Taxonomic, systematic, morphological and biological studies on Palaemon Weber, 1795 (Crustacea: Decapoda: Palaemonidae)

Ashelby, Christopher William

Abstract

Palaemon is a large genus of marine, brackish and freshwater shrimps and are amongst the most familiar shrimps to scientists and amateur naturalists. Despite being familiar and widely reported they remain understudied. A series of studies was conceived to address some of the outstanding questions on the taxonomy, systematics, functional morphology and invasive biology of Palaemon and related genera.

As previous studies have revealed that there is a lack of stable morphological characters to define both species and the genus Palaemon itself traditional taxonomic studies were conducted to highlight the potential of novel characters to delineate species and genera within the subfamily Palaemoninae. Previous phylogenetic work, both morphological and molecular, has also suggested that the genus probably does not represent a monophyletic lineage. The previous molecular phylogenetic studies were expanded on to give a phylogeny of the subfamily Palaemoninae based on the genes 16SrRNA and Histone (H3), representing the largest dataset to date on this subfamily. As the structure and morphology of an animal may also be heavily influenced by its function, an investigation into the structure of the mandible, a feature that has been attributed phylogenetic significance, was conducted via SEM to test the theory that its structure is actually a result of its function rather than its evolutionary relationships. Two previously reported forms of P. longirostris from brackish waters in Atlantic Europe were subjected to a geometric morphometric analysis to investigate alleged differences in carapace and rostrum shape. Two different studies were instigated to investigate the invasive species Palaemon macrodactylus. The first of these scrutinised and synthesised available data and presented hypotheses on modes of spread, patterns of spread, potential impacts, factors favouring its introduction and made predictions of areas at risk of future invasion. The second study investigated the potential impact of the introduction of P. macrodactylus on the native P. longirostris in the River Thames by analysing dietary overlap from stomach content analysis.

As a result of the taxonomic studies two new species of Palaemon were described from the tropical eastern Atlantic: Palaemon powelli and P. vicinus and a further, poorly known species was redescribed. A new genus, Rhopalaemon, was also erected to accommodate the rather aberrant, Indian species Leander belindae which had previously been included in Palaemon for many years. Several characters that had not previously been used or had not been attributed much significance were used in the definition of these taxa. A taxonomic guide to the marine Palaemoninae fauna of Taiwan was created reporting thirteen species belonging to seven genera. One of these species, P. serrifer occurs in two distinct morphotypes in Taiwan, one of which is suspected to be a new species.

The molecular phylogenetic study indicated that Palaemon as presently defined is not a monophyletic genus with the genera Exopalaemon, Coutierella and Palaemonetes likely being synonyms of Palaemon. Palaemon concinnus, P. pandaliformis and P. gracilis do not form a clade with other Palaemon and should be removed from the genus. As well as the systematic implications, the molecular study also revealed that freshwater species do not form a clade with one another but rather are more closely related to geographically proximate species suggesting that they arise from multiple invasions of freshwaters rather than a single invasion and subsequent radiation.

The functional morphology of palaemonoid mandibles was investigated via scanning electron microscope. The presence of cuticular structures was noted on most mandibles with five distinct types being recognised. Each type of cuticular structure is presumed to have a different function. The results indicated that only a weak phylogentic signal is conveyed by the structure of the mandible but that its structure is influenced by the food sources ingested. Those species that consume harder prey items had mandibles designed for grinding with presumed mechanosensory cuticular structures whilst those consuming soft-bodied prey did not possess cuticular structures. Feeding on particulate or mucous diets has resulted in highly modified mandibles. In order to try to categorise the variety of morphological form of the mandible and of the cuticular structures a new classification system was developed.

The two reported morphological forms of Palaemon longirostris with a divide in the southern Bay of Biscay were supported by the geometric morphometric analysis. The forms chiefly differed in the form of the rostrum with the southern form having a more elongate and upwardly directed rostrum than the northern form. However, no biological grounding was found to support the morphological differences observed.

The global distribution of Palaemon macrodactylus appears to have originated through a combination of at least three primary introductions from Asia and three secondary introductions. The factors that have lead to P. macrodactylus being so widely transported are still unclear as many of its life history traits and environmental tolerances are similar to other species of Palaemon that have not invaded. It is suggested that its advantage may lie in the tolerances of its larvae. The competitive interactions of the species in newly invaded regions are poorly known but the data obtained here suggest strong trophic overlap with P. longirostris in the Thames during the winter but less so in the summer. This seasonal aspect to the overlap in the diet may also provide differing opportunities for successful invasion by P. macrodactylus with greater chance of success in the summer when competition for food is lower. The diet of P. macrodactylus in the Thames is slightly different to that reported for other regions to which it has been introduced being dominated by amphipods rather than mysids. If food becomes a limiting factor, particularly in summer, then the competition between P. macrodactylus and P. longirostris may increase leading to potentially adverse effects on the latter species.

This study has increased the number of species of Palaemon from 39 to 41 through the description of two new species and removed a further species to a new monotypic genus, Rhopalaemon. However, the molecular analysis indicates that the true number of species that belong in the genus is approximately double this number and that further species should be removed to other, new genera. It has demonstrated that the freshwater species have independently invaded freshwater and that species of the genus are prone to human-mediated introductions. A new system for the classification of caridean mandibles and cuticular structures is proposed and their form has been linked to diet and feeding mode.

Citation

Ashelby, C. W. (2012). Taxonomic, systematic, morphological and biological studies on Palaemon Weber, 1795 (Crustacea: Decapoda: Palaemonidae). (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4214945

• Thesis Type: Thesis
• Deposit Date: Jan 27, 2014
• Publicly Available Date: Feb 23, 2023
• Keywords: Biological sciences
• Public URL: https://hull-repository.worktribe.com/output/4214945
• Additional Information: Centre for Environmental & Marine Science, The University of Hull
• Award Date: Dec 1, 2012