Indian Ocean Catalogue

DICTYOPTERIS

The first species referable to the genus Dictyopteris was described as Fucus membranaceus by Stackhouse in 1795 ([1795--1801]: 13, pl. VI) on the basis of a collection from Sidmouth, Devon, England. The second representative of the genus was described as Fucus polypodioides by Desfontaines in 1799 ([1798--1799]: 421) on the basis of a collection from Algeria. These two species were dealt with for the first time by Lamouroux (1805: 32--37, pl. XXIV), who based his opinion of F. membranaceus on the protologue together with observations published by Turner (1802a: 141--144), and his opinion of F. polypodioides on specimens sent to him by Desfontaines. He decided that the two species should be united, but, for an unstated reason (possibly national pride), he adopted the later name. He distinguished var. minor from typical F. polypodioides by a suite of vegetative characters (small size, short stem, twisted frond, close branching). At that time, he had seen specimens of var. minor only from Corsica and the Catalonian coast of Spain.

After examining many more representatives of the species sensu lato, he found further differences that seemed to warrant the recognition of two distinct species. To accommodate these species, Lamouroux (1809b: 332; 1809c: 129) established the genus Dictyopteris, which he distinguished from Dictyota chiefly by the presence of a midrib in the frond. At this time, he changed his earlier opinion, deciding that Fucus polypodioides should be kept apart from F. membranaceus and associated instead with var. minor, which he placed in synonymy under Dictyopteris polypodioides. In transferring F. membranaceus to Dictyopteris, he continued to ignore Stackhouse's name, choosing instead to use a new epithet, calling the species D. elongata. Dissociation of F. polypodioides from F. membranaceus resulted in geographic separation of Lamouroux's species, D. polypodioides being restricted to the Mediterranean, D. elongata to England and Atlantic France. In view of subsequent taxonomic history, it is important to note that Lamouroux supplemented vegetative characters with a reproductive character in distinguishing these species, D. polypodioides having the ``fructifications'' much more numerous near the midrib than near the margins, whereas in D. elongata they were said to be scattered.

Turner (1808--1809: 41--43, pl. 87) followed Lamouroux's 1805 treatment in considering F. membranaceus and F. polypodioides conspecific, but restored Stackhouse's name. Turner, to judge from his citation of Lamouroux elsewhere in the same work (1808--1809: 1, 3), was presumably aware of var. minor, but he made no reference to it. C. Agardh (1820a: 142--143) followed Lamouroux's 1809 treatment in dissociating F. membranaceus from F. polypodioides, but only at the varietal level. In choosing a name for the species, he also passed over Stackhouse's name in favor of the epithet polypodioides. It may be noted that in Agardh's classification, var. minor includes the type of the name adopted for the species so that under present-day rules (Art. 26.1) it would be called var. polypodioides, leaving the other implied variety without an available epithet. (Incidentally, C. Agardh rejected the generic name Dictyopteris in favor of Haliseris, a name proposed in manuscript by G. Targioni-Tozzetti. The name that he should have used is Neurocarpus Weber & Mohr (1805: 300), which he cited in synonymy. Neurocarpus, which was based on F. membranaceus, was eventually ruled a nomen rejiciendum in favor of Dictyopteris.) J. Agardh (1848b: 117) recognized only one species without varieties (Haliseris polypodioides), and this treatment has been generally followed to the present time.

In a study of the Dictyotales of Libya, Nizamuddin (1981: 11--23) concluded that the early authors mentioned above had been dealing with a mixture of two species. On the basis of the arrangement of tetrasporangial sori on the blade, he distinguished D. membranacea from D. polypodioides and described an additional species, D. tripolitana. In the first species the sori are said to be scattered on either side of the midrib, in the second they are linearly arranged on either side of the midrib, while in the third they are obliquely transverse on either side of the midrib. Nizamuddin recorded all three species from the Atlantic as far north as England and in various parts of the Mediterranean. Moreover, the three species are supposedly sympatric at such localities as Rovinj, Croatia, and Devon, England. The fact that none of the three species recognized by Nizamuddin has geographic integrity suggests the desirability of reinvestigating the taxonomy of northeastern Atlantic Dictyopteris.

Any effort to determine how Nizamuddin's ideas relate to earlier literature is frustrated by lack of information on reproductive structures and by difficulty in interpreting the information that is available. As with all algae, before the life history of members of the Dictyotales was elucidated, the fructification was puzzling and described in a diversity of terms. Stackhouse merely said that the fructification was in clusters of dots. Turner (1802a: 141--144) was unable to confirm the reproductive function of these ``dark, roundish, scattered spots, disposed without regularity on both sides of the midrib''. Encouraged by observations made by Mrs. Griffiths, Turner (1808--1809: 41--44, pl. 87) wrote of ``largish, roundish, dark-brown seeds ... collected into elliptical or hemispherical clusters''. Lamouroux (1809b; 1809c) described the fructification as capsules joined to form rather large blackish spots scattered over both surfaces of the frond.

Greville (1830: 63--65) described the fructification for the genus as ``ovate seeds, forming distinct sori or groups (mostly arranged in longitudinal lines)'', and for the species as ``blackish ovate seeds, either solitary and scattered over the frond, or aggregated and forming oblong spots arranged on each side, and nearly close to the midrib, in a linear series: the two kinds always found on distinct individuals''. He gave credit to Mrs. Griffiths for discovering the ``single scattered seeds''. In a careful study of D. polypodioides, Reinke (1878) argued for the equivalence of these ``zerstreuten Sporen'' to oogonia, although he was unable to find male plants. Antheridial sori were described by T. Johnson in 1891 as being of variable size and shape and occurring alongside the midrib as well as scattered over the blade on each side of it. The large immobile spores clustered in sori (``Haufensporen'') are produced in tetrads in Dictyopteris, as in all Dictyotaceae except Zonaria and Lobophora, and thus were termed tetraspores by Reinke and earlier authors. Reinke observed that these spores developed directly into new individuals and therefore were equivalent to the zoospores released by unilocular sporangia in other Phaeophyceae. It remained for Williams (1904a; 1904b) to provide cytological proof of an alternation of isomorphic somatic phases in the Dictyotales.

Even after an understanding of the significance of ``spots'' in Dictyopteris led to the use of standard terms for the reproductive structures (aplanosporangia or tetrasporangia, oogonia, and antheridia), discrepancies appeared in the literature. For example, Setchell & Gardner (1925: 655) stated that aplanospores are arranged in sori along either side of the midrib while oogonia and antheridia are scattered. Lily Newton (1931: 216), on the other hand, noted that the sporangia were borne singly, the sexual organs in sori. According to Fritsch (1945: 311, 313, 315), the sporangia are grouped around tufts of hairs, the oogonia are scattered singly or in small groups (citing Reinke, 1878), and the antheridial sori are scattered and comprise between 3 and 100 antheridia (citing T. Johnson, 1891). Womersley (1987), in a monograph of southern Australian Phaeophyceae, provided an across-the-board comparison among five species of Dictyopteris. Oogonia were not observed, but antheridia were found in three species, in each case in scattered sori. Regarding sporangia, at the generic level Womersley stated that they are scattered or clustered in sori. For three species he described scattered sporangia, presumably occurring singly, and for a fourth species he described sporangial sori associated with tufts of hairs. In the fifth species (D. acrostichoides) the sporangia were said to be ``scattered, usually densely, in elongate sori along the blades''. This wording is puzzling since a sorus is defined as a confluent group of reproductive organs.

In presenting the foregoing taxonomic and nomenclatural history of northeastern Atlantic Dictyopteris, I avoided hopeless confusion by suppressing the fact that neither Fucus membranaceus nor F. polypodioides is a legitimate name, both being later homonyms. Fucus membranaceus Burman (1768: 32 [28 in error]; type locality: Cape of Good Hope, South Africa) is a name of unknown application, while F. polypodioides S. Gmelin (1768: 186; type locality: Mona [Anglesey] Island, Wales) was placed by Hudson (1778: 573) in the synonymy of Fucus ovalis Hudson (l.c.), a superfluous name for F. ovatus Hudson (1762: 468; type locality: Yorkshire) (Gastroclonium ovatum (Hudson) Papenfuss, 1944a: 344). Art. 58.3 can be invoked to find legitimate names for later homonyms among intended combinations. In the case of F. polypodioides Desfontaines, Ulva polypodioides De Candolle (in Lamarck & De Candolle, 1805: 15) is to be treated as a legitimate new name even though its intended basionym is illegitimate. In the case of F. membranaceus Stackhouse, however, all intended combinations based on this name must be considered superfluous and hence illegitimate new names because the species to which the binomials were applied included the type of Ulva polypodioides. A legitimate new name, Dictyopteris elongata, was inadvertently supplied by Lamouroux (1809a: 332; 1809b: 129), who changed Stackhouse's epithet while recognizing D. polypodioides as a distinct species.

In attempting to integrate the foregoing nomenclatural information with Nizamuddin's classification, I encountered difficulty resulting from that author's practice of basing nomenclature at least partly on circumscriptions. Thus, he recognized as distinct species Dictyopteris polypodioides (Desfontaines) Lamouroux and D. polypodioides [sensu] Lamouroux. For the species that he called D. membranacea (Stackhouse) Batters, the synonymy, after being purged of circumscriptions, includes F. membranaceus Stackhouse 1795 (illegitimate), F. polypodioides Desfontaines 1799 (illegitimate), Ulva polypodioides De Candolle 1805 (legitimate), and Dictyopteris elongata Lamouroux 1809 (legitimate). The correct name for this species is thus D. polypodioides (De Candolle) Lamouroux, despite the fact that according to Nizamuddin, Lamouroux did not have this species in hand when he made the combination. Art. 7.4 requires a combination to remain with the basionym regardless of the identity of the material in the hands of the author who made the combination. For the species that Nizamuddin called D. polypodioides Lamouroux, the synonymy, after being purged of misapplied names, includes only .NX "Dictyopteris polypodioides var. angustifrons" D. polypodioides (De Candolle) Lamouroux var. angustifrons Montagne (1846b: 29). This species thus remains without a name.

Recently, Cremades (in Cremades & Pérez-Cirera, 1990b: 489) revealed that Fucus ambiguus Clemente y Rubio (1807: 310; lectotype locality: Almeria, Spain) is a representative of Dictyopteris with sori adjoining the midrib. The application of the name D. ambigua (Clemente y Rubio) Cremades, especially with regard to Nizamuddin's treatment, remains to be determined.

It is obvious that a satisfactory treatment of Dictyopteris in the northeastern Atlantic and Mediterranean depends upon studies to determine the taxonomic value, if any, of patterns of size, shape, and arrangement of sporangial sori. It is essential to know the interrelationships between these patterns, on the one hand, and habitat, age, and geography on the other. Dictyopteris polypodioides and D. membranacea have been reported from various parts of the world. All such records outside the northeastern Atlantic and Mediterranean should be regarded with suspicion. In the present catalogue, records under both names are grouped under D. polypodioides.