Antennella cf. secundaria ( Gmelin, 1791 )

Calder, Dale R. & Faucci, Anuschka, 2021, Shallow water hydroids (Cnidaria, Hydrozoa) from the 2002 NOWRAMP cruise to the Northwestern Hawaiian Islands, Zootaxa 5085 (1), pp. 1-73 : 42-43

Antennella cf. secundaria ( Gmelin, 1791)

Fig. 13b–d

Sertularia secundaria Gmelin, 1791: 3854 . Antennella secundaria .— Coles et al., 2002a: 318; 2002b: 177; 2004: 73.— Carlton & Eldredge, 2009: 36.

Monostaechas quadridens .— Calder, 2020: 230, fig. 8h, i [not Monostaechas quadridens ( McCrady, 1859) ].

Type locality. “Habitat in mari mediterraneo” ( Gmelin 1791, as Sertularia secundaria ).

Voucher material. Midway Atoll, on algae, 23.ix.2002, one unbranched colony, 3 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5454 .— Midway Atoll, 23.ix.2002, one branched colony, 6.5 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5455 .— Kure Atoll , on algae, 25.ix.2002, one unbranched colony, 4 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5456 .— Nihoa , 06.x.2002, two branched colonies or colony fragments, to 6 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5457 .

Remarks. Originally described from the Mediterranean Sea, Antennella secundaria ( Gmelin, 1791) has now been reported numerous times from tropical to temperate waters worldwide ( Cornelius 1995b). This is reflected in the extensive distribution records and lengthy synonymy list of the species provided by Ansín Agís et al. (2001). It nevertheless appears certain now that cryptic species exist in hydroids assigned to A. secundaria , with multiple lineages having been distinguished through barcoding sequences ( Moura et al. 2018, 2019). Indeed, Cornelius (1995b) had suspected, based on observed morphological differences, that more than one species was represented under the name. Given their wide separation geographically, hydroids from the Hawaiian Islands may well be distinct from those of the Mediterranean. However, with the current taxonomy of A. secundaria being unsettled, traditional nomenclature has been maintained here.

Of multiple cryptic species identified by Moura et al. (2018) within the A. secundaria complex, one lineage, conjointly with A. confusa Ansín Agís, Ramil & Vervoort, 2001 , clustered with the genus Monostaechas Allman, 1877 . Differences between Antennella Allman, 1877 and Monostaechas therefore need to be explored, and if valid the two genera may need to be redefined. If congeneric, precedence would have to be established according to the First Reviser Principle in nomenclature (ICZN 1999, Art. 24.2) inasmuch as they were described in the same work by Allman (1877). As for the species A. secundaria and Monostaechas quadridens , little doubt exists that they are distinct.

While hydroids of A. secundaria are usually unbranched and arise directly from the hydrorhiza, occasional colonies of the species are branched in the form of a helicoid sympodium. Accounts of the branched form have been given in various works including those of Billard (1913), Van Gemerden-Hoogeveen (1965), Millard & Bouillon (1973), Millard (1975), Vervoort & Vasseur (1977), Ryland & Gibbons (1991), Calder (1997), and Schuchert (2003). Both unbranched (ROMIZ B5454; ROMIZ B5456) and branched (ROMIZ B5455; ROMIZ B5457) colonies were represented in the present collection.

In a previous paper on hydroids from the main islands of Hawaii ( Calder 2020), branched colonies of A. secundaria were misidentified as M. quadridens . Indeed, several records of M. quadridens in the Pacific and Indian oceans may likewise have been based on such misidentifications.Accounts of M. quadridens from the vicinity of its type locality near Charleston, South Carolina, USA, indicate that branching in the species is extensive and differs in typically being truly dichotomous. In it, the basal hydrocaulus usually gives rise to a pair of opposite and upwarddirected branches, with the continuation of the main stem often similarly re-branched ( Nutting 1900; Calder 1983). Branches are then re-branched in the form of a helicoid sympodium. Colonies of the species attain a height of as much as 15 cm ( Nutting 1900; Fraser 1944) and are light mahogany in colour. By contrast, the number of branches in colonies of A. secundaria , if there are any at all, are almost always limited to four or fewer ( Ryland & Gibbons 1991), colonies seldom reach 5 cm high, and the colour is greenish-yellow to red ( Cornelius 1995b; Gravili et al. 2015).

Nomenclaturally, the type species of Antennella Allman, 1877 , by monotypy, is A. gracilis Allman, 1877 . It, in turn, has been considered a junior subjective synonym of A. secundaria . That putative synonymy may need reassessment in light of the genetic diversity discovered in this morphospecies.

Reported Distribution. Hawaiian archipelago. Oahu: Kaneohe Bay, North Channel ( Coles et al. 2002a); Pristine Reef ( Coles et al. 2002a; Calder 2020, as Monostaechas quadridens ); Waikiki, Outside Pop’s, scoured reef, 6 m ( Coles et al. 2002b); Waikiki, Atlantis submarine artificial reef, 20 m ( Coles et al. 2002b); Waikiki, Ala Wai buoy ( Coles et al. 2002b); Waikiki, Kapua Channel, 5 m ( Coles et al. 2002b).—Molokai: Kaunakakai Harbor main dock ( Coles et al. 2004).

Elsewhere. Considered cosmopolitan in temperate to tropical waters ( Millard 1975; Ansín Agís et al. 2001), but this is likely subject to revision.

Figures

FIGURE 13. Kirchenpaueriidae and Halopterididae. a, Pycnotheca producta (Bale, 1881), part of a hydrocladium with two hydrothecae, Laysan Island, ROMIZ B5481. Scale equals 0.1 mm. b, Antennella secundaria (Gmelin, 1791), part of hydrocaulus with two hydrothecae, Midway Atoll, ROMIZ B5454. Scale equals 0.1 mm. c, Antennella secundaria (Gmelin, 1791), part of a colony to show branching, Midway Atoll, ROMIZ B5455. Scale equals 0.5 mm. d, Antennella secundaria (Gmelin, 1791), part of hydrocaulus, and base of a hydrocladium with a hydrotheca, Midway Atoll, ROMIZ B5455. Scale equals 0.1 mm. e, Halopteris alternata (Nutting, 1900), part of a hydrocladium with two hydrothecae, Laysan Island, ROMIZ B5458. Scale equals 0.1 mm. f, Halopteris alternata (Nutting, 1900), part of hydrocaulus, with a cauline hydrotheca and proximal end of a hydrocladium, Laysan Island, ROMIZ B5458. Scale equals 0.1 mm g, Halopteris longibrachia, nom. nov., part of a hydrocladium with two hydrothecae, Gardner Pinnacles, ROMIZ B5459. Scale equals 0.1 mm. h, Halopteris longibrachia, nom. nov., female gonotheca, Pearl & Hermes Atoll, ROMIZ B5461. Scale equals 0.1 mm. i, Halopteris longibrachia, nom. nov., part of hydrocaulus with a male gonotheca, Gardner Pinnacles, ROMIZ B5459. Scale equals 0.1 mm.