Systellaspis lanceocaudata
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publication ID |
https://doi.org/10.1093/zoolinnean/zly039 |
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persistent identifier |
https://treatment.plazi.org/id/770D87FB-FFBE-8D4E-37BB-FEF7AFE1FECC |
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treatment provided by |
Plazi |
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scientific name |
Systellaspis lanceocaudata |
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SYSTELLASPIS LANCEOCAUDATA View in CoL GROUP
Diagnosis: Carapace without lateral ridge extending from orbit to posterior margin, both dorsal carina on posterior half of midline and strong ventral carina absent.Abdomen with first somite bearing a sharp tooth on anterior margin; fourth and fifth somites each with a single spinule on posterolateral margin. Telson with more than 1–3 pairs of dorsal spines and ten or fewer lateral spinules. Fifth pereopod with ischium unarmed on posterior margin.
Species included: S. lanceocaudata Spence Bate, 1888 , S. guillei Crosnier, 1987 and S. pellucida (Filhol, 1884)
CRYPTIC SPECIATION?
Cryptic speciation was found in this work in distant populations of O. gracilirostris (Atlantic vs. Pacific) and S. debilis (Atlantic vs. Pacific and Indian Oceans, Fig. 6 View Figure 6 ). This is not new for Oplophoroidea as it has been recognized earlier for J. spinicauda ( Taiwan vs. Gulf of Mexico) and M. mollis ( Taiwan vs. Spain) ( Wong et al., 2015). More intriguing are significant differences between populations of S. debilis from the North Atlantic and the Gulf of Mexico found in the present work ( Fig. 6 View Figure 6 ). Similar molecular differences between shrimp populations from the same areas were found in A. pelagica specimens ( Wong et al., 2015). We propose that the North Anticyclonic Gyre, from where the North Atlantic specimens were obtained, provides genetic isolation from the Caribbean waters for some shrimp populations. Evidence for such isolation is seen in other pelagic shrimps, e.g. Phorcosergia wolffi and Challengerosergia hansjacobi , which largely occur in Caribbean waters only. They have practically not been recorded in the North Anticyclonic Gyre (with the exception of two specimens) where both these species are replaced by closely related species ( Vereshchaka, 2000; Vereshchaka et al., 2014).
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10. - KEY TO GENERA, SPECIES GROUPS AND SPECIES OF OPLOPHORIDAE Carapace strongly chitinized, subtriangular in cross-section. Abdomen with sixth somite not longer than fifth; third to fourth somites with strong dorsomedial spines (at least half of segment length) ...............2 Carapace moderately chitinized, suboval in cross-section. Abdomen with sixth somite nearly twice as long as fifth; third to fourth somites without strong dorsomedial spines ..................................................... .................................................................................................................. 6 ( Systellaspis Spence Bate, 1888 ) Abdomen with strong dorsomedial spines on second to fourth somites. Telson with terminal spiny endpiece .................................................................................... Janicella spinicauda (A. Milne-Edwards, 1883) Abdomen with strong dorsomedial spines on third to fifth somites. Telson pointed posteriorly, without terminal spiny endpiece...........................................................................3 ( Oplophorus H. Milne Edwards ) Carapace with sharp tooth near posterior end of ventral margin...............................................................4 Carapace with ventral margin unarmed ......................................................................................................5 Rostrum distinctly over-reaching scaphocerite, with 10–15 preorbital dorsal teeth and seven or more ventral teeth. Abdomen with first pleuron bearing a small ventral tooth.................................................... ........................................................................................ Oplophorus gracilirostris A. Milne-Edwards, 1881 Rostrum not over-reaching scaphocerite, with 7–9 preorbital dorsal teeth and 2–6 ventral teeth. Abdomen with first pleuron ventrally unarmed ...................................... Oplophorus typus H. Milne Edwards, 1837 Scaphocerite with lateral side unarmed ................................ Oplophorus novaezeelandiae (de Man, 1931) Scaphocerite with lateral side serrate ........ Oplophorus spinosus (Brullé, 1839) H. Milne-Edwards, 1837 Abdomen with fourth and fifth somites serrate on posterolateral margins above pleura ........................... .............................................................................................................. 7 ( Systellaspis debilis species group) Abdomen with fourth and fifth somites bearing at most a single spinule on posterolateral margins above pleura..............................................................................................................................................................8 Telson with two regular rows of dorsal spines only ............ Systellaspis debilis (A. Milne-Edwards, 1881) Telson with two regular rows of dorsal spines and additional spinules in between .................................... ................................................................................................................. Systellaspis liui Sha & Wang, 2015 Carapace without lateral ridge extending from orbit to posterior margin. Abdomen with first somite bearing a sharp tooth on anterior margin; telson with 1–3 pairs of dorsal spines; fifth pereopod with ischium unarmed on outer side ...................................................... 9 ( Systellaspis pellucida species group) Carapace with lateral ridge extending from orbit to posterior margin. Abdomen with first somite bearing a blunt barb on anterior margin; telson with four or more pairs of dorsal spines; fifth pereopod with ischium bearing a row of spines on outer side............................................................................................11 Abdomen with sixth somite dorsally sulcate ...................... Systellaspis lanceocaudata Spence Bate, 1888 Abdomen with sixth somite dorsally flattened...........................................................................................10 Rostrum over-reaching scaphocerite by a quarter of its length or more. Abdomen with fifth somite bearing a sharp tooth on posterior margin of pleuron. Meri of third and fifth pereopods each bearing inner row of spines.......................................................................................... Systellaspis pellucida (Filhol, 1884) Rostrum , if over-reaching scaphocerite, not more than by a quarter of its length. Abdomen with pleuron of fifth somite unarmed on posterior margin. Meri of third and fifth pereopods without inner rows of spines ........................................................................................................ Systellaspis guillei Crosnier, 1987
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14. - 15. - Carapace with dorsal carina on posterior half of midline and with strong ventral carina. Fifth pereopod with elongate dactyl (≥3 times as long as wide) .............................12 ( Systellaspis cristata species group) Carapace without dorsal carina on posterior half of midline and without strong ventral carina. Fifth pereopod with short dactyl (as long as wide).................................. 13 ( Systellaspis braueri species group) Rostrum with 2–6 ventral teeth. Abdomen with third somite bearing ordinary dorsal tooth (tip directed backward). Third to fifth pereopods with spinose carpi ........................ Systellaspis cristata (Faxon, 1893) Rostrum with seven or more ventral teeth. Abdomen with third somite bearing recurved dorsal tooth (tip directed upward). Third to fifth pereopods with carpi unarmed. Systellaspis curvispina Crosnier, 1987 Rostrum <half times as long as carapace ..................................................................................................14 Rostrum> half times as long as carapace ..................................................................................................15 Telson with 15–30 lateral spinules arranged in a single row of on each side............................................... ........................................................................................................ Systellaspis paucispinosa Crosnier, 1987 Telson with 40–50 lateral spinules arranged in two or more rows of on each side ...................................... .................................................................................................................... Systellaspis braueri (Balss, 1914) Rostrum >0.7 times as long as carapace, with anterior dorsal teeth more spaced than posterior teeth
............................................................................................................... Systellaspis eltanini Wasmer, 1986 Rostrum <0.7 times as long as carapace, with dorsal teeth equally spaced .......... Systellaspis intermedia
Crosnier, 1987
Conversely, North and South Anticyclonic Gyres and the Equatorial waters in between are linked by a meridional water transfer in the meso- and bathypelagic, possibly resulting in significant gene flow between these waters. Similar gene exchange over North and South Anticyclonic Gyres was observed in meso- and bathypelagic populations of another dominant plankton species, the chaetognath Pseudosagitta maxima (Kulagin & Neretina, 2017) .
Future efforts should focus on collecting and analysing more abundant and diverse material from the Atlantic pelagic to investigate population structure of pelagic species across the oceans and to gain statistically more representative results.
MORPHOLOGICAL TRAITS IN OPLOPHORIDAE
In the pelagic realm, where shelters are absent, a protective strategy (passive defence and escaping predators) is common. For example, such strategies have been described for numerous genera of the pelagic shrimp family Sergestidae ( Vereshchaka et al., 2014) . Our analyses show that this strategy is also common in Oplophoridae and may account for a significant amount of synapomorphies. Indeed, the three most important groups of synapomorphies (rostrum, carapace and abdomen/telson-linked) are associated primarily with the defence mechanisms.
The abdomen/telson-linked characters account for 40% of the morphological diversity of Oplophoridae (34 of 85 observed characters). Part of these characters may be linked either to passive defence (the presence of large dorsal and pleural spines on the abdomen or dorsal and terminal spines on the telson) or to an active defence, i.e. escape function (e.g. the elongated sixth abdominal segment for more efficient backward flips). Characters such as serration of the pleura, keels or sulci on the telson may enhance both the passive defence and the escape function owing to the reduction of turbulence by the presence of serration (like a spoiler on a hydrofoil) or to the better rudder control by keels during the flips. As a result, the abdomen/telson-linked characters represent the most numerous synapomorphies within the Oplophoridae . They provide at least a quarter of the synapomorphies supporting all the genus and species-group clades except the S. cristata species group ( Table 4). Moreover, this group of characters accounts for over 40% support of either species-group or genus level clades on average ( Table 4).
The rostrum-related characters also significantly contribute to oplophorid diversity (11%, nine of 85 characters). Similar to the abdomen/telson, the rostrum may also be used as a passive defensive structure (long, acute and spiny) and/or as a rudder during the backward escape flips (active protection). The rostrum-related characters significantly support branching of S. pellucida and S. cristata species groups (over 50% of supporting synapomorphies, Table 4). These characters are not important at a generic level (7%) but significant at the species-group and species level (24–25% of synapomorphies, Table 4). It is noteworthy that the S. cristata species group, which is not supported by the abdomen-related synapomorphies, is instead supported by the rostrum-linked characters, thus indicating that a rostrum in this clade has a defensive role.
The carapace-related characters account for 12% (ten of 85) of the observed morphological characters in Oplophoridae and have probably been significant in the diversification of Oplophorus and S. cristata species group (33% and 50%, respectively; Table 4). Like the abdomen and the rostrum, the carapace holds morphological characters that may be used in a passive defense (spines) and escape function (keels). Along with the rostrum-linked characters, this group of synapomorphies may provide a defense function in the S. cristata species group, instead of the abdomen-related characters.
Characters related to the armature of the last three pereopods refer to presence, position and a number of specialized movable spines on various pereopodal segments. These characters represent 24% (20 of 85) of a total number of characters. Unlike the previous three groups of characters, the armature of pereopods is unlikely to be associated with defence functions. Instead, they are possibly linked to grooming and/or to mating.
Grooming is an anti-fouling adaptation in many decapods and is especially common in shrimps, as the consequences of fouling (interference with swimming) is more severe in natantians than in reptantian forms ( Bauer, 1989). Bauer (1978) showed that most carideans have a specialized brush of setae on the propodus or dactyl of the fifth pereopod, which is used for general body grooming. A similar brush is present on a short or elongate dactyl of Oplophoridae ( Fig. 3D, E View Figure 3 ), thus indicating possible adaptations to grooming. However, the more regular armature of the other segments of the last pereopods is most likely an adaption to another function, possibly mating.
Direct observations of pelagic shrimp mating are lacking. The most detailed observations on benthic species were made by Misamore & Browdy (1996), and, according to these authors, the mating includes four general phases:
1. The male rapidly approaches the female from behind, positions itself beneath the female mimicking the swimming path of the female. The pursued shrimp frequently changes directions and swimming velocities, which often results in ‘eluding’ the trailing male.
2. The male probes the ventral surface of a swimming female with its antennules, which may serve to prepare the female for spermatophore placement, determine female receptivity and maturity for mating, and/or assist in species recognition.
3. The male rolls horizontally, juxtaposing ventral surfaces with the female, wrapping its pereopods around the posterior half of the female’s carapace and anterior two abdominal segments.
4. The male embraces the female, curls its abdomen and rotates 90° with respect to the midline of the female; the rotation is facilitated by adduction of the pereopods grasping the female. Upon completion of the rotation, the male rapidly contracts its abdomen and is propelled away from the female.
Observations on the caridean shrimp Rhynchocinetes typus , by Correa et al. (2000) support the general scheme above, such as highlighting the significance of the last pereopods in the process. During the second phase, the male probes the posterior part of the female cephalothorax (possible species recognition) and the armature of last pereopods of the female may be important in this process. During the last stage, the male embraces the female with the last pereopods and their armature serves in this process.
In penaeid shrimps it was observed that only six out of 22 copulation flexes between male and thelycum and petasma is not a guarantee for success. It probably can be assumed that mating in caridean shrimps, which lack an elaborate petasma and thelycum, is a challenge in the turbulent water column ( Lunina & Vereshchaka, 2017). For Oplophoridae we suggest that the complexes of movable spines on various segments of the last three pereopods act as a holding structure. Comparable mechanisms may be found in breeding males of cambarid crayfishes, which have copulatory hooks on the ischia of the third and fourth pereopods and use them to hook on to the pereopods of the females during copulation (e.g. Holdich, 2002).
Recent analyses have shown that in the turbulent and hydrographically dynamic pelagic zone, successful copulation depends on a perfect fixation and possibly on mutual stimulation of mates during spermatophore transfer and thus on the presence of suitable copulatory structures ( Lunina & Vereshchaka, 2017). In pelagic taxa such as Sergestidae , Benthesicymidae and Euphausiacea, perfect fixation and possible stimulation are provided by an elaborate petasma consisting of numerous lobi and processi. For Oplophoridae and other Caridea , lacking true petasma and having only an appendix masculina, fixation and stimulation of mates are much more problematic than for the abovementioned taxa. The armature of the last three pereopods possibly plays the same role in Oplophoridae (as possibly in other pelagic Caridea ) as the petasma and thelycum do in dendrobranhchiates and euphausiaceans. These structures are not homologous but have the same function and occur in the same area, which is in the last three thoracic and first pleonic somites. This area seems to be most suitable for fixing mates during copulation in the water column, probably because its position in the centre of the body.
The armature of the last three pereopods may thus be considered as a mating structure, which can explain its phylogenetic importance at the species level (45% of the characters’ average; Table 4). In addition, this character group significantly contributes to the diversification of the genus Systellaspis and the S. braueri species group (38% and 75%, respectively; Table 4).
The exopods of the third maxilliped and first pereopod are expanded in a very characteristic way and support the genus Oplophorus . This genus is composed of voracious predators occurring shallower (100–500 m) than the rest of Oplophoridae in the Atlantic ( Foxton, 1970), Pacific ( Vereshchaka, 1990a) and Indian ( Vereshchaka, 1990b, 1995) Oceans. Like other pelagic genera exhibiting an offensive (predatory) strategy ( Neosergestes , Parasergestes, Allosergeste and Deosergestes : Vereshchaka, 2009; Vereshchaka et al., 2014), Oplophorus is regularly recorded but always in low numbers, which suggests that schooling behaviour is absent. We propose that the expanded exopods may favour Oplophorus to manoeuvre in the water column and thereby more effectively pursue its prey.
CO-EVOLUTION OF MORPHOLOGICAL CHARACTERS
Oplophoridae show a spectacular co-evolution between various characters associated with the carapace, the abdomen and the exopods ( Fig. 5C View Figure 5 ). As discussed above, these parts bear characters that are linked together and serve as a part of the locomotory process, either backward or forward. Backward flips may be facilitated by characters such as shape of abdominal segments serving as an engine, keels on the carapace and abdomen serving as rudders, spines and teeth acting as spoilers in a hydrofoil. The steering (manoeuvring) function is accomplished mainly by the exopods acting as propellers. Thus, the carapace, the abdomen and the exopods in Oplophoridae all probably co-evolved together to function for locomotion.
Conversely, the armature of the last three pereopods is not grouped with any other characters, which may suggest separate functions, e.g. mating, as hypothesized above and, to a lesser extent, grooming ( Bauer, 1978, 1989).
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