Pseudomma, G. O. Sars, 1870

Meland, Kenneth, 2004, Species diversity and phylogeny of the deep-sea genus Pseudomma (Crustacea: Mysida), Zootaxa 649 (1), pp. 1-30 : 16-19

Pseudomma phylogeny

The genus Pseudomma is found world wide in the deep sea, and consists of species that are apparently limited in their distribution within well­defined biogeographical regions. In view of this distribution pattern, morphological variability between species is surprisingly low.

Apomorphic character states associated with feeding were spread across the phylogeny, but in some instances showed support for shallower nodes. Consequently, mouthpart characters proved to be non­informative in resolving deeper nodes in the trees. As an example, the absence of cusps on the first maxilliped dactylus, with the exception of P. marumoi , defines the GMNP group, but this character seems to have evolved independently in P. truncatum . Similar homoplastic characters are also seen in the mandibles and mandible palp. The only unique synapomorphy in mouthpart characters is the serration of the frontal lamina supporting a monophyletic P. heardi and P. californica . Considering that the bottom conditions in the deep sea are sufficiently homogeneous, coupled with the fact that Pseudomma species are highly adapted to a deep­sea habitat, retention of a plesiomorphic mouthpart morphology can be ascribed selective pressure resulting in phenotypic stasis.

Murano (1974a) recognized eight Pseudomma species groups based on characteristics of the ocular plate, antennal scale and telson. These phenetic species groups were not reflected in the resulting trees, except as parts of the " affine ­group" in the North Atlantic clade, which is defined by antennal scale characters. Failing to establish robust branching patterns in early lineages, character state distribution of the ocular plate, uropod, and telson resembled the homoplastic distribution observed in mouthpart morphology.

Although support in deeper nodes was generally low, characters holding the largest amount of phylogenetic information in defining early branching patterns were observed in both female and male pleopods characters. Considering the idea that these are size­related characters that may have evolved in concert to maintain swimming functionality, one can certainly question the usefulness of pleopod characters, such as the number of annulations, in phylogenetic analyses. Despite the potential bias of a forced size dimension in the ordered multistate characters, none of the clades in the resulting trees were defined by body size. Also, such arguments of functional morphology, taken as a priori assumptions, do not seem particularly helpful in the generation of phylogenetic hypotheses.

It has been shown that the modified setae on the third and fourth male pleopods are used in grasping and holding the female during copulation ( Mauchline 1980). Assuming that similar sexual traits might be attributed to other pleopod characters, it is possible that these also are prone to sexual selection. A detailed study on pleopod morphology in the light of mysid reproductive biology would be valuable for a better understanding of the sexually linked nature of the pleopods, and in this context might prove useful for inferring phylogeny.

A general interpretation of low variation in morphology suggests phenotypic stasis in Pseudomma through stabilizing selection. It should also be noted that mysids brood their young and have no planktonic stages, dispersal is therefore limited to either swimming or walking. Selective force could then be working in concert with dispersal, maintaining a well adapted organism to the deep sea environment that is coupled with active migration into new similar habitats as they are made available for intrusion. If this is the case, we can then ask "How old are these deep­sea mysids and when did they speciate"? The fossil record of Lophogastrida and Mysida is relatively rich. Despite their age Lophogastrida from the Triassic and Jurassic strata of France ( Schram 1986; Secretan & Riou 1986) show a remarkable resemblance to extant congeners. A close resemblance to mysids of today is also observed in the Mysida genus Siriella from the Upper Middle Jurassic of France ( Secretan & Riou 1986). These records suggest a conservative evolutionary history for Mysidacea and it seems that the evolutionary stage reached by the Jurassic has not been appreciably modified later. In effect, the cosmopolitan distribution of the genus Pseudomma might be reflecting an early origin, and the ambiguous resolution of character evolution might be the result of more recent speciation events in separate geographical regions.

The idea of an essentially widely distributed fauna in the ocean basins, resulting from the absence of barriers and a confluence of the ocean floor is highly recognized ( Gage & Tyler 1991). Support for widespread distribution of faunal assemblages in the deep sea has been demonstrated in comparisons at the generic and higher levels of peracarids such as deep­sea endemic asellotan isopods, where distributions tend to have a global uniformity ( Hessler & Wilson 1983; Wilson 1998). From an evolutionary and biogeographical point of view one can therefore expect a certain degree of agreement between distribution and phylogenetic relationship ( Humphries 1992). Consequently, due to existence in all ocean basins of the deep sea and being restricted to a hyperbenthic lifestyle on soft­bottom sediment within low temperature regimes, comparable observations of species radiation and distribution were expected to be demonstrated in the mysid genus Pseudomma .

Despite alternative coding strategies and implied assumptions on character evolution, the morphological phylogeny presented in this study did not allow for a clear historical interpretation of species distribution in Pseudomma . When comparing the alternative tree topologies, conflicting branching patterns in deeper nodes became evident, and a meaningful interpretation on origin and radiation in early lineages proved to be difficult. On the other hand, congruence was observed in several shallower nodes that permitted for the recognition of consistent branching patterns emerging in four consistent lineages. These monophyletic species groups are confined within three major geographic areas, North Atlantic (NA), Northern Pacific (NP), and Antarctic (AN).

In accordance with geographical distribution, this study suggests both an Antarctic and North Atlantic clade. Although relationships between the east and western Pacific are not so evident, a "Gulf of Mexico and northern Pacific" clade persisted in almost all analyses. The sister­group relationship between P. heardi (Gulf of Mexico) and P. californica (East Pacific) can be interpreted as a divergence resulting from the closing of the Isthmus of Panama approximately three million years ago ( Knowlton & Weigt 1998). Similar explanations through recent geological events is observed in the monophyly of the North Atlantic species P. frigidum , P.roseum , P. affine I, and P. affine II. The modern biogeography of the Arctic and Norwegian Sea is largely the product of Iceland –Faeroes ridge submergences, leading to the connection of the Norwegian Sea with the Atlantic Ocean in Oligocene and Miocene. In effect, the deep­basin fauna in the boreal North Atlantic is considered no more than two to three million years old ( Dunton 1992; Svavarsson et al. 1993; Crame 1997). It has additionally been shaped by the Quaternary glaciations ( Kennett 1982). The high degree of morphological similarity between North Atlantic Pseudomma species ( Meland & Brattegard 1995) suggests a young fauna and recent speciation in the Norwegian Sea. As an example we have P. roseum resembling a smaller version of P. frigidum . While P. roseum has an amphi­Atlantic distribution on the continental shelves and is also found in Norwegian fjords, P. frigidum is endemic to the cold basin water in the Norwegian Sea. Considering the fact that North Atlantic shelf areas were covered by ice in Quaternary glacial periods and not open for colonization until after deglaciation ( Thiede et al. 1990), speciation of P. frigidum and P. roseum could have taken place less than 10,000 years ago. Comparable events of possible recent speciation are observed in the northern Pacific where P. latiphthalmum is identified as a larger version of P. japonicum . In Antarctic waters P. belgicae shares most morphological traits with, but is larger than, its sister species P. armatum .

Although partial agreement between phylogeny and distribution was achieved, distributional polyphyly was indicated in several lineages (e.g. P. matsuei and/or P. longisquamosum / P. surugae (NP) + P. antarcticum , P. armatum , P. belgicae (AN) and P. izuensis (NP) included with NA species). As already discussed, phenotypic stasis within the genus is suggested, and the ambiguous grouping of Pseudomma across biogeographic regions, partially due to low resolution in early lineages, is therefore not surprising. In suggesting a hypothesis that emphasizes the effect of ongoing dispersal and stabilizing selection it is interesting to note the close affinity of P. jasi from the Faeroe Islands and Iceland Basin with Antarctic Pseudomma species. These findings might be reflecting a common ancestry and recent divergence between Antarctic and North Atlantic deep­sea mysids. In this regard, the newly discovered mysid fauna in the Iceland Basin in the BIOICE­project and possible new Mysidacea records in the ANDEEP­project might contribute additional data for future phylogenetic studies on the order Mysidacea .

In summary, low level morphological variation in Pseudomma gives ambiguous results when inferring phylogeny. Therefore, in an attempt to pursue questions concerning the genus Pseudomma 's evolutionary history, the present study has been expanded to also include DNA­sequence data. Phylogenetic hypotheses based on molecular data is presented in a separate study ( Meland & Willassen 2004). Future investigations concerning the evolution of deep­sea mysids will also benefit from emphasizing on bipolar distribution patterns and geographic variation between the Antarctic and Arctic fauna ( Brandt 2001). In pursuing these issues, future research on deeper living mysids can contribute valuable information on speciation, origin, and maintenance of diversity in the deep sea.