Tulpa costata Gravier-Bonnet, 1979

Tulpa costata Gravier-Bonnet, 1979 View in CoL

Tulpa costata Gravier-Bonnet, 1979: 31 View in CoL , fig. 6A–F.

Description [elaborated from Gravier-Bonnet’s (1979) description and figures]. Stolonal colony. Stolons giving rise to long pedicels with distal hydrotheca; pedicel length twice or equal to hydrothecal height; pedicel diameter constant, with one to three randomly arranged regeneration nodes. No distinct articulation between pedicel and hydrotheca.

Hydrotheca tubular, with distinct bulbous basal part, diameter strongly increasing at annular thickening, reaching its maximum by the middle, then decreasing to minimum at distal fifth and increasing again at aperture. Distal third of hydrothecal wall sinuous. Rim of hydrothecal aperture with eight to 11 large, rounded cusps, corresponding to external troughs. Hydrothecal perisarc totally or partially finely transversally striated. Hydrothecae without longitudinal ridges. Hydrothecal rim may have numerous renovations.

Gonothecae oval-shaped, with a narrow basal part, and distally projecting into a long neck bearing a circular, distal aperture. Swollen part with eight distinct longitudinal ridges. Gonothecal perisarc totally finely transversally striated.

Cnidome including very large, bean-shaped nematocysts.

Remarks. Gravier-Bonnet (1979) indicated that she was unable to observe longitudinal ridges as those described in T. diverticulata and T. tulipifera . According to her, the presence and prominence of longitudinal ridges depends on the degree of undulation of the hydrothecal wall: when the ridges are acute, the longitudinal lines are present (e.g., in T. tulipifera or T. diverticulata ), but when the ridges are less pronounced, the lines are absent (as in T. costata ).

The most striking feature of T. costata is the relatively large size of its nematocysts (67 × 20 µm), especially in comparison with other known Tulpa species (see Table 2), or even with species from other hydrozoan genera that are typically known to have large nematocysts. For example, in Eudendrium megaloarmatus Peña Cantero, 2023 , a species characterised by having unusually large nematocysts within the genus Eudendrium Ehrenberg, 1834 , they are 41.5–46 × 19–21 µm ( Peña Cantero 2023). This initially led me to believe that the author may have made an error in measurement. However, Gravier-Bonnet (1979) also provided measurements for three other species in her study, and these were within the expected range for these genera. They are Synthecium megathecum Billard, 1925 (33–35 × 10–11 µm), Synthecium sp. (38–44 × 8–9 µm), and Thecocarpus flexuosus ( Lamouroux, 1816) (36–38 × 4 µm). This led me to conclude that her measurements were correct, and that T. costata is indeed characterised by exceptionally large nematocysts.

As mentioned above, I believe that the material assigned to T. diverticulata by Vervoort & Watson (2003) actually belongs to a different species. While it shares with T. costata the relatively small size of the hydrothecae, their shapes are different. In addition, the two species differ in the gonothecae, as those studied by Vervoort & Watson have a smooth perisarc and lack the longitudinal ridges characteristic of T. costata .

Ecology and distribution. Tulpa costata is known only east of Cape Sainte Marie , south of Madagascar (25°09'2"S – 47°22'6"E), where it was collected at a depth of 460–465 m. Gonothecae were observed in March GoogleMaps .