Plexaurella dichotoma (Esper, 1791)

Castro, C. B., Medeiros, M. S. & Loiola, L. L., 2010, Octocorallia (Cnidaria: Anthozoa) from Brazilian reefs, Journal of Natural History 44 (13 - 14), pp. 763-827 : 791-793

Plexaurella dichotoma (Esper, 1791)

( Figures 1 , 12A,A′ , 13)

For the synonymy previous to 1961, see Bayer (1961: 170–172, text-fig. 50, pl. VI, figs. 6,7, pls. XXIII–XXV) and Grasshoff 1991: 334.

Plexaurella dichotoma: Tixier-Durivault 1970: 155–156 ; Medeiros and Castro 1999: 11.

Diagnosis

Colonies dichotomously branching ( Figure 12A ). Branches long or short, tips slightly clavate or not ( Figure 12A′ ). Coenenchymal mounds absent or coenenchyme elevated around polyps, sometimes on opposite sides of aperture, forming two “lips”. Outer layer of coenenchyme with six radiates, slender butterflies or “antler-shaped bodies” (up to about 0.1 mm long), with rays weakly ornamented. Middle layer of coenenchyme mainly with spindles, “triradiates” and “quadriradiates” (butterfly-form) (all up to about 0.35 mm long) ( Figure 13 ); all middle-layer sclerites strong, compact, and with tubercles well developed and densely distributed ( Bayer 1961: 170).

Description

For a complete description see Bayer (1961: 170–172, text-fig. 50, pl. VI, figs. 6,7, pls. XXIII–XXV).

Material

USA: Florida ( MNRJ 1269 [ex-USNM 50319]). Brazil: Maranhão ( MNRJ 01534, 01541); Rio Grande do Norte ( MNRJ 00443); Pernambuco ( YPM 4509, USNM 5278, MNRJ 00980).

Type depository

Senckenberg Museum, Frankfurt ( SMF 5808 ) ( Grasshoff 1991: 334) .

Type locality

“Von den südlichen americanischen Inseln” (Esper 1791: 59).

Geographic distribution

Western Atlantic: Bermudas ( Verrill 1906 –1907), Florida ( Bayer 1961), Bahamas ( Bayer 1961), Antilles ( Bayer 1961), off northern Brazil (Parcel do Manuel Luiz (MA), Atol das Rocas, Fernando de Noronha) ( Figure 1 ).

Remarks

Plexaurella dichotoma shows conspicuous morphological variation and this has contributed to its many synonyms in the literature. Among the species of Plexaurella considered valid by Bayer (1961), it is worth noting two later contributions. Alcolado (1985) included P. fusifera Kunze, 1916 , in the synonymy of P. dichotoma , based on the variability of diagnostic characters. Gerhart (1983) referred to the study by Tursch et al. (1978), which showed that P. dichotoma , P. fusifera and P. grisea Kunze, 1916 , did not differ chemically (in terpenoid compounds), although they were all different from P. nutans ( Duchassaing and Michelotti, 1860) .

Currently, the following species are considered valid: Plexaurella dichotoma , P. grisea , P. nutans , P. grandiflora Verrill, 1912 (see below) and P. regia Castro, 1989 (see below).

However, P. grisea was originally described as a variety of P. dichotoma , which suggests it is a closely related form, and that these species are chemically similar ( Gerhart 1983). Plexaurella grandiflora is also close to P. dichotoma , differing in a few aspects (see Remarks under the heading P. grandiflora ). It is possible that detailed studies will establish that these three species are synonymous.

Verrill (1906 –1907) described some dry specimens of P. dichotoma as whitish colonies, with the surface finely granulated. This “condition” is seen in all specimens from Brazil (Maranhão State, Rocas Atoll, and Fernando de Noronha Archipelago) and imparts a finely porous appearance to the colony surface.

Specimens from Fernando de Noronha had middle layer sclerites a little more slender than those from Rocas Atoll and Manuel Luiz Reefs, Maranhão State. They also differed in colony growth form: short colonies with short branches at Fernando de Noronha and tall with long branches at the other localities. Specimens from Fernando de Noronha were collected in shallow water (c. 2 m) and high hydrodynamism (off Leão Beach), while those from Rocas Atoll were found loose from the substratum and, therefore, it is not possible to establish the depth at which they lived. However, no attached colonies were observed in 20 days of intensive dive collecting in the Rocas Atoll, which suggests that these colonies came from much deeper areas than those from Fernando de Noronha. It is possible that the variation seen on the sclerites is related to the variation of colony growth form and to depth/hydrodynamism/ light characteristics at the areas where they grow. Such correlation has been demonstrated for other plexaurids ( Muricea Lamouroux, 1821 ) by Grigg (1970).

Figures

Figure 1. Map of distribution of reef octocorals in different regions off the Brazilian coastline. The Espírito Santo State area (*) includes the Vitória- Trindade seamount chain. “BA Sector” refer to a division of reef areas in Bahia State by Leão (1996).

Figure 12. Plexaurella dichotoma (Esper, 1791) (MNRJ 00980): (A) colony, (A′) branch. Plexaurella grandiflora Verrill, 1912 (MNRJ 04342 – long branches): (B) colony, (B′) branch. Plexaurella grandiflora Verrill, 1912 (MNRJ 02752 – short branches): (C) colony, (C′) branch. Plexaurella regia Castro, 1989 (MNRJ 02748): (D) colony, (D′) branch. Scale bars: A–D, 2 cm; A′–D′, 5 mm.

Figure 13. Sclerites of Plexaurella dichotoma (Esper, 1791) (MNRJ 00980): (A) anthocodia; (B) outer layer of coenenchyme; (C) middle layer of coenenchyme; (D) axial sheath.