Margelopsis gibbesii McCrady 1859

Margelopsis gibbesii (McCrady, 1859)

Figs. 1, 2

not Nemopsis gibbesii McCrady, 1859: 160, pl. 10, figs. 1–3 [medusa] [= Nemopsis bachei L. Agassiz, 1849].— Bedot, 1925: 295 [binomen N. gibbesii encompasses two species].— Stephens & Calder, 1992: 44, fig. 3, in part (reproduction of McCrady’s pl. 10, figs. 1–3) [mention of medusa; historical discussion] [= Nemopsis bachei].— Sanders & Anderson, 1999: 58 [medusa; historical discussion] [= Nemopsis bachei].

Nemopsis gibbesii McCrady, 1859: 162 –163, pl. 10, figs. 4–7 [description of hydroid; description of medusae newly liberated from hydroid].— Allman, 1864: 370; 1872: 362 [mention of hydroid; not adult medusa].— Bedot, 1910: 333 [synonymy].— Schuchert, 2006: 361 [taxonomic discussion].

Nemopsis bachei.— L. Agassiz, 1862: 345 [in part; hydroid of Nemopsis gibbesi included in synonymy with N. bachei] [not Nemopsis bachei L. Agassiz, 1849].

not Nemopsis bachei.— L. Agassiz, 1862: 345 [in part; medusa of Nemopsis gibbesi included in synonymy with N. bachei].— A. Agassiz, 1865: 149 [medusa] [Nemopsis gibbesi included in synonymy list] [incorrect subsequent spelling].— Haeckel, 1879: 93 [mention of medusa] [Nemopsis gibbesii included in synonymy list].— Brooks, 1883: 468 [mention of medusa] [Nemopsis gibbesii included in synonymy list].

Nemopsis gibbesi.— Frech, 1897: 565 [mention of hydroid].— Hartlaub, 1899: 221, fig. 4; 1903: 28 [mention of hydroid].— Bedot, 1918: 358 [synonymy: N. gibbesi = Margelopsis gibbesi in part] [incorrect subsequent spelling].

Margelopsis gibbesii.— Hartlaub, 1899: 224 [mention of hydroid].— Mayer, 1910, pl. 9, figs. 4–7 [illustration of medusa].— Stephens & Calder, 1992: 44, fig. 3, in part (reproduction of McCrady’s pl. 10, figs. 4–7) [mention of hydroid & young medusa; historical discussion].— Sanders & Anderson, 1999: 58 [historical discussion].— Cairns et al., 2002: 14 [listing of hydroid]; 17 [listing of medusa].— Schuchert, 2006: 361 [taxonomic discussion].— Johnson & Allen, 2012: 94, fig. p. 95 [report of medusa].

Margelopsis gibbesi.— Hartlaub, 1903: 28 [mention of hydroid].— Hartlaub, 1907: 92 [mention of hydroid].— Mayer, 1910: 82 [report of medusa]; 83 [mention of hydroid].— Ritchie, 1915: 563 [mention of hydroid].— Bedot, 1918: 184 [literature].— Bedot, 1925: 279 [literature].— Uchida, 1927: 196 [mention of medusa].— Rees, 1941: 134 [mention of hydroid].— Fraser, 1944: 104, pl. 18, fig. 77; 1946: 156 [description of hydroid and young medusa based on McCrady 1859].— Miner, 1950: 102, figs. pl. 35 [guidebook; mention of hydroid; mention of medusa].— Werner, 1955a: 8 [mention of medusa].— Kramp, 1959: 92, fig. 48 [account of medusa from literature].— Kramp, 1961: 49 [literature: review of medusa stage].— Allwein, 1967: 122 [report of medusa].— Gosner, 1971: 111, fig. 5.18B [guidebook; mention of medusa].— Sandine & Swiecicki, 1975: 83 [technical report; report of medusa].— Hester, 1976: 19, pl. 1, fig. 4 [thesis; report of medusa].— Tatham et al., 1977: 141 [technical report; report of medusa].— Calder & Hester, 1978: 88 [mention of hydroid; mention of medusa].— Harrison et al., 1979: 37 [mention of hydroid; mention of medusa; species of “special concern”].— Cairns et al., 1991: 16 [listing of hydroid]; 20 [listing of medusa].— Bouillon & Boero, 2000: 145 [listing of hydroid; listing of medusa].— Bouillon et al., 2006: 245, fig. 118B [listing of hydroid; listing of medusa; figure of medusa, from Kramp 1959 after Mayer 1910] [incorrect subsequent spelling].

not Margelopsis gibbesi. — Thiel, 1938: 294 [misidentification of medusa] [= Margelopsis australis Browne, 1910].

Margelopsis.— Werner, 1954: 144 [reference to McCrady’s 1859 hydroid].

M. (as Margelipsis) gibbsi.— Zamponi, 1983: 178 [mention of medusa] [incorrect subsequent spelling].

Type Locality. USA: Charleston Harbor, South Carolina (McCrady 1859).

Material examined. Virginia Beach, Virginia, from aquaria operated by Hampton Roads Sanitation District, May 1989, 22 ‰, several dozen hydroids, some with medusa buds, coll. Ms. Butterworth, ROMIZ B1027.— Chincoteague, Virginia, Station 199, about 5 km offshore, depth 10 m, 24.iv.2006, epibenthic sled, <20 cm off bottom, two male medusae, coll. W. Johnson, ROMIZ B4077.—Assateague Island, Virginia, Station 208, just off beach near southern tip of island, surf zone, depth 1–2 m, 10.vi.2006, epibenthic sled, <20 cm off bottom, three female medusae, some with nascent polyps, coll. W. Johnson, ROMIZ B4078.—Assateague Island, Virginia, Station 221, ca. 200 m off beach near southern tip of island, depth 5 m, 10.vi.2006, epibenthic sled, <20 cm off bottom, one male medusa, coll. W. Johnson, ROMIZ B4079.—Assateague Island, Virginia, ca. 400 m offshore, depth 5 m, 10.vi.2006, epibenthic sled, <20 cm off bottom, about 35 female medusae, some with nascent polyps, and four male medusae, coll. W. Johnson, ROMIZ B4080.

Description of material. Hydroids solitary, naked, monomorphic, free-floating. Hydranths all young, minute (<1 mm long), typically vasiform to pear-shaped, with dome-shaped hypostome at distal end and vestigial hydrocaulus at proximal end; mouth at tip of hypostome. Hydrocaulus with base hollowed out, having a sucker-like appearance. Tentacles appearing moniliform, in widely separated oral and aboral whorls; oral tentacles 5–7 in number; aboral tentacles 7–9 in number, slightly larger than oral ones. Some hydranths appearing to undergo asexual reproduction by transverse fission (Fig. 1 e) and budding (Fig. 1 f).

Gonophores medusa buds arising on hydranth distal to aboral tentacular whorl; all observed medusa buds very early in development.

Medusae thimble-shaped with rounded apex, up to 2.3 mm high, 1.9 mm wide; mesoglea of moderate thickness. Exumbrellar surface with scattered nematocysts, possibly subspherical heteronemes, and nematocyst patches. Apical canal present, narrow, inconspicuous. Manubrium vasiform, extending nearly to velar opening, proximal end with vacuolated gastrodermal cells. Mouth irregularly circular, without lips or oral tentacles. Radial canals four, simple. Ring canal present. Marginal bulbs four, pyriform. Ocelli lacking. Marginal tentacles 5–8 per marginal bulb, moniliform, with nematocysts arranged in rings and with an elongated terminal bulb. Velum welldeveloped. Gonad surrounding manubrium except at upper and lower extremities. Female medusae with prominent subitaneous eggs extruded from gonad, some developing as nascent polyps (actinulae). Developing polyps subspherical to discoidal at release, about 0.3 mm in diameter, with approximately 12–13 short tentacles.

Cnidome of hydroid.

Desmonemes (n = 10): 5.0–5.8 µm long × 2.6–3.3 µm wide (undischarged)

Haplonemes (n = 10): 6.3–7.4 µm long × 2.5–3.6 µm wide (undischarged)

Microbasic heteronemes (n = 10): 7.0–8.4 µm long × 2.8–3.6 µm wide (undischarged)

Cnidome of medusa.

Desmonemes (n = 10): 3.7–4.0 µm long × 2.8–3.2 µm wide (undischarged) Haplonemes (n = 5): 5.0–6.4 µm long × 1.6–2.5 µm wide (undischarged) Microbasic heteronemes (n = 10): 5.0–6.1 µm long × 2.8–3.6 µm wide (undischarged) Subspherical heteronemes (n = 10): 8.4–9.5 µm long × 7.5–8.6 µm wide (undischarged) Stenoteles (n = 10): 5.8–7.3 µm long × 4.0–5.9 µm wide (undischarged)

Historical account. Under the name Nemopsis gibbesii, McCrady (1859) described and illustrated a medusa from Charleston Harbor, South Carolina, that he regarded as a new species. Included with it was an account of a hydroid, also illustrated, that he took to be its polyp stage. McCrady recognized that the medusa was referable to the same genus as Nemopsis bachei L. Agassiz, 1849 from New England, but considered it to be specifically distinct. The hydroid he associated with the species bore medusa buds, and medusae were liberated from it. Apparently seen only once by him, it was found during January as a free-floating, solitary polyp having a rudimentary hydrocaulus and two whorls of tentacles. Medusae attributed to N. gibbesii were observed during winter and spring.

Without comment, but surely based on the medusa stage only, L. Agassiz (1862: 345) included Nemopsis gibbesii (spelled therein as N. gibbesi) in the synonymy of N. bachei. Following studies on the medusa stage of N. bachei in New England, A. Agassiz (1865) also included N. gibbesii (again spelled N. gibbesi) as a subjective synonym. The only reservation he expressed in doing so was based on seasonality of populations at the two locations, with those from Charleston occurring in winter and those from Vineyard Sound being common in September. After searching for the hydroid stage without success, the younger Agassiz concluded that McCrady’s planktonic polyp had simply been a detached hydranth of a benthic species.

Brooks (1883) was first to unequivocally establish that the hydroid and medusa stages attributed to Nemopsis gibbesii by McCrady (1859) were different species. He raised medusae identical to the original account of N. gibbesii from a benthic colonial hydroid reportedly resembling species of Eudendrium Ehrenberg, 1834 or Bougainvillia Lesson, 1830, both morphologically quite unlike the polyp described by McCrady. His observations on development of the species supported the hypothesis that medusae of N. gibbesii and N. bachei were conspecific. However, that confirmation did not fully resolve the taxonomic identity of the nominal species N. gibbesii. No type specimen had been designated to objectively define it, whether hydroid or medusa, and the true affinities of McCrady’s remarkable hydroid remained unknown. As with A. Agassiz (1865) earlier, Allman (1872) questioned whether the trophosome was indeed a normally free-floating polyp.

The generic identity of the hydroid McCrady (1859) had assigned to Nemopsis gibbesii was finally resolved by Hartlaub (1899) through life cycle studies of the European medusa Margelopsis haeckelii Hartlaub, 1897. He discovered that its hydroid stage was a solitary, planktonic polyp much like that described by McCrady. Hartlaub thereupon assigned McCrady’s hydroid to Margelopsis Hartlaub, 1897, as M. gibbesii. The medusa stage of the American hydroid remained unknown until Mayer (1910) discovered and described specimens from North Carolina and South Carolina, referring to them as Margelopsis gibbesi in the text (p. 82) and as M. gibbesii in captions accompanying Plate 9. While the link between McCrady’s hydroid and Mayer’s medusa is subjective, not having been based on a complete life cycle study, it seems probable that they are conspecific. For more than a century, the name M. gibbesi has commonly been applied to the two stages (see synonymy list above). Both hydroid and medusa appear endemic to the southeast and mid-Atlantic coasts of the United States. A report of the species from the South Atlantic Ocean between South Georgia and Bouvet Island by Thiel (1938) is believed to have been based instead on M. australis Browne, 1910 (Kramp 1961: 49).

Nomenclature and taxonomy. As apparent from the synonymy list above, the specific name of McCrady’s species has been spelled in the literature as both gibbesii and gibbesi. Following provisions of the International Code of Zoological Nomenclature (ICZN) (International Commission on Zoological Nomenclature 1999), gibbesii is the original and correct spelling (ICZN Art. 32). The name honours noted South Carolina mathematician and naturalist Lewis Reeve Gibbes (1810–1894) (Sanders & Anderson 1999; Stephens 2000).

Although the identities of the hydroid and medusa described by McCrady (1859) have long been known, nomenclatural uncertainty persists over the concept of the species to which they were assigned. The taxonomic identity of Nemopsis gibbesii remains ambiguous because of the absence of a name-bearing type for a binomen that originally included two species. McCrady did not designate a holotype of N. gibbesii, and no syntype specimens are known to exist. His collections, including types, are thought to have been destroyed by fire during the American Civil War (Stephens & Calder 1992: 44). We are also unaware of any previous lectotype or neotype selections.

To stabilize the taxonomic identity of Nemopsis gibbesii, we select as lectotype the hydroid specimen illustrated by McCrady (1859) in Plate 10, Figure 7 following ICZN Arts. 74.7.1, 74.7.2, and 74.7.3. Our designation is not invalidated by “…the fact that the specimen no longer exists or cannot be traced” (ICZN Article 74.4). The lectotype chosen here upholds prevailing usage of the name Margelopsis gibbesii for both hydroid and medusa stages of the species since the turn of the 20th century. Had one of the medusae illustrated in McCrady’s work (Plate 10, Figures 1 –3) been chosen as lectotype, N. gibbesii would become a subjective synonym of N. bachei. A new specific name for the species widely known as M. gibbesii would then be required, an act we consider unnecessary and undesirable.

Margelopsidae, the name of the family to which M. gibbesii belongs, is sometimes credited in error to Uchida (1927). Under the Principle of Coordination in nomenclature (ICZN Art. 36), authorship and date should be credited instead to Mayer (1910). Although Mayer established the name as Margelopsinae, for a subfamily, he is deemed under that article of the code to have simultaneously established coordinate names for all other ranks of the family group, including the name Margelopsidae.

Two other genera, Pelagohydra Dendy, 1902 and Climacocodon Uchida, 1924, are usually included with Margelopsis in the family Margelopsidae (e.g. Uchida 1924; Kramp 1961; Petersen 1990; Bouillon et al. 2006; Schuchert 2006). When Pelagohydra and Margelopsis are included in the same family, a nomenclatural issue arises because the infrequently used family name Pelagohydridae Dendy, 1902 predates the more familiar name Margelopsidae Mayer, 1910. The two had earlier been recognized as distinct subfamilies by Rees (1941), with Margelopsinae (including Margelopsis and Climacocodon) having whorls of tentacles on the hydranth and lacking a float, and Pelagohydrinae (including only Pelagohydra) having scattered tentacles over the entire hydranth and having the proximal portion of the hydranth modified into a float. Unfortunately, the adult medusa stage of Pelagohydra is as yet unknown for comparison with that of Margelopsis. Following Schuchert (2006: 356), prevailing usage of the name Margelopsidae is maintained here pending a taxonomic or nomenclatural resolution of the problem.

The family Margelopsidae has generally been classified within the superfamily Tubularioidea Fleming, 1828 (e.g. Rees 1957; Bouillon 1985; Petersen 1990). The latter group has recently been transferred from the suborder Capitata Kühn, 1913 to Aplanulata Collins, Winkelman, Hadrys & Schierwater, 2005 (Cartwright & Nawrocki 2010; Nawrocki et al. 2013). Molecular evidence supporting inclusion of Margelopsidae in Aplanulata, however, is still lacking (Peter Schuchert, personal communication, 30 June 2015). The six species commonly included in the family (Margelopsis gibbesii, M. haeckelii, M. hartlaubii, M. australis, Pelagohydra mirabilis, and Climacocodon ikarii) all appear to be of infrequent occurrence, and specimens have been difficult to obtain for analysis. As for the name Tubularioidea, authorship of it should be credited to Fleming (1828). While the name “Tubulariae” had been used earlier in both Goldfuss (1818) and Fischer von Waldheim (1823), it was not rendered available in either of those works (ICZN Art. 11.7.1.1). As used in them, “Tubulariae” was merely a descriptive term applied to groups that excluded the genus Tubularia Linnaeus, 1758 (see Calder 2010: 45).

Purported morphological differences between M. gibbesii from the western North Atlantic and M. haeckelii from the eastern North Atlantic need re-examination. In a key to medusa stages of species of Margelopsis, Kramp (1959) highlighted three characters based on then-existing descriptions that seemed to discriminate the two. Margelopsis haeckelii was distinguished by having 3–4 tentacles on each marginal bulb, a wide axial canal above the manubrium in adults, and eggs that developed into actinula-like larval stages on the manubrium. In M. gibbesii, 5–6 tentacles were said to arise from each marginal bulb, no wide axial canal was known to exist above the manubrium in adults, and no actinula-like larval stages were thought to occur on the manubrium. The latter distinction has been shown here to be inaccurate; some female medusae of M. gibbesii studied by us were observed with developing larval polyps just as in M. haeckelii. Meanwhile, the other two characters require further evaluation. Several medusae of M. gibbesii in our material possessed an axial canal, although it was narrow in each case. As for numbers of marginal tentacles, Werner (1955a) reported as many as 8–9 on one or more of the marginal bulbs of M. haeckelii. Up to eight were seen on the bulbs of a large male of M. gibbesii examined during this study (ROMIZ B4077). The hydroid stage of M. gibbesii is still too poorly known to reliably assess how it may differ, if at all, from that of M. haeckelii. Overall, it seems best to maintain the two as distinct species for now, an opinion expressed earlier by Schuchert (2006).

Live medusae of M. gibbesii were examined and described by Mayer (1910). The umbrella was said to be highly contractile. When relaxed, the manubrium reached little more than half-way along the length of the subumbrellar cavity but when contacted, the mouth often extended beyond the velar opening. The endoderm of the marginal bulbs was dull-yellow in colour, while that of the manubrium varied from dull-yellow to dull-green. In M. haeckelii, marginal bulbs are brown while the manubrium is gray with dark-brown pigment granules (Schuchert 2006).

Life cycle. The life cycle of Margelopsis gibbesii has yet to be adequately described. Werner (1954, 1955a, b, 1956) traced the remarkable life cycle of M. haeckelii, and it seems likely that M. gibbesii follows a similar pattern inasmuch as hydroids of both species are predominantly planktonic. Notably too, female medusae of both species extrude subitaneous eggs that develop into nascent polyps before being released into the water. Present also in M. haeckelii are resting eggs that settle on bottom and become encysted as a dormant stage. Existence of such eggs in M. gibbesii seems likely but has yet to be confirmed. Meanwhile, male medusae are said to be extremely rare in M. haeckelii, and development is thought to be largely parthenogenetic (Werner 1954, 1955a, 1956; Schuchert 2006). By contrast, males appear to be at least occasional in populations of M. gibbesii (Mayer 1910; this study).

Hydroids of Margelopsis gibbesii examined here were received for identification in 1989 from the Hampton Roads Sanitation District, Virginia Beach, Virginia (ROMIZ B1027). Discovered in aquaria, they were described as a pest by the collector and donor of the material (Ms. Butterworth, personal communication, May 1989). Remains of brine shrimp, their likely prey in the tanks, were observed together with the hydroid specimens. Hydrozoans are a frequent problem in closed system culture tanks and aquaria. Successful as competitors for food, flourishing under favourable environmental conditions, and lacking predators, populations may rapidly increase in abundance. Some stages may also prey directly on organisms under culture, such as larval decapod crustaceans (Sandifer et al. 1974). Resistant or resting stages, known to occur in many hydrozoans (Calder 1990) including M. haeckelii (Werner 1954, 1955a, b) and the related Climacocodon ikarii Uchida, 1924 (Kubota 1993), make them especially persistent. Meanwhile, M. gibbesii was considered a species of “special concern” by Harrison et al. (1979) in South Carolina because of its restricted distribution, infrequent occurrence, and the seeming vulnerability of the hydroid stage.

Remarkably, this is the first record of the hydroid of M. gibbesii since it was first described and illustrated by McCrady (1859). While small and easily overlooked, it might also be readily dismissed during examination of plankton samples as an insignificant and unidentifiable detached hydranth. The medusa stage is also rarely seen, having been reported only six times earlier (Mayer 1910; Allwein 1967; Sandine & Swiecicki 1975; Hester 1976; Tatham et al. 1977; Johnson & Allen 2012).

Cnidome. Studies were undertaken to document the cnidome of M. gibbesii. With few discharged capsules being found in preserved specimens, and with all observed nematocysts being small, their identification was problematic. Categories observed in medusae of the species included desmonemes (Figs. 2 a, b), haplonemes (Figs. 2 c, d), microbasic heteronemes (Figs. 2 e–g), subspherical heteronemes (Figs. 2 h, i), and stenoteles (Figs. 2 j–l). Of all these types, stenoteles were by far the most abundant. It could not be determined with certainty whether microbasic heteronemes were euryteles or mastigophores. The nematocyst complement of the hydroid stage of M. gibbesii was conspicuously different from that of the medusa, most notably in the absence of stenoteles. Categories present included desmonemes (Fig. 2 m), haplonemes (Figs. 2 n, o), and microbasic heteronemes (Fig. 2 p), with desmonemes and microbasic heteronemes being most abundant. Again, heteronemes may have been either mastigophores or euryteles. By comparison, Schuchert (2006) reported desmonemes, microbasic mastigophores and stenoteles in medusae of Margelopsis hartlaubii Browne, 1903, with stenoteles apparently occurring only around the mouth opening. According to Bouillon (1974: 37), the nematocyst complement in hydroids of M. haeckelii comprises desmonemes, basitrichs, microbasic euryteles, and stenoteles. Nematocysts of the closely related Climacocodon ikarii from Japan were identified as large and small stenoteles and microbasic euryteles in both medusa and hydroid stages (Kubota 1976, 1979). Curiously, no desmonemes were found. Finally, the cnidome of Pelagohydra mirabilis Dendy, 1902, a margelopsid from New Zealand, comprised stenoteles and desmonemes in the hydroid, while that of the young medusa comprised stenoteles, desmonemes, heteronemes, and haplonemes (Schuchert, 1996). The apparent absence of stenoteles in our hydroids of M. gibbesii was unexpected inasmuch as these nematocysts are almost universally present in capitate and aplanulate hydrozoans.

Seasonality. Margelopsis gibbesii has been reported during colder months of the year along its known range (January, Charleston, South Carolina, hydroid, McCrady 1859; November, Oregon Inlet and Southport, North Carolina, medusa, Mayer 1910; December, Beaufort, South Carolina, medusa, Mayer 1910; March and April, Beaufort, North Carolina, medusa, Allwein 1967; winter-spring, off Little Egg Inlet, New Jersey, Sandine & Swiecicki 1975; December, 17.2° C, 32.19‰, Deveaux Bank, South Carolina, medusa, Hester 1976; February and March, Barnegat Bay, New Jersey, medusa, Tatham et al. 1977; April and June, eastern shore of Virginia, medusa, this study). The species tolerates brackish conditions (Mayer 1910), and polyps examined here had been thriving in aquaria at 22‰.

Reported distribution. Charleston Harbor, South Carolina: planktonic hydroid (McCrady 1859: 163, as Nemopsis gibbesii).–Oregon Inlet, North Carolina: medusa; Southport, North Carolina: medusa; Beaufort, South Carolina: medusa (Mayer 1910: 82, as Margelopsis gibbesi in text; as M. gibbesii in figure captions).–Beaufort, North Carolina: medusa (Allwein 1967: 122, as M. gibbesi).–Off Little Egg Inlet, New Jersey: medusa (Sandine & Swiecicki 1975: 129, 146).–Deveaux Bank, South Carolina: medusa (Hester 1976: 60, as M. gibbesi).–Barnegat Bay, New Jersey: medusa (Tatham et al. 1977: 141, 143, 331, as M. gibbesi).–Maryland to South Carolina: medusa (Johnson & Allen 2012: 94).–Virginia Beach, Virginia: hydroid in aquaria; off Chincoteague, Virginia, and Assateague Island, Virginia: medusa (reported herein).