Flora of North America species comparison

Myriophyllum spicatum is considered one of the worst nuisance aquatic weeds in North America. Identification of this species is critical for management of lakes. Until the early 1900s, the widely accepted view was that M. spicatum was native to North America and was conspecific with European M. spicatum. M. L. Fernald (1919c) described M. exalbescens to distinguish all North American specimens from European M. spicatum, with the former name subsequently being changed to M. sibiricum due to nomenclatural precedence (S. G. Aiken and A. Cronquist 1988). The first to recognize the presence of both species in North America was apparently C. F. Reed (1970b). E. Hultén (1941–1950), B. C. Patten (1954), and S. A. Nichols (1975) proposed alternatively that M. spicatum and the native M. sibiricum form a continuum of variation, suggesting the two taxa may simply represent varieties or subspecies of a highly variable cosmopolitan species. Based on a study of herbarium collections, R. Couch and E. Nelson (1985, 1992) believed that M. spicatum was introduced from Europe in the 1940s and subsequently spread throughout the United States and Canada. A recent biogeographic study of cpDNA haplotypes indicates this species was introduced to North America from China and Korea (M. L. Moody et al. 2016).

Based upon examination of specimens for this treatment, and as pointed out by A. E. Orchard (1981), most of the characters initially proposed by M. L. Fernald (1919c) and expanded upon by S. G. Aiken (1981) that are thought to be reliable for distinguishing the two species, such as size and shape of floral bracts and bracteoles, anther length, swollen base of inflorescence, color of the stem in dried material, extent of branching, and differences in mericarps, break down when a wide range of North American herbarium material is examined. One of the few useful vegetative characters to distinguish these species in the northern regions of North America is that M. sibiricum often produces turions in the latter part of the growing season, whereas M. spicatum does not (E. Hultén 1947). The most commonly used vegetative character to distinguish the two species is the number of leaf segments in submersed leaves (Fernald). When attempting to distinguish plants of the latter two species, this is a reliable character, but only when specimens have low (6–18) or high (24–42) segment numbers; plants often have submersed leaves with intermediate segment numbers.

Molecular studies have shown that the overlap seen in morphological characters, such as leaf segment number, between Myriophyllum sibiricum and M. spicatum may be the result of frequent and widespread hybridization (M. L. Moody and D. H. Les 2002, 2007b; A. P. Sturtevant et al. 2009). Hybrids between these two species can have leaf segment numbers from 16–28 (Moody and Les 2007b), which overlaps with leaf segment numbers for both M. sibiricum (6–18) and M. spicatum (24–36). A reliable method to distinguish these taxa when there is overlap in this character state is DNA fingerprinting (Moody and Les 2002).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 68 (14 in the flora).

M. L. Moody and D. H. Les (2010) realigned some species of Myriophyllum to conform to the results of their molecular analyses, formally recognizing three subgenera, five sections, and five subsections. Because of strong levels of independent convergence, the subgenera almost completely overlap in vegetative characters. Although we recognize the phylogenetic merits of the work of Moody and Les, use of their classification here would serve only to confuse and not clarify our attempts to present the most straightforward taxonomic arrangement for Myriophyllum.

Although Myriophyllum is easily recognized in the field, positive identification at the species level has been problematic. Much of this difficulty can be attributed to over-reliance on submersed vegetative material for identification. As noted by some authors (for example, C. D. Sculthorpe 1967; G. E. Hutchinson 1975), phenotypic plasticity can greatly alter leaf characters such as size and segment number of aquatic plants in different environments. In some cases, vegetative character states used in taxonomic treatments have been based largely on information repeated from regional floras. As a result, circumscriptions of species have often not encompassed the full extent of variability observed in some taxonomic characters. Reliance on these older treatments for identification, coupled with plasticity in vegetative characters, has led to misidentifications, particularly when flowering and fruiting materials are absent. Most misidentified herbarium specimens of Myriophyllum examined for this flora did not possess reproductive characters. Currently, the most effective method for identifying vegetative specimens of Myriophyllum is by analysis of ITS and cpDNA sequences (see discussion under 8. M. spicatum).

Floral structures and fruits offer good characters to distinguish Myriophyllum species. The inflorescences in Myriophyllum are described here as racemes because most flowers are short-pedicellate. The term spike often has been used in Myriophyllum implying that the flowers are sessile. Admittedly, the distinction is largely semantic, where the pedicels are so short that the flowers appear sessile. It is important to note that mericarp characters used to distinguish these species are not fully expressed morphologically until late in their development. As a result, mericarps that are farthest from the shoot apex typically best display diagnostic characters.

Many authors have mentioned that bisexual flowers often occur on the raceme where there is a transition from pistillate to staminate flowers. Bisexual flowers are more common in Myriophyllum than realized, and we found that many staminate flowers possess pistillodes at greater or lesser stages of development. This condition is most pronounced in M. humile, in which staminate flowers often have complete pistils rather than pistillodes.

Leaf characters in Myriophyllum often have been described using terms for compound leaves, such as pinnate in form, with a central rachis and leaf divisions referred to as pinnae. Because the leaves of Myriophyllum are simple, we use pectinate, central axis, and segments as descriptors.

Many Myriophyllum species will occasionally produce a small emergent form when plants become stranded along shorelines by wave action or when water levels decrease. Only those species that produce an emersed form as part of their normal life history are discussed in some detail.

Small ascidiate (flask-shaped) trichomes are found on the stems and in the axils, or at the bases of leaves and leaf segments. They have often been referred to as hydathodes (A. E. Orchard 1979; S. G. Aiken 1981), but they have been referred to also as enations, myriophyllin glands, pseudostipules, or scales (Orchard). Because the function of these structures is unknown and they do not appear to be secretory, the best approach would seem to be to refer to them as trichomes.

Correct identification of Myriophyllum species in North America is critical in the conservation and management of aquatic habitats because some species are introduced and highly invasive (for example, M. aquaticum and M. spicatum). This issue is further complicated by hybridization of M. spicatum with native M. sibiricum (see 8. M. spicatum discussion). The native M. heterophyllum is considered invasive in the northeastern United States and Pacific Northwest, where there is evidence of hybridization with other Myriophyllum species (see 13. M. heterophyllum discussion).

(Discussion copyrighted by Flora of North America; reprinted with permission.)