Aiptasia couchii ( Cocks, 1851 )

Grajales, Alejandro & Rodríguez, Estefanía, 2014, Morphological revision of the genus Aiptasia and the family Aiptasiidae (Cnidaria, Actiniaria, Metridioidea), Zootaxa 3826 (1), pp. 55-100 : 59-64

Aiptasia couchii ( Cocks, 1851)

( Figs. 2 –4, Table 1)

Anthea Couchii [sic] Cocks, 1850 (nomen nudum) Anthea Couchii [sic] Cocks, 1851

? Actinea biserialis Forbes, 1840

? Entacmaea biserialis Milne Edwards & Haime, 1851 ? Dysactis biseralis Milne Edwards, 1857 Aiptasia amacha Gosse, 1858

Aiptasia Couchii [sic]: Gosse 1860

Aiptasia Couchii [sic]: Johnson 1861

Aiptasia Couchi [sic] Gosse: Andres 1883 ( 1884) Non Aiptasia couchii Cocks : Pax 1909

Aiptasia couchi [sic]: Stephenson 1920

Aiptasia mutabilis Form I : Schmidt 1972 Aiptasia mutabilis Form II: den Hartog & Ates 2011

Material examined. (See Appendix 1).

Description. External anatomy ( Fig. 2 ): Pedal disc to 15 mm diameter, wider than column in living specimens. Column smooth, to 15 mm height and 5 mm diameter in preserved specimens. Cinclides in mid-column, relatively conspicuous, in FIGURE 4. Cnidae of Aiptasia couchii . A, C, F, I, K, L, N, O) Microbasic p -amastigophores. B, D, G, J, M, P) Basitrichs. E) Microbasic b -mastigophore. H) Spirocyst.

three rows, corresponding with endocoels of first two cycles of mesenteries ( Fig. 2 C); sometimes scattered cinclides in proximal column ( Figs. 2 D, E). Mesenterial insertions visible. Oral disc to 15 mm diameter. Tentacles to 96, smooth, tapering towards tips; inner tentacles longer than outer ones, to 15 mm and 5 mm length, respectively.

Internal anatomy and microanatomy ( Fig. 3 ): Mesogleal marginal sphincter muscle diffuse, relatively strong to the size of animal, short, restricted to column margin, slightly alveolar ( Fig. 3 F). Same number of mesenteries proximally and distally. Mesenteries hexamerously arranged in four cycles ( Figs. 3 A, B). Only first cycle perfect; first three cycles fertile, including directives. Two pairs of directives each associated with a well-developed siphonoglyph. Gonochoric. Retractor muscles slightly restricted. Parietobasilar muscles differentiated, weak. Longitudinal muscles of tentacles ectodermal ( Fig. 3 C). Strong longitudinal ectodermal muscles in distal end of column ( Fig. 3 D). Basilar muscles well differentiated, relatively weak, with fibers on short mesogleal pennon ( Fig. 3 E). Acontia numerous, well developed.

Color ( Fig. 2 ): In living specimens, column translucent proximally and greyish-brownish distally; tentacles and oral disc translucent greyish, the latter often with radial whitish stripes corresponding with mesenterial insertions. Mouth and actinopharynx whitish. Preserved specimens uniform tan to yellowish in color.

Cnidom: Spirocysts, basitrichs, microbasic b -mastigophores and p- amastigophores (Fig. 4). See Table 1 for size and distribution.

TABLE 1. Size ranges of the cnidae of Aiptasia couchii . x, mean; SD, standard deviation; S, ratio of number of specimens in which each cnida was found to number of specimens examined; N, Total number of capsules measured; F, frequency; +++, very common; ++, common; +, rather common; Abbreviations: M, Microbasic.

Geographic and bathymetric distribution. Aiptasia couchii is known from the eastern Atlantic coast of Europe, the Canary Islands and Madeira, and in the Mediterranean Sea ( Schmidt 1972; Manuel 1981; Ocaña et al. 1994; den Hartog & Ates 2011). Aiptasia couchii is a shallow water species present in subtidal waters and usually found in crevices, between 1– 5 m.

Taxonomic remarks. Schmidt (1972) revised the European species of Aiptasiidae , synonymizing A. couchii with A. mutabilis and stating that they were different ecotypes of the same species ( A. mutabilis Form I = A. couchii and A. mutabilis Form II = A. mutabilis , see Schmidt 1972). However, A. couchii and A. mutabilis can be differentiated by size (to 15 mm vs. to 30 mm length), color pattern (particularly that of the tentacles and oral disc, see Figs. 2 , 5 ), number of mesenteries distally and proximally (equal numbers vs. more cycles distally) and tentacles (48–96 vs. 96–192), cnidae, in specific, the size of the microbasic p -amastigophores 2. In the acontia, although sizes ranges overlap, these cnidae are generally shorter in A. couchii than in A. mutabilis , which has a more restricted size range; a similar situation in found in the column and the tentacles; see Tables 1, 2). Additionally, the two differ in ecology (clonal vs. solitary), geographical distribution ( A. mutabilis is restricted to the Mediterranean Sea), depth (intertidal vs. up to 30 m), type of symbionts ( Symbiodinium vs. Amphidinium : see Grajales 2014), and mode of asexual reproduction (transversal vs. longitudinal fission). Schmidt (1972) based the synonymy on his observation that one specimen of A. couchii kept in an aquarium could grow more than 96 tentacles, and he interpreted this to represent an intermediate form between both species. In our opinion, morphological and ecological evidence clearly distinguishes Schmidt's (1972) ecotypes as different species. Furthermore, molecular evidence supports this differentiation (Grajales 2014).

Our cnidae size range data differ from those of Schmidt (1972), particularly in the presence of microbasic b - mastigophores of the column and the shorter microbasic p -amastigophores (2) of the acontia (Tables 1, 2). Schmidt (1972) did not report these two categories in his specimens. Both categories are easily overlooked or confused with other types present in the tissue because of similarities in length and width of the capsules. The category Schmidt (1972) named as "anisorhize haplonemes" in the column probably corresponds partly with what we call basitrichs. Den Hartog and Ates (2011) identified one small specimen from the northern Atlantic coast of Spain as Aiptasia mutabilis type II sensu Schmidt (1972), stating that this corresponded with the previously known as A. couchii . However, A. couchii corresponds with A. mutabilis Form I sensu Schmidt (Schmidt 1972 ; Manuel 1981). Den Hartog and Ates (2011) provided only cnida data from the sole examined specimen from Spain. Similarly to those of Schmidt (1972), their cnida data differ from ours in the absence of the microbasic b -mastigophores of the column and the shorter microbasic p -amastigophores (1) of the acontia. The sizes ranges provided for cnida categories in all examined tissues overlap for their specimen and those of A. couchii in this study but are narrower than those in this study (Table 1, den Hartog & Ates 2011).

Figures

FIGURE 2. External anatomy of Aiptasia couchii. A, B) Oral view of living specimens. C) Lateral view of living specimen; notice rows of cinclides (arrows). D) Detail of proximal column of preserved specimen; notice scattered cinclides (arrows). E) Lateral view of preserved specimen; notice cinclides and acontia (arrows). Scale bars: A – E, 10 mm.

FIGURE 3. Internal anatomy of Aiptasia couchii. A) Cross section of the column at the level of the actinopharynx showing the cycles of mesenteries (indicated by roman numberals between pairs). B) Detail of fourth cycle of mesenteries. C) Cross section through a tentacle showing the ectodermal longitudinal musculature. D) Cross section through distal column showing the strong ectodermal longitudinal musculature. E) Detail of a longitudinal section of the proximal end showing the basilar muscles (arrows). F) Longitudinal section of distal column showing mesogleal marginal sphincter muscle. Abbreviations: ep, epidermis: ga, gastrodermis; me, mesoglea. Scale bars: A – F, 0.1 mm.

FIGURE 5. External anatomy of Aiptasia mutabilis. A) Oral view of living specimen. B) Detail of the color pattern of the tentacle. C) Lateral view of preserved specimen. D) Detail of pedal disc of preserved specimen. Scale bars: A – D, 10 mm.