Orchistoma pileus ( Lesson, 1843 )

Schuchert, Peter & Collins, Richard, 2021, Hydromedusae observed during night dives in the Gulf Stream, Revue suisse de Zoologie 128 (2), pp. 237-356 : 291-294

Orchistoma pileus ( Lesson, 1843)

Figs 34 A-F & 35A-G

Mesonema pileus Lesson, 1843: 317 , pl. 6 fig. 1.

Orchistoma pileus . – Haeckel, 1879: 139. – Kramp, 1959a: 139, fig. 163. – Kramp, 1961: 144. – Segura-Puertas et al., 2009: 376. – Bouillon, 1984b: 90. – Gershwin et al., 2010: table 7.

Orchistoma steenstrupii Haeckel, 1879: 139 , pl. 15 figs 3-5. – Mayer, 1910: 211, pl. 25 figs 1-4. – Kramp, 1955a: 157, re-examination of type specimens, synonym.

Orchistoma agariciforme Keller, 1884: 418 , pl. 21 figs 1-3. n. syn.

Orchistoma agariciforme . – Kramp, 1959a:140. – Kramp, 1961: 144. – Bouillon, 1984b: 88, figs 29-30, redescription. – Bouillon et al., 2004: 170, fig. 91E. – Gershwin et al., 2010: table 7.

Tetracannota collapsum Mayer, 1900: 46 , pls 7-8 figs 14-16. n. syn.

Dipleurosoma collapsum . – Mayer, 1910: 226, pl. 27 figs 1-3 & 7. – Kramp, 1961: 134. – Goy, 1979: 274, fig. 11. – Kramp, 1959a: 132, fig. 147.

Dipleurosoma collapsa . – Vanhöffen, 1913a: 420.

Orchistoma collapsum . – Bouillon, 1984b: 90, genus transfer. – Pagès et al., 2006: 373, fig. 7A-C. – Gershwin et al., 2010: table 7.

Examined material: BFLA3785 ; 1 specimen; 20-SEP- 2018; size 12 mm; preserved in ethanol for DNA extraction; 16S sequence MW528651 . – BFLA3810 ; 1 specimen; 20-OCT-2018; size 18 mm; preserved in ethanol for DNA extraction; 16S sequence MW528652 . – BFLA3813 ; 1 specimen; 20-OCT-2018; size 20 mm; preserved in ethanol for DNA extraction; 16S sequence MW528653 . – BFLA3816 ; 1 specimen; 20-OCT-2018; size 10 mm; preserved in ethanol for DNA extraction; 16S sequence MW528654 . – BFLA4132 ; 1 specimen; 11-JUN-2019; size 34 mm; preserved in ethanol for DNA extraction; 16S sequence MW528680 . – BFLA4183 ; 1 specimen; 12-AUG-2019; size 26 mm; preserved in formalin and deposited as UF-013823 , no alcohol sample. – BFLA4383 ; 1 specimen; 09-MAY- 2020; size 15 mm; part preserved in formalin and deposited as UF-014033 , small part preserved in ethanol for DNA extraction; 16S sequence MW528716 . – BFLA4387 ; 1 specimen; 09-MAY-2020; size 18 mm; part preserved in formalin and deposited as UF-014035 , part preserved in ethanol for DNA extraction; 16S sequence MW528717 . – 9 specimens, photographed but not collected; dates 10-APR-2019, 26-MAY-2020, 05-MAY-2017, 17-OCT-2017, 28-JUN-2018, 26-MAR- 2019, 07-MAY-2019, 20-MAY-2019, 17-MAY-2020; sizes 15-30 mm.

Observations: Medusa bell diameter 10 to 34 mm when mature, almost hemispherical or somewhat shallower ( Fig. 34A, D ), evenly rounded exumbrella, mesoglea thick, jelly at apex about 2/5 of total height; with thick, tapering gastric peduncle that protrudes through velar opening. Velum broad. Stomach complex, base in centre H- or cross-shaped ( Fig. 35A, D ), then subdividing dichotomously or irregularly into elongate diverticula attached to the distal part of the gastric peduncle, walls of diverticula contain gonads and are continued on gastric peduncle as radial canals ( Fig. 35 A-D). Stomach wall follows the branching of the diverticula, mouth opening with complexly folded rim, mouth rim corners at position of diverticula drawn out into lips of variable length, 10 to 16 lips more elongated, sometimes also absent due to apparent damage and subsequent healing ( Fig. 35F ). Usually several radial canal in formation ( Fig. 35E ) and not reaching circular canal. Circular canal broad. Along circular canal on adaxial side numerous dark ocelli, not correlated with tentacle positions. 16 to 29 tentacles, extensible but also able to contract to short length ( Fig. 34F ), tentacles not in phase with the radial canals, with distinct, ovoid tentacular bulbs that taper rapidly into tentacles, bulbs divided bilaterally by a more transparent tissue in median line ( Fig. 34C ). Between each pair of tentacles 4 to 12, usually around 5 to 6, short thin tentacles ( Fig. 34C ), these secondary tentacles arise from circular canal, proximal half adnate to or embedded in umbrella, free part with nematocysts, contractile. Colours: stomach and diverticula with gonads can be brown-yellow in some specimens ( Fig. 35B, E, F ), otherwise whitish or with a yellow hue ( Fig. 35D, G ).

16S Data: The seven haplotypes found showed high divergence rates ( Table 1 ). The sequences form a monophyletic group in the maximum likelihood tree ( Fig. 28 ) but fell into two well separated subclades with relatively high divergence (up to 8.1%, Table 1 ), while the differences within the subclades are low.

Distribution: Western Atlantic from Maine ( Pagès et al., 2006) to Brazil ( Goy, 1979), but mainly Bahamas, Florida and Caribbean ( Haeckel, 1879; Vanhöffen, 1913a; Mayer, 1910; Kramp, 1959a), Gulf of Mexico ( Segura-Puertas et al., 2009), also Mediterranean ( Bouillon, 1984b) and coast of Western Africa. Type locality: West Africa ( Haeckel, 1879).

Remarks: Although the 16S data showed that our material separates in two lineages that might qualify as two distinct species ( Table 1 , Fig. 28 ), we referred all our samples to Orchistoma pileus .

Orchistoma pileus was first described by Lesson (1843), presumably based on a specimen from western Africa (as “mers d’Afrique?”). Lesson’s drawing was rather simple, but the species is recognizable. He did not show the small tentacles and there are about 30 normal tentacles or their bulbs depicted. Haeckel (1879) restricted the type locality to Western Africa and described concomitantly O. steenstrupi based on material from Cuba. The type material of O. steenstrupi was later re-examined by Kramp (1955a) and referred to O. pileus as it matched Mayer’s (1910) description of this species. Mayer (1910) had material from the Bahamas and Tortugas. Kramp gives the tentacle number as 64, which must be meant as a maximal number because the type specimen of Lesson had only 30.

Tetracannota collapsum was described by Mayer (1900) using medusae from the Tortugas and Bahamas. Despite the strong resemblance to Orchistoma , he placed it in a separate genus because it had visibly branched radial canals. His animals were mature but had only 16 tentacles at a size of 7 mm and only 8 manubrial lips. Mayer’s branched radial canals are actually the same structure as here described for the stomach diverticula, only that the radial canals showed a grouping into four sets close to the manubrium, but more distally they are evenly distributed (comp. Fig. 35A ). Later, Vanhöffen (1913a) found Mayer’s species in the West Indies and noted that it is more variable than given in Mayer. Vanhöffen found them to have 16 manubrial lips beginning at a size of 3 mm.

Another nominal species resembling O. pileus was described by Keller (1884) as Orchistoma agariciforme . He had a single mature medusa from the Mediterranean with only 7 manubrial lips, but which was otherwise similar to O. pileus . Bouillon (1984b) described more specimens of O. agariciforme from the Mediterranean and considerably widened its morphological scope, notably he observed that the number of the manubrial lips was usually 16. Bouillon concludes that O. agariciforme is very close to both O. pileus and O. collapsum . He gives as distinguishing traits that in O. collapsum the radial canals remain in four groups near the manubrium even in adults and it has shorter manubrial lips. Orchistoma pileus differs by attaining a larger size of up to 30 to 40 mm, with up to 32 manubrial lips and radial canals as well as up to 64 marginal tentacles.

Gershwin et al. (2010) favoured splitting and retention of all these nominal species, even of O. steenstrupi , but used outdated values for O. agariciforme in their table.

The material we examined in this study was very variable and referable to either O. pileus and O. agariciforme and we think these names should be synonymized, including also O. steenstrupi and O. collapsum .

While we had no specimen that exactly matched O. collapsum as described by Mayer (1900, 1910), BFLA4383 ( Fig. 35E ) comes close to it in terms of tentacle and lip numbers. We nevertheless think that this nominal species was based on growth stages of O. pileus . Gonads mature in O. pileus at a size of less than 12 mm bell size ( Fig. 34A ) and the medusa continues its growth and the multiplication of radial canals and tentacles. Manubrial lips numbers and sizes are variable, usually there are about 16 somewhat longer lips. The lips can be damaged and regenerated and their number is not a reliable diagnostic feature ( Fig. 35F ). In general O. pileus appears as a rather variable species, mostly due to a long growth and different developmental stages.

This synonymy appears to be to some degree in conflict with the DNA data obtained in this study because the 16S sequences separate into two distinct lineages, although they are sister lineages ( Fig. 28 ). We could not find convincing morphological traits that would allow to identify the lineages, except that in one the manubrium and gonads are brownish-yellow while in the other they are whitish (compare Fig. 35E & 35F versus 35G). BFLA4383 ( Fig. 35E ) was also in this “yellow” clade and was mentioned above as being close to the morphotype of O. collapsum . However, the other sample from this “yellow” clade (BFLA4132, Fig. 35F ) is referable to O. pileus .

At this stage we cannot resolve the problem of the two lineages. The difference in colour could also be a random result due to low sample numbers. If it can be confirmed by a more detailed genetic analysis of more samples that indeed two species are involved, the two lineages are likely not separable so that they correspond to the nominal species synonymized here.

The high genetic subdivision is reminiscent to the one observed above for Zancleopsis dichotoma and Laodicea undulata .

Figures

Fig. 34. Orchistoma pileus. (A-B) BFLA3785, bell diameter 12 mm, female. (C) BFLA4183, bell diameter 26 mm, bell margin of a mature animal, note irregular radial canals, likely due to healed damage. (D) BFLA3810, bell diameter 18 mm, male. (E) BFLA3813, bell diameter 20 mm. (F) BFLA4132, note curling of long tentacles.

Fig. 35. Orchistoma pileus. (A-D) Stomach details. (A) Young animal without gonads, O. collapsum stage, oral view, photo 10- APR-2019. (B) BFLA4183, mature female, bell diameter 26 mm, reticulate radial canals are likely due to a healed damage. (C) BFLA3785, bell size 12 mm, subadult female. (D) BFLA3810, bell diameter 18 mm, male, view from aboral side, note branching of radial canals (stomach diverticula), originating from a H-shaped figure, compare to A. (E) BFLA4383, size 15 mm. (F) BFLA4132, size 34 mm, manubrium likely damaged and in process of regeneration. (G) BFLA4387, size 18 mm.

Fig. 28. 16S maximum likelihood phylogenetictree of Laodiceidae,Hebellidae,and Orchistomatidae,obtainedwith PhyML (GTR+G+I model) and based on about 600 bp positions of the mitochondrial 16S gene. Node-support values are bootstrap values of 100 pseudoreplicates (shown only if> 70%).All sequence labels start with the GenBank numbers (except for identical haplotypes) permitting the retrieval of more information. Red labels are new sequences from this study.