudofilaments unbranched, arising from a discoid and pseudoparenchymatous base. Cells discoid or cubic, 5–17.5 μm in length and 15–19 μm in diameter, length/diameter 0.5–1.0, contiguous or separate, arranged within a broad gelatinous matrix. Rhizoidal outgrowths occur at points of contact among cells. Each cell contains several blue-green, parietal, discoid chloroplasts, without a pyrenoid. Vegetative reproduction by fragmentation and asexual reproduction by monosporangia shed by release from the sheath. Presumptive sexual reproduction with small colorless spermatia and large, pigmented carpogonia with tubular projections. Postfertilization structures unknown.

Remarks: only one species (K. latvica) has been described in the genus (Kumano 2002; Sheath and Vis 2015) and occur usually as epiphyte on macroalgae or macrophytes in softwater streams in North America and Europe.

Class Compsopogonophyceae—Order Compsopogonales: Genera Boldia, Compsopogon

Boldia Herndon (Fig. 4.1e, f)

Thalli consisting of pink to reddish brown, rarely olive green, monostromatic sac, or tube, occurring singly or in clusters arising from a microscopic disc or an irregular aggregation of cells. The disc or aggregation produces a cushion-like structure that functions as a perennial holdfast. Vegetative cells are isodiametric or rectangular in surface view, containing several peripheral, ribbon-like chloroplasts, and a large central vacuole. Secondary filaments arise as outgrowths from vegetative cells, elongating between and above vegetative cells. Reproduction by monosporangia, formed from vegetative cells, 5–9 μm in diameter.

Remarks: a single species (B. erythrosiphon) occurs in scattered streams in eastern North America from Alabama to Ontario and Quebec (Sheath and Vis 2015). In the southern range, it is often reported as epizoic on snails, whereas in the north it is largely epilithic.

Compsopogon Montagne (Fig. 4.1g–i)

Thalli consisting of branched, bluish to violet-green, uniseriate filaments with mature portions corticated. Cortical filaments consisting of a cortex with one to several layers, produced by vertical division of axial cells. Thalli up to 20–50 cm long and 250–2000 μm in diameter, may be free floating or benthic. Axial cells large and evident by slight constrictions in mature portions, usually barrel shaped. Axial cells may break down, leaving a hollow cylinder. Attached thalli fixed by rhizoids or discoid base, formed by outgrowths of lower cortical cells. Cortical cells 7–22 × 10–48 μm, polygonals, containing several peripheral, discoid chloroplasts. Reproduction by fragmentation or monosporangia, which are formed by oblique divisions of cortical or uncorticated axial cells. Monospore divides into creeping, branched filament; a central cell eventually elongates vertically and divides to form erect stage. Sexual reproduction not confirmed; microsporangia reported may represent spermatia but not confirmed.

Remarks: a recent study involving molecular data in a global sampling (Necchi et al. 2013) revealed a low genetic diversity in the genus, with a single species recognized (C. caeruleus) within the monotypic family Compsopogonaceae. The genus is reported under a wide range of environmental variables, ranging from hard to softwaters and clean to moderately polluted water bodies, mainly from tropical and subtropical regions, but there are also few reports from temperate habitats.

Class Bangiophyceae—Order Bangiales: Genus Bangia

Bangia Lyngbye (Fig. 4.1j, k)

Thalli filamentous, filiform, red or dark-purple, consisting of unbranched cylinders of cells embedded in a firm gelatinous matrix. Attachment by down-growing rhizoids, usually in dense clumps. Basal region uniseriate becoming multiseriate in upper portions. Cells spherical or cubic, containing a large, axial stellate chloroplast with prominent pyrenoid; cell number and filament length correlated in uniaxial filaments. Asexual reproduction by monosporangia produced in apical packets. Sexual reproduction not observed in freshwater habitats.

Remarks: according to the recent generic revision by Sutherland et al. (2011), Bangia is a distinct genus among the filamentous Bangiales, occurring exclusively in freshwaters and with only one species (B. atropurpurea). It is mostly found in hardwater lakes (as the Great Lakes in North America, Sheath and Vis 2015) but there are some reports from streams in North America and Europe.

Class Florideophyceae—Order Acrochaetiales: Genus Audouinella

Audouinella Bory (Figs. 4.1l and 4.2a)

Fig. 4.2

(a) Audouinella : filament with monosporangia; (b, c) Balbiania; (b) thalli (arrows) epiphytic on Batrachospermum; (c) filaments with monosporangia (arrows); (d–k) Batrachospermum: (d–g) whorls and carposporophytes (arrows); (h–j) carpogonia (arrows) and trichogynes (arrowheads); (k) carposporophytes (arrows). (l, m) Kumanoa: whorls and carposporophytes (arrows). Scale bars: (e) = 500 μm; (b, d, f, l, m) = 250 μm; (g) = 100 μm; (k) = 50 μm; (a, c) = 25 μm; (h–j) = 10 μm. Image authors: (a–c) C. Carter

Thalli filamentous, uniaxial, reddish, growing typically in dense tufts. Basal portion composed of rhizoidal outgrowth, simple or parenchymatous disc. Erect filaments consisting of cylindrical cells with unilateral, opposite, or alternate branches. Cells contain reddish, parietal, ribbon-like chloroplasts. Cell diameter 6–26 μm. Asexual reproduction by monosporangia formed at branch tips, obovoidal to spherical. Sexual reproduction by colorless spermatangia arising in clusters in branch tips; carpogonia have a cylindrical base and thin trichogyne. Carposporophytes are spherical to subspherical, consisting of a compact mass of short gonimoblast filaments; carposporangia are obovoid to subspherical. Tetrasporangia are cruciate, formed at branch tips. Asexual reproduction by monosporangia, obovoidal, elliptical, spherical, or subspherical, arising from short branches on gametophytes or tetrasporophytes.

Remarks: species of Audouinella can be misidentified, by the morphological resemblance, with the acrochaetioid “Chantransia ” stage of members of the freshwater orders Batrachospermales, Balbianiales, and Thoreales. Presence of sexual reproductive structures in true species of Audouinella (Necchi et al. 1993a) is the unequivocal diagnostic character. However, these structures are not often found and other characters are helpful to distinguish these two entities. Freshwater members of Audouinella are usually reddish (Necchi et al. 1993a), whereas the “Chantransia” stages are bluish (Necchi et al. 1993b; Zucchi and Necchi 2003). Molecular evidence was used to prove that two widely distributed bluish species (A. macrospora and A. pygmaea) are “Chantransia” stages (Necchi and Oliveira 2011). Six reddish species have been recognized as good species (A. eugenea, A. hermannii, A. huastecana, A. meiospora, A. scopulata, and A. tenella), whereas others remain uncertain. The genus is widespread in streams, ranging from Alaska to Costa Rica in North America (Sheath and Vis 2015), Brazil in South America (Necchi and Zucchi 1995), and all over Europe (Eloranta et al. 2011). Audouinella hermannii , the most common species in North America, tends to occur in cool waters (11 °C), with low ion content (specific conductivity 104 μS cm⁻¹) and mildly alkaline pH (7.5). In contrast, A. eugenea tended to occur in warm (23 °C), mildly alkaline waters (pH 7.6) with high ion content (530 μS cm⁻¹) in Mexico and Brazil (Carmona and Necchi 2001a).

Class Florideophyceae—Order Balbianiales: Genera Balbiania, Rhododraparnaldia

Balbiania Sirodot (Fig. 4.2b, c)

Thalli microscopic, composed of red-purple, uniseriate filaments, epiphytic on thalli of Batrachospermum . Filaments with a prostrate and an erect part; prostrate part composed of creeping filaments with short cylindrical or barrel-shaped cells, 1–4 times longer than large; erect part formed by elongate cylindrical cells, 10–15 times longer than wide. Asexual reproduction by obovoidal or subspherical monosporangia, arising terminally on short lateral branches. Spermatangia formed in clusters, stalked, terminal on short branchlets. Carpogonia terminal on two to three celled branches, ovoidal with a long and thin trichogyne. Carposporophytes are spherical to subspherical, consisting of a compact mass of short gonimoblast filaments; carposporangia are obovoid or subspherical.

Remarks: two species were described in the genus (B. investiens and B. meiospora) but the latter is recognized within the genus Audouinella (see comments above), as it does not develop the specialized spermatangial branch characteristic of the genus. Balbiania investiens is known from several countries in Europe (Eloranta et al. 2011). Reports of this species from Australia (Skinner and Entwisle 2001) require further investigation (Entwisle pers. comm.).

Rhododraparnaldia Sheath, Whittick, and Cole

Thalli composed of crimson, uniseriate filaments, composed of large barrel-shaped axial cells (17.3–30.1 μm in diameter) and determinate lateral branches with smaller cells (4.3–8.5 μm in diameter). Spermatangia are complex, formed at the tips of colorless stalks, consisting of two to three cells: the central cell is spherical, like a typical spermatangium and the outer one or two cells are more cylindrical. Carpogonium is borne on an undifferentiated branch and has a swollen, cylindrical base and thin trichogyne. Carposporophyte spherical, consisting of a compact mass of short gonimoblast filaments. Carposporangia spherical, 5.5–8.0 μm in diameter. Carpospores germinate into a “Chantransia” stage with composed of cylindrical cells 16–38 μm in length, 5–7 μm in diameter.

Remarks: the single species (R. oregonica) combines characteristics of both the Acrochaetiales (carpogonium shape) and Batrachospermales (thallus with main axes and determinate laterals; presence of a “Chantransia” stage). It is known only from two mountain streams in Oregon (Sheath et al. 1994); the type locality has moderate current velocity (35–61 cm s⁻¹), temperatures 8–11 °C, pH 8.3, and specific conductivity 30 μS cm⁻¹.

Class Florideophyceae—Order Batrachospermales: Genera Batrachospermum, Kumanoa, Lemanea, Nothocladus, Paralemanea, Petrohua, Psilosiphon, Sheathia, Sirodotia, Tuomeya

Batrachospermum Roth (Fig. 4.2d–k)

Thalli poorly to abundantly mucilaginous, delicate or rigid, with beaded appearance, uniaxial, formed by a large axial cell surrounded by rhizoidal filaments with cylindrical to barrel-shaped cells; pericentral cells in variable number around the nodes, producing from the upper side one to five primary fascicles of limited growth and from the lower side rhizoidal filaments growing attached to axial cells and forming one to four layers; branching irregular or pseudodichotomous. Whorls well developed or reduced, usually distinct, but sometimes becoming confluent and indistinct in older parts. Primary fascicles straight or curved with 2–35 cell-storeys; branching di- or trichotomous; fascicle cells variable in shape, generally distinct between proximal and distal parts. Secondary fascicles usually abundant, covering the entire internode and as long as primary fascicles, less often few, sparse covering part of the internode or lacking. Monosporangia occur in few species. Monoecious, less often dioecious or polyoecious. Spermatangia usually spherical to obovoid, terminal or subterminal on primary or secondary fascicles, rarely on involucral filaments. Carpogonial branches ranging from well differentiated to little modified from the fascicle cells, straight, composed of 1–22 cells; it usually arises from pericentral cells, less often from fascicle cells, carpogonial branches, or rhizoidal filaments; involucral filaments short to long, one to ten cell-storeys. Carpogonia symmetric, straight, inserted on the center of the carpogonial branch distal cell; trichogynes variable in shape, unstalked, or stalked. Carposporophytes one to two per whorl, dense or loose, spherical or hemispherical, lower or higher than the whorl radius; gonimoblast filaments with one to ten cell-storeys; cells cylindrical, elliptical, or barrel shaped; carposporangia variable in shape; usually of one type, but two types (small and large) and bisporangia rarely observed; propagules observed in one species.

Remarks: Batrachospermum is morphologically the most diverse genus in the Batrachospermales and due to this wide variation in vegetative and reproductive features it is usually split in sections. The genus has been demonstrated to be paraphyletic in phylogenetic analyses (Entwisle et al. 2009), resulting in a recent trend of raising monophyletic sections to the genus rank, such as Kumanoa, for the former sections Contorta and Hybrida (Entwisle et al. 2009) and Sheathia, former section Helminthoidea (Salomaki et al. 2014). The genus has been reported in a wide range of environmental conditions (see Kumano 2002; Sheath and Vis 2015; and references therein).

Key to the sections currently recognized within the genus Batrachospermum

1a	Carposporophyte lacking; “Chantransia” stage developing directly from the fertilized carpogonium onto the gametophyte	Acarposporophytum
1b	Carposporophyte present; “Chantransia” stage developing from the germination of the carposporangia, free living	2
2a	Whorls reduced; carposporophytes inserted on the axis, forming wart-like structures on the nodes	Setacea
2b	Whorls well developed; carposporophytes inserted on the axis or stalked	3
3a	Carpogonial branches short; carposporophytes inserted on the axis	4
3b	Carpogonial branches long; carposporophytes stalked and distributed at different positions within the whorl	6
4a	Carposporophytes with segmented propagules	Gonimopropagulum
4a	Carposporophytes with regular, nonsegemented carposporangia	5
5a	Carposporophytes dense with one type of gonimoblast filaments (upright) radiating from the fertilized carpogonia	Virescentia
5b	Carposporophytes dense or loose, sometimes abortive, with two types of gonimoblast filaments (upright and prostrate), radiating from the fertilized carpogonia or spreading on the rhizoidal filaments	Turfosa
6a	Carpogonial branches little differentiated from the fascicles	Batrachospermum
6b	Carpogonial branches well differentiated from the fascicles	7
7a	Carposporangia large (≥30 μm long and 20 μm in diameter)	Macrospora
7b	Carposporangia small (≤25 μm long and 15 μm in diameter)	Aristata

Kumanoa Entwisle, Vis, Chiasson, Necchi, and Sherwood (Figs. 4.2l, m and 4.3a–c)

Fig. 4.3

(a) Kumanoa : (a, b) carpogonial branches (arrows) and trichogyne (arrowhead); (c) gonimoblast filaments with carposporangia (arrowheads). (d–g) Lemanea: (d) nodes with spermatangia in patches (arrowheads); (e) mid portion showing carposporophytes (arrows) and spermatangia in patches (arrowhead); (f) detail of spermatangia and cortical cells; (g) gonimoblast filaments with carposporangia. (h, i) Nothocladus: (h) general view of a dried thallus; (i) detail of primary fascicles. (j–l) Paralemanea: (j) general view of thalli with spermatangia in rings (arrows); (k) mid portion showing rhizoidal filaments (arrows) and carposporophyte (arrowhead); (l) detail of spermatangia and cortical cells. Scale bars: (h–j) = 1 cm; (d) = 500 μm; (e) = 250 μm; (k) = 100 μm; (c, f, l) = 50 μm; (a, b, g) = 10 μm. Image authors: (d–g) C. Carter; (h) Museum of New Zealand; (i) T. Entwisle; (j–l) J. Carmona

Thalli poorly, moderately, or abundantly mucilaginous, delicate or rigid, usually with evident beaded appearance; branching irregular or pseudodichotomous. Whorls well developed or reduced, usually distinct, but sometimes becoming confluent and indistinct in older parts. Pericentral cells in variable number around the nodes, each forming one to four primary fascicles of limited growth and rhizoidal filaments growing attached to axial cells forming one to four layers; rhizoidal filaments homocorticated (with cylindrical cells only); primary fascicles straight or curved with 3–20 cell-storeys; branching di- or trichotomous; fascicle cells variable in shape. Secondary fascicles usually abundant, covering the entire internode and as long as primary fascicles, less often few, sparse covering part of the internode or lacking. Monosporangia occur in few species. Monoecious, rarely dioecious. Spermatangia usually spherical to obovoid, terminal or subterminal on primary or secondary fascicles, rarely on involucral filaments or specialized branches. Carpogonial branches helically twisted (more than one turn), less often curved or slightly curved (less than one turn), usually arising from pericentral cells, less often from fascicle cells, carpogonial branches or rhizoidal filaments, composed of 2–26 disc- or barrel-shaped cells; involucral filaments short, one to five cell-storeys. Carpogonia asymmetric, twisted or straight, inserted off-center on the carpogonial branch distal cell; trichogynes variable in shape, unstalked, or stalked. Carposporophytes one to two per whorl, dense or loose, hemispherical, rarely spherical, lower or higher than the whorl radius; gonimoblast filaments with three to ten cell-storeys; cells cylindrical, elliptical, or barrel shaped; carposporangia variable in shape; usually of one type, but two types (small and large) and bisporangia rarely observed.

Remarks: Kumanoa is the most species rich genus in the Batrachospermales, with 35 species recognized worldwide by Necchi and Vis (2012); at least three new species have been described or reinstated recently. The genus occurs mainly in tropical and subtropical regions of the world.

Lemanea Bory (Fig. 4.3d–g)

Thalli cartilaginous, rigid, pseudoparenchymatous, with beaded appearance, uniaxial, hollow, formed by a long axial cell lacking rhizoidal filaments not connected to the outer cortex; branching irregular and sparse. Pericentral cells consisting of T- or L-shaped ray cells closely applied to the outer cortex. Outer cortex formed by two to four cell layers with closely appressed cells; the inner layers formed by large and colorless cells; the outer layer formed by small and pigmented cells. Monoecious. Spermatangia formed in patches at the nodes. Carpogonial branches internal, straight, short, composed of three to four cells. Carpogonia symmetric, inserted on the center of the carpogonial branch distal cell; trichogynes thin, protruding beyond the outer cortex layer. Carposporophytes microscopic, forming dense spherical masses; all cells of the gonimoblast filaments produce carposporangia; carposporangia ellipsoid to subspherical, formed in chains within the central cavity.

Remarks: Lemanea is widespread in lotic systems of temperate and boreal regions of Europe (Eloranta et al. 2011) and North America (Vis and Sheath 1992). Kumano (2002) recognized 11 species in the world, whereas two new species were described later from China (Shi 2006) and one from India (Ganesan et al. 2015), totaling 14 species in the genus.

Nothocladus Skuja (Fig. 4.3h, i)

Thalli cartilaginous or mucilaginous, rigid, more or less pseudoparenchymatous, with beaded appearance when young, but cylindrical or barrel shaped when mature, uniaxial; branching irregular and abundant. Whorls reduced, distinct on the apices, but becoming confluent and indistinct in older parts. Pericentral cells in variable number around the nodes, each forming one to three primary fascicles of limited growth and rhizoidal filaments growing attached to large axial cells, forming several layers; primary fascicles straight with four to nine cell-storeys; branching di- or trichotomous; fascicle cells variable in shape; elongate distal cells loosely arranged. Secondary fascicles abundant, covering the entire internode and as long as primary fascicles. Monoecious. Spermatangia obovoid to spherical, terminal on primary and secondary fascicles or on specialized branches arising from pericentral cells. Carpogonial branches curved or twisted (more than one turn), arising from pericentral cells, proximal fascicle cells, or pericentral cells, composed of 4–14 disc- or barrel-shaped cells; involucral filaments short, one to three cell-storeys. Carpogonia asymmetric, inserted off-center on the carpogonial branch distal cell; trichogynes club shaped or cylindrical, unstalked. Carposporophytes indefinite in shape; gonimoblast filaments of unlimited growth, spreading among fascicles; carposporangia obovoid to ellipsoidal.

Remarks: Nothocladus has three species, two occurring in southeastern Australia and one endemic to Madagascar (Entwisle and Foard 2007).

Paralemanea (Silva) Vis and Sheath (Fig. 4.3j–l)

Thalli cartilaginous, rigid, pseudoparenchymatous, with beaded appearance, uniaxial, hollow, formed by a long axial cell surrounded by dense rhizoidal filaments not connected the outer cortex; branching irregular and sparse. Pericentral cells consisting of simple ray cells not abutting the outer cortex. Outer cortex formed by two to four cell layers with closely appressed cells; the inner layers formed by large and colorless cells; the outer layer formed by small and pigmented cells. Monoecious. Spermatangia formed in rings at the nodes. Carpogonial branches internal, straight, short to long, composed of three to tend cells. Carpogonia symmetric, inserted on the center of the carpogonial branch distal cell; trichogynes thin, protruding beyond the outer cortex layer. Carposporophytes forming dense spherical masses; all cells of the gonimoblast filaments produce carposporangia; carposporangia ellipsoid to subspherical, formed in chains within the central cavity.

Remarks: Paralemanea is widespread in lotic systems of temperate, subtropical, and alpine regions of Europe (Eloranta et al. 2011) and North America (Vis and Sheath 1992). An accurate estimation on the number of species in the genus is presently difficult, as Kumano (2002) recognized four species in the world, but additional species have been accepted or reinstated more recently for some regions, e.g., China (Shi 2006) and Central Europe (Eloranta et al. 2011).

Petrohua G.W. Saunders

Thalli cartilaginous, rigid, pseudoparenchymatous, stalked, lacking beaded appearance, uniaxial, hollow, formed by a long axial cell surrounded by rhizoidal filaments not connected to the outer cortex; branching irregular and rare. Pericentral cells consisting of simple ray cells not abutting the outer cortex. Outer cortex formed by two to three cell layers of densely arranged cells; the inner layers formed by large and colorless cells; the outer layer formed by small and pigmented cells. Monoecious? Putative spermatangia formed in patches on thallus surface. Putative carpogonial branches internal, composed of a cluster of small cells, arising from elongated cells of the cortical cells. Carpogonia asymmetric, inserted on the center of the carpogonial branch distal cell; putative trichogynes club shaped, projecting toward the outer cortex layer. Carposporophytes microscopic, forming spherical masses; gonimoblast filaments arising from clusters of small cells; all cells of the gonimoblast filaments apparently produce carposporangia; carposporangia formed in chains within the central cavity, carposporangia released by an ostiole, formed by modified outer cortical cells.

Remarks: Petrohua bernabei is the single Petrohua (Saunders, G.W.)species in the genus, known only from the type locality in Chile, South America (Vis et al. 2007).

Psilosiphon Entwisle (Fig. 4.4a)

Fig. 4.4

(a) Psilosiphon : general view of thalli. (b–d) Sheathia: (b) whorls with carposporophytes (arrowheads); (c) rhizoidal filaments heterocorticated with bulbous cells (arrow); (d) carpogonium with attached spermatium (arrow); (e, f) Sirodotia: whorls; (f) carpogonia with protuberance (arrow). (g, h) Tuomeya: (g) whorls; (h) carpogonia (arrow) and trichogyne. (i–m) Nemalionopsis: (i) mid part showing medulla (me) and assimilatory filaments (af); (j) cross section showing medulla (me) and assimilatory filaments (af); (k) carpogonia (arrow) and trichogyne; (l) assimilatory filaments with spermatangia (arrow); (m) gonimoblast filaments with carposporangia (arrows). (n–r) Thorea: (n) mid part showing medulla (me) and assimilatory filaments (af); (o) longitudinal section showing (me) and assimilatory filaments (af); (p) cross section showing medulla (me) and assimilatory filaments (af); (q) carpogonia (arrow) and young trichogyne; (r) short assimilatory filaments with spermatangia (arrow). Scale bars: (a) = 5 mm; (b, e, g, i, j, n) = 250 μm; (c, l, m) = 25 μm; (d, h, q, r) = 10 μm. Image authors: (a) T. Entwisle; (b–d) C. Carter; (g, h) Sheath and Vis (2015, Elsevier)

Thalli cartilaginous, rigid, pseudoparenchymatous, unstalked, lacking beaded appearance, uniaxial, compact, formed by a long axial cell surrounded by medullary filaments connected to the outer cortex; branching irregular and sparse. Four pericentral cells arising from axial cells. Medullary filaments occupying the area between the axis and the cortex, forming an open tangle mass; medullary cells with numerous chloroplasts. Outer cortex formed by cortical filaments, radially branched, compact, and cohesive; cortical filaments arising laterally and adventitiously from medullary cells. Sexual reproduction poorly known. Putative spermatangia scattered on thallus surface. Apparently asexual sporangia arising from apical cells of cortical filaments. Adventitious plantlets produced vegetatively, apparently from indeterminate growth of some cortical filaments.

Remarks: there is one species only, Psilosiphon scoparium , from southeastern Australia and northern New Zealand, restricted to rocky streams in or around waterfalls (Entwisle and Foard 2007).

Sheathia Salomaki and Vis (Fig. 4.4b–d)

Thalli moderately or abundantly mucilaginous, delicate, usually with evident beaded appearance; branching irregular. Whorls well developed, usually distinct, but sometimes becoming confluent and indistinct in older parts. Pericentral cells in variable number around the nodes, each forming two to four primary fascicles of limited growth and rhizoidal filaments growing attached to axial cells forming one to three layers; rhizoidal filaments usually heterocorticated (with bulbous and cylindrical cells) but with regular cortication in one species; primary fascicles straight with 6–23 cell-storeys; branching di- or trichotomous; fascicle cells variable in shape. Secondary fascicles usually abundant, covering the entire internode and as long as primary fascicles, less often sparse covering part of the internode or lacking. Monoecious or dioecious. Spermatangia usually spherical to obovoid, terminal, or subterminal on primary or secondary fascicles, rarely on involucral filaments. Carpogonial branches straight, usually arising from fascicle cells, rarely from pericentral cells; involucral filaments undifferentiated from fascicle cells; long, one to five cell-storeys. Carpogonia symmetric, inserted on the center of the carpogonial branch distal cell; trichogynes club shaped or lanceolate, unstalked. Carposporophytes 1 to several per whorl, dense, spherical, or subspherical, stalked, distributed at variable positions within the whorl or exerted; gonimoblast filaments with two to five cell-storeys; cells cylindrical; carposporangia obovoid to spherical.

Remarks: Sheathia was recently proposed as a genus, resulting from raising of the former section Helminthoidea to genus (Salomaki et al. 2014), with eight species recognized worldwide. The genus occurs mainly in temperate regions of Europe and North America.

Sirodotia Kylin (Fig. 4.4e, f)

Thalli moderately or abundantly mucilaginous, delicate, with evident nodes and internodes; branching irregular and abundant. Whorls well developed or reduced, obconic or pear shaped, distinct, but sometimes becoming confluent and indistinct in older parts. Pericentral cells in variable number around the nodes, each forming one to three primary fascicles of limited growth and rhizoidal filaments growing attached to axial cells forming one to three layers; primary fascicles straight or curved with 5–13 cell-storeys; branching di- or trichotomous; fascicle cells variable in shape. Secondary fascicles usually abundant, covering the entire internode and as long as primary fascicles. Monoecious, dioecious or polyoecious. Spermatangia usually spherical to obovoid, terminal or subterminal on primary or secondary fascicles, less often on involucral filaments or specialized branches. Carpogonial branches straight or slightly curved, usually arising from pericentral cells, less often from fascicle cells, carpogonial branches or rhizoidal filaments, composed of two to nine disc- or barrel-shaped cells; involucral filaments short, one to four cell-storeys. Carpogonia asymmetric, inserted off-center on the carpogonial branch distal cell with a protuberance on one side; trichogynes variable in shape, unstalked or stalked. Carposporophytes indefinite in shape, extending along the internodes; gonimoblast filaments of two types: prostrate filaments creeping among primary and secondary fascicles and rhizoidal filaments; erect filaments arising from prostrate filaments, producing carposporangia; carposporangia variable in shape.

Remarks: Sirodotia is widespread in tropical and subtropical regions of North (Necchi et al. 1993c) and South America (Necchi 1991), but one species (S. suecica) is commonly found in temperate or boreal regions of North America (Necchi et al. 1993c) and Europe (Eloranta et al. 2011). Kumano (2002) recognized six species in the genus, but other (probably new) species have been reported in recent studies using molecular data (Lam et al. 2012; Paiano and Necchi 2013).

Tuomeya W.H. Harvey (Fig. 4.4g, h)

Thalli cartilaginous, rigid, pseudoparenchymatous, with beaded appearance when young, but cylindrical when mature, uniaxial; branching irregular and abundant. Whorls reduced, distinct on the apices, but becoming confluent and indistinct in older parts. Pericentral cells in variable number around the nodes, each forming one to three primary fascicles of limited growth and rhizoidal filaments growing attached to large axial cells, forming two to three layers; primary fascicles straight with four to seven cell-storeys; branching di- or trichotomous; fascicle cells variable in shape; small distal cells densely arranged, forming an outer cortex with two to three layers. Secondary fascicles lacking. A cavity formed between two whorls limited by the inner cortex and the axis with rhizoidal filaments. Monoecious. Spermatangia usually elongate obovoid or ellipsoidal, terminal on primary fascicles. Carpogonial branches curved or slightly twisted (less than one turn), arising from pericentral cells, composed of four to seven disc- or barrel-shaped cells; involucral filaments short, one to three cell-storeys. Carpogonia asymmetric, inserted off-center on the carpogonial branch distal cell; trichogynes club-shaped, unstalked. Carposporophytes consisting of a globular and dense mass; gonimoblast filaments radially spreading from the fertilized carpogonia, with three to five storeys of cylindrical cells; carposporangia obovoid to ellipsoid.

Remarks: Tuomeya is a monotypic genus, with the single species T. americana occurring in eastern North America, although there are some unsubstantiated reports from Europe (Eloranta et al. 2011) and Asia (Shi 2006).

Class Florideophyceae—Order Thoreales: Genera Nemalionopsis and Thorea

Nemalionopsis Skuja (Fig. 4.4i–m)

Thalli mucilaginous, cord-like, multiaxial, composed of a central medullary region and an outer layer of lateral filaments of limited growth; branching irregular and abundant. Medullary region composed of interwoven branched and twisted, colorless or yellowish filaments with cylindrical short to elongate cells, occupying half to two thirds of the thallus diameter. Cortical region consisting of densely arranged assimilatory filaments, in an outer whorl at a right angle to the main axis, disposed along the entire thallus, with cylindrical or barrel-shaped cells containing chloroplasts. Assimilatory filaments densely arranged at the distal portion composed of cylindrical, elliptical or barrel-shaped cells. Spermatangia arising from terminal or subterminal cells of assimilatory filaments at the thallus surface, in clusters, pairs or single; spermatangia can regenerate from old empty spermatangial cells. Carpogonia bottle shaped or ovoid, inserted directly on the basal cell of assimilatory filaments or on short carpogonial branches with cylindrical or barrel-shaped cells; trichogynes elongate and filiform, reaching half to two thirds of the outer layer. Gonimoblast filaments spreading among the assimilatory filaments, radially arranged in the distal portion, composed of cylindrical cells. Carposporangia in clusters, near the thallus surface, larger than spermatangia. Monosporangia arising from the long assimilatory filaments, similarly to spermatangia and carposporangia.

Remarks: two species are widely recognized in the genus (N. shawii and N. tortuosa) occurring mostly in warm and hard water from Asia and North America. Reproductive structures usually described as monosporangia are possibly spermatangia or carposporangia, as observed in the genus Thorea (Necchi et al. 2016). Carpogonia, spermatangia and carposporangia were recently reported in N. shawii from Indonesia and Nepal (Johnston et al. 2014; Necchi et al. 2016).

Thorea Bory (Fig. 4.4n–r)

Thalli mucilaginous, cord-like, multiaxial, composed of a central medullary region and an outer layer of lateral filaments of limited growth; branching irregular and abundant. Medullary region composed of interwoven, colorless filaments, occupying one to two thirds of the thallus diameter. Outer region consisting of loosely arranged assimilatory filaments, disposed along the entire thallus, with cylindrical or less often barrel-shaped cells containing chloroplasts; assimilatory filaments are of two types: short and restricted to the proximal portion of the outer layer; long and reaching the thallus surface. Monoecious or dioecious. Spermatangia arising on the basal part of the outer region, from the short assimilatory filaments. Carpogonia consisting of short carpogonial branches, with few cells, arising from the short assimilatory filaments; trichogynes elongate and filiform, reaching half to two thirds of the outer layer. Gonimoblast filaments spreading among the short assimilatory filaments, arising directly from the fertilized carpogonia. Carposporangia in clusters, arising from short, erect gonimoblast filaments. Monosporangia arising from the short assimilatory filaments, similarly to spermatangia and carposporangia.

Remarks: more than ten species are currently recognized in the genus, based on morphological (Kumano 2002) or molecular data (Vis, pers. comm.). Species occur predominantly in warm and hard waters in tropical or subtropical regions of most continents. Reproductive structures usually described as monosporangia by some authors are possibly spermatangia or carposporangia, as noted by Necchi (1987) and Necchi and Zucchi (1997). However, the coexistence of asexual monosporangia with sexual reproductive structures (carpogonia and spermatangia) and carposporangia has also been confirmed in some species (Carmona and Necchi 2001b).

Class Florideophyceae—Order Hildenbrandiales: Genus Hildenbrandia

Hildenbrandia Nardo (Fig. 4.5a–e)

Fig. 4.5

(a–e) Hildenbrandia : (a) surface view of the crust; (b) erect filaments; (c) marginal part of the crust; (d) surface view of the crust with a gap formed by release of a gemma; (e) released gemma with germinating filaments (arrow). (f–i) Bostrychia: (f) uncorticated part with polysiphonous axis; (g) hapteron; (h) apical part with stichidium; (i) uniseriate terminal branches. (j, k) Caloglossa: leafy branches with prominent midribs (arrow); (k) young leafy branch with prominent midrib (arrow). (l, m) Polysiphonia: (l) mid part with polysiphonous axis; (m) apical part with polysiphonous and uniseriate axes. (n–q) Sterrocladia: (n) mid portion showing cortical cells; (o) apical portion showing apical cell (arrow); (p) longitudinal section showing axial cells (arrow) and cortical cells (arrowhead); (r) cross section showing axial cell (arrow) and cortical cells (arrowhead). Scale bars: (j) = 500 μm; (h) = 250 μm; (f, g, i, k–p) = 100 μm; (d, q) = 50 μm; (e) = 25 μm; (a–c) = 10 μm. Image authors: (j) M. Kamiya; (k) J. West

Thalli crustose, bright red, firmly attached to the substratum, with a smooth surface or wart-like protuberances, composed of a basal layer and erect filaments. Basal layer thin and formed by branched filaments that are densely aggregated laterally. Erect filaments formed by cubic or short-cylindrical cells. Sexual reproduction and tetrasporangia are unknown in freshwaters. Asexual reproduction by gemmae, consisting of a small and dense packet of filaments, that release and germinate into new crusts. Vegetative reproduction by fragments of older thalli.

Remarks: two species are usually recognized in freshwater habitats, with H. angolensis distributed in Africa, Australia, North and South America, and H. rivularis in Europe and Asia (Eloranta et al. 2011; Sheath and Vis 2015; Entwisle pers. comm.). The two species are usually reported in shaded and fast-flowing streams.

Class Florideophyceae—Order Ceramiales: Genera Bostrychia, Caloglossa and Polysiphonia

Bostrychia Montagne (Fig. 4.5f–i)

Thalli dark reddish, pseudodichotomously or dichotomously branched, corticated or uncorticated, attached to substrata by haptera (specialized rhizoidal branches). Vegetative branching bilateral; apical branches are usually curved. Axis composed of polysiphonous axes and branches consisting of axial cells having two to six whorls of four to eight periaxial cells. Tetrasporangia are the most usually reproductive structures observed in freshwater collections, consisting of inflated, multichambered structures termed stichidia, on the upper portion of the thalli. Carpogonia and cystocarps described in one species.

Remarks: six species have been reported in freshwater habitats (rivers, streams, and ponds; Kumano 1979, 2002). D’Lacoste and Ganesan (1987) described carpogonia, cystocarps and stichidia in B. moritziana from a freshwater population in Venezuela.

Lotic populations tend to occur in streams or rivers with relatively high temperatures (17–26 °C), neutral to alkaline (pH 7.0–8.4) and a wide range in specific conductivity (38–440 μS cm⁻¹) (Sheath et al. 1993; Necchi et al. 1999).

Caloglossa (Harvey) Martens (Fig. 4.5j, k)

Thalli reddish, dichotomously, rarely trichotomously branches, consisting of flat, articulate, narrow leafy segments; a prominent midrib is evident and is composed of a broad axial cell, surrounded by four periaxial cells, appearing like three rows of cylindrical or barrel-shaped cells in surface view. Outer portions of the leafy segments monostromatic, with oblique series of hexagonal cells. Rhizoids and new leafy segments arise at constrictions, either from the midrib area or peripheral layer of cells. Freshwater populations vegetative; reproductive structures unknown.

Remarks: Five species have been reported from freshwaters (Kumano 2002), but two species (C. leprieurii and C. ogasawarensis) are found most often. Current velocities are usually moderate (33–43 cm s⁻¹), temperature high (23–24 °C), pH alkaline (7.6–8.4) and specific conductivity moderate (100–200 μS cm⁻¹) in North America (Sheath et al. 1993).

Polysiphonia Greville (Fig. 4.5l, m)

Thalli dark reddish, consisting of erect, polysiphonous filaments with a single tier of pericentral cells around the axial cell. Freshwater collections with no additional layer of cortication that could be present in few marine species. Rhizoidal branches arise from pericentral cells. Delicately branched hairs (trichoblasts) formed in upper portions of the thalli. Freshwater populations vegetative; reproductive structures unknown.

Remarks: only one species (P. subtilissima) has been reported from stream habitats, from warm (22–26 °C), alkaline (pH 7.6–8.8) and high conductivity (1150–2700 μS cm⁻¹) waters (Sheath et al. 1993; Lam et al. 2013).

Class Florideophyceae—Order Gigartinales: Genus Sterrocladia

Sterrocladia F. Schmitz (Fig. 4.5n–q)

Thalli pseudoparenchymatous, cylindrical and irregularly branched, with main axes and branches of similar size. Apices rounded and slightly mucronated due to the occurrence of a prominent apical cell, which undergoes divisions in early stages to form axial and pericentral cells. Uniaxial structure evident in cross section, with the presence of an axial cell surrounded by two to three inner layers of large and irregular medullary cells and one, rarely two, outer layers of small cortical cells. Medulla entire, with all cells abutting in cross section and axial cell adherent to the outer cortex. One row of cylindrical to elliptical axial cells revealed in longitudinal section, surrounded by medullary and cortical cells. Small, irregularly shaped, polygonal and densely arranged cortical cells viewed in surface view. Reproductive structures developed in nemathecia, forming wart-like protuberances on the thallus surface. Nemathecia composed of short, branched filaments, producing terminal “sporangia”; it is not clear if these structures are sporangia or spermatangia, but their large size (16–20 μm long, 15–19 μm in diameter) and presence of chloroplasts suggest they are sporangia; in this case they could be asexual monosporangia, sexually related carposporangia or young (undivided) tetrasporangia.

Remarks: Sterrocladia is the only completely freshwater genus belonging to the order Gigartinales (Sherwood et al. 2012) with an incertae sedis status at the familial level. Two species were described in the genus: the type species S. amnica from Guyana and French Guyana and the recently described S. belizeana. Both are rarely reported in these areas.