Subdivision of the Genus Sergestes

The subdivision of this rather unwieldy genus has been a constant point of difficulty since Hansen's review in 1896. He divided the species into two groups (I and II) on the relative length and strength of the 3rd maxilliped as compared with the 3rd pereiopod, and further divided each group into several subgroups on other morphological characters. This division he maintained with minor alterations in his later papers (1903, 1919, 1922, etc.).

In 1924, Gurney stated (p. 88) that, because of the several strikingly different types of early larvae (representing "groups of species"), "it is permissible to suggest that the genus Sergestes may be a composite of two or more really distinct genera which have reached a great similarity of form by convergence".

Burkenroad (1937a) drew attention to the great resemblance in critical features between the "S. corniculum" group of species in Hansen's Group I and the "S. sargassi" group of species in Group II. He finds that they can be distinguished from one another only by "the form of the third maxillipeds, among characters of more than specific significance". He then called attention to the fact that Pesta (1918, quoted in Hansen, 1922: 21) had described internal pigmented organs within the cephalothorax of S. corniculum Kröyer, S. arcticus Kröyer and S. vigilax Stimpson, and had suggested for them a luminous function. These three species are illustrated in colour by Hansen (1922, Pl. I). This plate shows that they are transparent species with scattered red chromatophores, which tend to be concentrated on the anterior part of the body. The two figures of S. arcticus also show clearly the internal organs described by Pesta as a red mass within the transparent cephalothorax. A mature S. robustus Smith, figured on the same plate, is an all-red species, while immature specimens of S. robustus and S. tenuiremis (non Kröyer, now regarded as S. kröyeri Bate) appear to have orange or pink cuticular pigment rather than chromatophores. Burkenroad demonstrated that the internal pigmented organs were modified areas of the gastrohepatic gland and that in life a bright blue pigment was associated with them, though this was obscured by a dense layer of carmine chromatophores on the surface of the gland itself. However, manipulation of living page 5specimens of several species with these organs was not effective in producing light. These "organs of Pesta", as they were termed by Burkenroad, varied in number and position in various species, up to 10 being present in S. corniculum, and were present in all members of the genus other than Hansen's groups "S. challengeri", "S. robustus", "S. mollis" and "S. tenuiremus". He noted that these groups, containing then about 16 species in all, included all those species with dermal lens-bearing photophores (the "S. challengeri" group) and all those with dermal, lens-less organs of possible luminous function (the "S. robustus" group). He suggested that the presence or absence of the organs of Pesta might mark "the genus off into natural groups", the enlargement of the third maxillipeds having possibly occurred "independently in two different stocks".

In 1940 Burkenroad described 18 new species of Sergestes, distributing them between the subgenus Sergestes s.s. which he characterises in 1945 as including all those species with organs of Pesta, and the subgenus Sergia Stimpson, in which he includes those without these organs.

It has long been realised that two different types of pigmentation were present in the genus. These have been termed by Kemp in unpublished observations (Dennell, 1940: 315; 1955: 399) the "all-red" group and the "half-red" group, the latter having the posterior abdominal segments almost colourless. He found the pigmentation of these two groups to be basically different. In the all-red group, the colour is due to the distribution over the entire body of a red pigment carried in the cuticle, while in the half-red group the colour of the anterior part of the body is not due to cuticular pigment but to red subcuticular chromatophores (though faint cuticular pigments may also be present). All members of the half-red group have organs of Pesta and none have dermal photophores, while no members of the all-red group have organs of Pesta though dermal photophores, either with or without lenses, are present on most. Thus it can be seen that Kemp's half-red species belong to Burkenroad's subgenus Sergestes s.s. and the all-red to the subgenus Sergia, the pigmentation differences therefore being of subgeneric importance.

Zahl (1953) and Hardy (1956) have published excellent coloured illustrations of examples of these two subgenera. Zahl's colour photographs of specimens from the Straits of Messina show a species of Sergestes s.s. (p. 592) with extremely large stellate chromatophores on the carapace, and a species of Sergia of the "S. robustus" group (pp. 616–617) with the characteristic carmine photophores clearly visible on the scaphocerite and some of the thoracic appendages. Hardy's sketches, made during the Discovery Expeditions, illustrate S. arcticus and S. vigilax (Pl. 17) of the half-red subgenus Sergestes s.s. and the all-red S. (Sergia) robustus (Pl. 18) as well as other bathypelagic crustaceans of the same two colour types.

Although Hansen attempted to summarise all the data on this genus in his detailed review (1896) he did not cover the question of colour in any way. Indeed, during the preceding 60 odd years, since the genus was described by Milne-Edwards, only a few brief references to the colour of various species had been made (e.g., Wood-Mason and Alcock, 1891a: 190; 1891b: 353). However, meagre though these references were, the two widely different colorations had been seen: all-red Sergia species ("blood-red", "lurid-red", "deep crimson", etc.), and transparent Sergestes s.s. with red chromatophores (described as "white" with " red spots ", " pink blotches ", etc.).

The first species recognised as bearing dermal photophores, S. challengeri, was described by Hansen in 1903 from the Challenger Expedition material examined by Bate (1888). These photophores, over 117 were seen in the damaged, unique specimen, all possessed a prominent, circular, biconvex, chitinous lens and an internal reflecting layer, which in Hansen's preserved material was unpigmented. This discovery was the first definite record of luminous organs, as distinct from luminous secretions, in the Decapoda, though photophores of a highly developed type were well known in the closely allied Euphausiacea.

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Apparently the first record of the production of light by decapod photophores was that of Stanley Gardiner (Kemp, 1910b) who described specimens of S. challengeri taken in the Indian Ocean as "brilliantly phosphorescent on the occasion of their capture". Kemp, while demonstrating the detailed structure and histology of these photophores, was able to record that the cellular layer immediately beneath the lens contained a deep blue pigment in life (still present in specimens preserved in weak formalin) and also that the number of photophores present increases with size. In 1913 he gave further details of the distribution of photophores in S. challengeri, especially those on the ventral surface of the body.

Terao (1917) described in detail the distribution and histology of the lens-bearing photophores in the Japanese S. lucens Hansen (recorded as S. prehensilis Bate), closely related to S. challengeri. These photophores were red in life, numerous, 157 being found on one specimen, and were observed to emit a dim, intermittent, greenish-yellow light. Within the next three years, two more species of the "S. challengeri" group. S. fulgens Hansen, 1919, and S. splendens Hansen, 1920 (nom. preocc. now known as S. talismani Barnard) were described, each possessing large numbers of lens-bearing photophores on the body and appendages. The distribution of these organs in the latter species is given in detail by Hansen (1922).

In 1935 Gordon re-examined a number of the "S. challengeri" group of species and gave comparative illustrations of the photophore patterns on the ventral surface of the body in S. prehensilis, S. lucens, S. talismani (as S. splendens Hansen) and S. challengeri. Thus five species were then recognised in this group. She also recorded that Kemp, who had studied fresh material of S. prehensilis during the Discovery Expedition, found that the total number of photophores in this species "varied enormously according to the age and size of the specimens" (from 183 to 358). She was thus able to show that it was photophore pattern rather than number which would be of systematic significance. S. prehensilis (as S. gloriosus) had already been described as possessing lens-bearing photophores by Stebbing (1905), who also recorded this species as "red" in colour. Two further species of this group of the subgenus Sergia, S. scintillans and S. stellatus, were described by Burkenroad in 1940. Structures inside the coxae of the 5th pereiopods in S. tenuiremis, believed to be large lens-bearing photophores by Welsh and Chace (1938) have since been recognised as spermatophores by Burkenroad (in Dennell, 1940: 314).

In contrast to this type of photophore, pigmented "patches" (opaque white in preserved material) seen in some species of Sergia, being similarly arranged to the lens-bearing photophores of S. challengeri, led Sund (1920) to suggest that they were also luminous organs. Dennell (1940) examined and described histologically these lens-less dermal organs in S. regalis Gordon. He found these to be photophores of an entirely new type, consisting of horizontal sheets of interwoven fibres and scattered perpendicular "photogenic bodies". He was later able to observe (1955) luminescence from this type of organ in S. robustus and S. splendens Sund (recorded as S. richardi) and thus demonstrate that they were indeed photophores, though they "have no evolutionary relationship with the lensed organs occurring in S. challengeri and its allies". These lens-less photophores had been first recognised by Kemp (1910a) when he described S. robustus as an all-red species with "crimson spots" on various parts of the body and appendages. Illig (1914), Hansen (1919: 10; 1922: 21) and Sund (1920: 11, 15, 18) gave further details of these lens-less organs in S. robustus and the closely related species, S. gardineri Kemp, S. splendens Sund and S. grandis Sund. Burkenroad (1937a) described from life the lens-less photophores of S. splendens (recorded as S. crassus) as "transparent structures invested on their inner sides by a layer of vermilion pigment". Dermal organs of the same type were found in preserved specimens of S. phorcus Faxon, which also belongs to the "S. robustus" group. Within the next two years, Welsh and Chace (1938) figured the distribution of lens-less organs on the body and appendages of S. grandis page 7Sund (167 in a large specimen), and Gordon (1939) had described another species in this group, S. regalis, giving Kemp's notes on the distribution of the lens-less deep purple spots" on this otherwise bright scarlet shrimp.

In 1940 Burkenroad described seven new species of the subgenus Sergia as having lens-less dermal photophores and thus belonging to the "S. robustus" group. Lens-less photophores were also reported in S. bisulcatus Wood-Mason. In addition to observing the luminescence of these photophores in S. robustus and S. splendens, Dennell (1955) illustrated their pattern in these two species and in S. grandis. In all three species these photophores are described as "carmine" in life. Specimens of S. japonicus Bate (with which S. mollis Smith is synonymous, sec Burkenroad, 1940: 53) and S. kröyeri (with which S. tropicus Sund is synonymous, see Burkenroad. 1940: 50), though uniformly pigmented red in life, were found to lack photophores of any type. S. laminatus Burkenroad, 1940, is another species of Sergia belonging to the same photophore-less "S. japonicus" group, also characterised by having a relatively membraneous integument.

In 1940, Dennell made a thorough histological examination of the organs of Pesta in S. seminudus Hansen (recorded as S. corniculum), S. sargassi Ortmann, S. diapontius Bate and S. edwardsi Kröyer. He described possible lens and reflector cells and concluded that these organs were, as had been suggested before, almost definitely luminous organs. This opinion he was later able to confirm from his own observations (1955) on living S. vigilax and S. atlanticus M.-Edw. In addition to observing the organs of Pesta luminescent in the two species mentioned above, he recorded that in these two as well as in S. cornutus Kröyer, S. armatus Kröyer, S. "hanseni" (presumably S. henseni Ortmann) and "S. sundi" (unknown to me), the organs in life were dark blue with a carmine pigment cap.

Thus it is clear that the genus Sergestes may be divided into two subgenera on both morphological and physiological grounds. Those species possessing organs of Pesta and lacking cuticular pigmentation are referred to the subgenus Sergestes s.s. and those without organs of Pesta, with cuticular pigmentation and often possessing dermal photophores, to the subgenus Sergia. This systematic division may not reflect the differences between the two groups adequately and in the future, when we have a more intimate knowledge of the sergestids, it may be necessary for this primary separation to be at the generic level.