Solmissus incisa

Solmissus incisa is a midwater narcomedusa of the family Cuninidae ^[1], a gelatinous cnidarian of the genus Solmissus that is regarded as a predator mainly of other gelatinous zooplankton ^[2]. The name actually covers a complex of several cryptic species, which often overlap geographically and whose main niche difference is depth ^[3]. In the records of the genus Solmissus compiled in that study, the median depth of daytime observations was 685 m ^[3].

The genus Solmissus is defined by undivided, perradial stomach pouches; the tentacles arise opposite the centre of each pouch and equal them in number, and there is no peripheral canal system and no otoporpae ^[2]. The tentacle count of Solmissus incisa is highly variable: among 814 individuals counted it ranged from 12 to 41, with a median of 26 and frequency peaks around 23 and 29 ^[3].

That variation reflects the name covering several species. Fewkes's original description (1886) rested on broken medusae from more than one station, with 24 to 32 tentacles and bells 50 to 100 mm across ^[2]. A specimen collected in Monterey Submarine Canyon had a 50 mm bell, 20 purplish tentacles and rectangular marginal lappets ^[4].

A morphotype observed in Japan, with 22 to 24 tentacles, has a flattened, disc-like exumbrella that is concave at the apex, and the distal quarter to two fifths of the tentacles is whitish, presumably because of a high concentration of nematocysts ^[2].

Similar species

• Solmissus marshalli: It has a thick, rigid bell and its maximum tentacle count is usually given as 16 (although some records report more), against the 22 to 24 or more of Solmissus incisa; the authors note that the taxonomy of the genus is confused and that the two are easily mistaken ^[2].

The species was described from material collected all around 39° N and 79° W, in the Gulf Stream region ^[2]. It is a midwater animal: distribution models for the genus place it mostly in the North Pacific between 400 and 1200 m, with lower probabilities in the Atlantic and Indian oceans and very low ones in the Southern Ocean, although the authors warn that the available data are biased toward the North Pacific ^[3].

In the compiled data for the genus Solmissus there was no appreciable daily vertical migration: the median depth was 685 m by day (n = 1259) and 671 m by night (n = 124) ^[3]. In the models, the presence probability of the morphotype with fewer than 28 tentacles peaks near 400 m and that of the one with 28 or more near 750 m; in both, at very low dissolved oxygen and a salinity of 34 ^[3].

Off California, in Monterey Canyon, one specimen was collected at 230 m, shallower than the 400 m below which the species had been thought to live ^[4]. In the western Pacific, individuals were filmed between 573 and 619 m depth ^[2].

Solmissus is considered to prey mainly on gelatinous zooplankton. In an earlier study of 82 individuals of Solmissus in Monterey Bay, 88.6 % of the prey were gelatinous zooplankton and none was a fish ^[2]. In Japan, however, one individual with 24 tentacles was filmed having ingested a fish at 573 m, another with salps at 614 m and another with a narcomedusa at 619 m, all ingested whole ^[2]. The authors suggest that either this morphotype captures fish, or the fish arrived as prey of another predatory cnidarian ^[2].

The gonads of that morphotype are globular, with a single egg, and lie in 2 to 4 opposed pairs on the margin of each pouch ^[2]. In the northern hemisphere, individuals of the genus with visible oocytes were found all year round, on average 38.4 % per season, and reproducing individuals generally had fewer tentacles (median 26) than non-reproducing ones (28) ^[3].

On the Monterey specimen, about 45 juveniles 0.2 to 1.0 mm across, with 2 to 6 tentacles, moved freely beneath the epidermis of the bell; it is not known whether they were parasites or brooded offspring ^[4]. In situ, the medusa swam with its velum upward and, on stopping, rotated and hung upside down for 1 minute ^[4].

Narcomedusae act as top-down regulators of the midwater food web ^[3]. The genus Solmissus has many records in open databases: 87.4 % of the 2914 OBIS records come from an underwater video database of the US west coast, mostly Monterey Bay ^[3]. A hyperiid amphipod was also seen associated with the bell surface of the Monterey specimen ^[4].

With increasing depth, signs of predation on these medusae rise and the presence of prey in their stomachs falls ^[3]. The authors suggest that, with ocean deoxygenation and global warming, the genus may expand its distribution and reproduce more; they note that this projection rests on data biased toward the North Pacific ^[3].