Macrorhynchia allmani ( Nutting, 1900 )

Galea, Horia R., 2013, New additions to the shallow-water hydroids (Cnidaria: Hydrozoa) of the French Lesser Antilles: Martinique, Zootaxa 3686 (1), pp. 1-50 : 41-44

Macrorhynchia allmani ( Nutting, 1900)

(Fig. 10 I –N)

Aglaophenia ramosa Allman, 1877 : pl. 23 figs 1–4 [invalid junior secondary homonym of Aglaophenia ramosa (Busk, 1852) ]. Aglaophenia allmani Nutting, 1900: 100 , pl. 22 figs 2–3.

Macrorhynchia allmani ― Calder, 1997: 64, fig. 19; 2013: 50, fig. 14F.

Material examined. Stn. 1, 26.ii.2012, 10– 15 m, M266: three fertile colonies, 7.5–10 cm high (MHNG-INVE- 82948); M269: several colonies 7–14 cm high, some fertile (MHNG-INVE-82889). Stn. 14, 20.ii.2012, 10– 13 m, M239: a large (almost 30 cm high), profusely branched, fertile colony (MHNG-INVE-82887); M240: two fragmented colonies, of which one is sterile and attains ca. 11 cm high, the other fertile, may have attained ca. 30 cm in life; M241: a large (ca. 30 cm high) fertile colony; M242: two fragmentary, fertile colonies, largest fragment 17 cm high (MHNG-INVE-82888).

Description. Colonies large, up to 30 cm high (even larger in the field), arising from a complex mesh of intertwined hydrorhizal fibers firmly attached to a hard substrate. Color dark brown basally, changing to light horn distally. Stems alternately to irregularly branched, with side branches given off at almost 90° in the largest colonies, then gradually curving backwards; branches generally branched in like manner several times. Stem and branches thick, highly polysiphonic, except for their very distal parts. Side branches given off from secondary tubes, with the new main tubes bearing a row of large, scoop-shaped to triangular, frontal nematothecae basally, then giving off alternately placed hydrocladia, the latter being generally present only on the less thickest parts of stem and branches. Monosiphonic parts divided by slightly oblique nodes into moderately long internodes, each of which bears two frontal nematothecae, as well as a lateral apophysis (supporting a cladium), carrying basally a mamelon provided with an almost circular foramen (reduced nematotheca). Stem nematothecae, more or less triangular, with a large foramen communicating with the internode behind, and wide, two-spined aperture having the abaxial wall inwardly rolled distally. Hydrocladia composed of up to 17 short cormidia delimited by transverse nodes; each internode with two internal septa (one adjacent to intra-hydrothecal ridge, the other beneath lateral nematothecae), and a hydrotheca. Hydrotheca cone-shaped, basal fourth with a slightly convex internal ridge, aperture oblique to long axis, margin with nine cusps: a median one and four pairs of laterals, of which the former and the three proximal pairs have slightly thickened perisarc, giving the margin a wavy appearance when seen from above, and the fourth pair hidden behind the lateral nematothecae. Median cusp rather prominent, inclined towards lumen of hydrotheca, the others triangular with rounded tips, except for the second pair, which may be comparatively wider, more blunt or even with an incised apex (see variation in Fig. 10M). Three nematothecae associated to a hydrotheca: a mesial and a pair of laterals. Mesial nematotheca curved, adnate for much of its length to abaxial wall of hydrotheca, leaving a rather short, free, tubular end, provided with a distal aperture and an adaxially one near the axil with the abaxial hydrothecal wall. Lateral nematothecae ovoid to horn shaped, bearing two apertures, one distal, and another one ovoid, opened within the hydrotheca. Length and structure of nematothecae variable, according to the cormidia to which they belong: proximal most hydrothecae with short lateral nematothecae, barely surpassing the rim, and having the apertures mostly fused; mesial nematotheca with a minute free part, guttershaped, opened adaxially through fusion of its apertures. Distalmost hydrothecae with horn-shaped lateral nematothecae, extening beyond margin of hydrotheca, and mesial nematothecae with generally a longer free part, and the apertures distinct; occasionally, the mesial nematotheca may be short. Fertile specimens produce pseudocorbulae, the latter up to 11 mm long. They are given off from modified side branches, whose main tubes carry basally a sequence of up to 8 internodes, each of which with a frontal, scoop-shaped to triangular nematotheca, followed by up to 17 internodes being the rachis of the pseudocorbula, and ending in a normal sequence of internodes bearing hydrocladia. Internodes of rachis with the same structure as the stem and branches, with two frontal, triangular nematothecae and a lateral apophysis supporting a corbulacosta, the apophysis carrying basally a reduced nematotheca. Costae given off alternately from the rachis, curving over the middle region to form a partially open structure; composed of up to 12 internodes, of which the first bears a normal hydrotheca; it is followed by 2–5 segments carrying three nematothecae (one median inferior and two latero-distal) and a centrally placed apophysis supporting a lenticular gonotheca; the remaining internodes carry alternately either a single, dorsal nematotheca, or a pair of lateral nematothecae.

Remarks. The above description is based mainly on sample M242. Some macro- and microscopic differences, with no major importance, are apparent among the colonies present in other samples examined. For instance, decimeter high colonies are branched more or less regularly, with side branches given off laterally or in front of the main stem (samples M266 and M269), while the largest colonies exhibit a fully irregular branching pattern, with side branches given off in all directions (samples M239–242). Variation also occur in the shape and size of the nematothecae as, for example, in samples M241 and M269, in which the mesials have an almost constant length, never reaching the hydrothecal rim, even in the distalmost hydrothecae. Their apertures are generally fused, a situation also met with in the well-developed, lateral nematothecae of the distal most cormidia. In addition, the cormidia in sample M241 exhibit a second internodal ridge situated at level of the insertion of the lateral nematothecae, a structure absent in other specimens.

The identification of the present material faced considerable difficulties, since there is a stupefacient confusion over the identity of seven tropical, western Atlantic species of hydroids, namely Macrorhynchia allmani ( Nutting, 1900) , M. grandis ( Clarke, 1879) , M. ramosa ( Fewkes, 1881) , M. racemifera ( Allman, 1883) , M. clarkei ( Nutting, 1900) , M. mercatoris ( Leloup, 1937) , and M. longiramosa ( Fraser, 1945) . Nearly all original descriptions and illustrations are rather poor and occasionally incorrect, and the relationships between the new species added in time and those described earlier were either not discussed or, when provided, they are based on less than reliable morphological characters. In addition, numerous subsequent authors included their materials in one or another species, mostly without providing any argument for doing so, thus further complicating the general confusion.

Judging from their microscopic trophosomal and, when known, gonosomal features, all seven nominal species are more or less related, but their degree of intraspecific variation is as yet insufficiently known. Their differentiation has been done mainly owing the shape of the hydrotheca (more or less deep and/or wide) and the length of the mesial nematotheca.

In all the available records, M. allmani is depicted as a species with hydrothecae having mesial nematothecae of varied development, though never surpassing the hydrothecal rim ( Allman 1877, Ritchie 1909, Van Gemerden- Hoogeveen 1965, Vervoort 1968, Calder 1997). In contrast, both M. grandis and M. ramosa were originally reported as having well-developed mesial nematothecae that may considerably overtop the aperture of hydrotheca. Reexamination of the type material of the former species by Calder (1997), and of the latter by both Nutting (1900) and Calder (1997), confirmed the original statements. However, variation in length of the mesial nematotheca is evident from the accounts of Versluys (1899), Ritchie (1907), and Bedot (1921) on M. grandis , and of Nutting (1900) on M. ramosa . In addition, the description and illustration provided by Versluys (1899) for the gonosome of M. grandis , and by both Fewkes (1881) and Nutting (1900) for that of M. ramosa , raise the question as to their possible conspecificity, an opinion expressed earlier by Bedot (1921), Bogle (1975) and Calder (1997).

Among other features allowing distinction of M. racemifera from its congeners, the side branches shifted on to the anterior side of the colonies are reportedly characteristic ( Allman 1883, Vervoort 1968). However, such a character was already found in specimens assigned by Ritchie (1907) to a new variety, unilateralis, of L. grandis , as well as in the present material of M. allmani , showing that at least two other taxa could display the "typical" habit of M. racemifera . The hydrothecae of the latter exhibit an intermediate morphology between those of M. allmani and M. grandis and/or M. ramosa , especially in the length of mesial nematotheca, which may slightly overtop ( Allman 1883) or just reach ( Vervoort 1968) the hydrothecal rim. Its gonosome is composed of only 3–6 pairs of corbulacostae, each of which bears as much as 5 to 10 gonothecae ( Allman 1883, Vervoort 1968), in contrast with that of its congeners, which is more profuse and carries lesser gonothecae [e.g. 7 pairs of costae, each bearing 2–3 gonothecae in M. grandis ( Versluys 1899) , 8–9 pairs of costae in M. ramosa ( Fewkes 1881) , 7–16 pairs of costae and 2–5 gonothecae in M. clarkei ( Ansín Agís et al. 2001) ]. Last but not least, the gonophore of M. racemifera is said to bear distally "a wreath of highly refringent spherules" ( Allman 1883), suggesting that it could produce a medusoid, thus setting apart this species with respect to the other western Atlantic Macrorhynchia , until their gonophores are described properly.

According to Nutting (1900), the colonies of M. clarkei exhibit one distinguishing feature in that they are "very dark colored when fresh", though it is recognized that some differences arose between shallow and deep-water specimens, with colonies having the "coenosarc crowded full of black pigment" in the first case, and others that "do not show so many of these granular bodies". Similar observations on the pigmentation were made by Ansín Agís et al. (2001), who even found colorless colonies. Dark-colored species, among the group under discussion here, were scantly reported in the literature, as for instance in M. grandis , whose stems are "black and thickest at the base, changing to light horn-color" distally ( Clarke 1879).

Although reportedly coming close to M. grandis, Nutting (1900) distinguished his M. clarkei from the former on the account of its mesial nematothecae, comparatively shorter and never reaching the hydrothecal aperture. The same is demonstrated in subsequent reports of this species ( Vervoort 1959, 1968), though a little variation in length could be noted ( Ansín Agís et al. 2001), nevertheless without facing the situation met with in M. grandis .

Among the seven species discussed here, M. mercatoris is immediately distinguished from its congeners through its lateral nematothecae of the first cormidium, showing a pronounced asymmetry, with one of them being considerably hypertrophied, tubular in shape, outwardly and backwardly directed with respect to the long axis of its corresponding hydrotheca ( Leloup 1937, Vervoort 1968). In addition, the first hydrotheca is smaller than the following ones, and is provided with a longer mesial nematotheca ( Leloup 1937).

Macrorhynchia longiramosa exhibits characteristically exceedingly long cladia, "up to 2 cm or even more" ( Fraser 1945), though the shape of its hydrothecae was found close to that of M. mercatoris by Bogle (1975). However, a reexamination of the type of Fraser's species is imperative, in order to check whether an asymmetry of its lateral nematothecae possibly occurs.

The present material from Martinique is assigned to M. allmani on the account of its mesial nematothecae not surpassing the hydrothecal rim, in contrast with those of both M. grandis and M. ramosa , which greatly overtop the aperture of hydrotheca. It is different from M. racemifera in having pseudo-corbulae with numerous costae, carrying lesser gonothecae, and having the gonophores apparently devoid17 of the belt of refringent corpuscles reported by Allman (1883). It also differs from M. clarkei in having comparatively longer (7.5 vs. 4.0 mm), more widely-spaced cladia (10 vs. 13 cladia/cm of branch), and brownish vs. black colonies in live [similar arguments in favor of their distinction were also expressed by Calder (2013)]. Macrorhychia mercatoris was excluded on the account of the pronounced asymmetry occurring in the first cormidium, as well as M. longiramosa , due to its, at least, three times longer hydrocladia, and mesial nematothecae with a very short free portion, barely reaching the hydrothecal rim.

Geographical distribution. Western Atlantic, as summarized by Calder (2013).