Comparative feeding behaviour and morphology of mysids (Crustacea: Mysidacea)
Creators: name: Metillo, EB
Publisher: name: University Of Tasmania
Year: 2023

description: Behavioural and morphological studies were made to determine whether feeding niche partitioning occurs in three sympatric coastal mysids, Anisomysis mixta australis, Paramesopodopsis rufa, and Tenagomysis tasmaniae. All three species showed optimal chemoreception as judged from stereotyped food capturing response to mixtures of feeding stimulant (either betaine-HCl or glycine) and suppressant (ammonia). The strongest responses were to 50:50 mixtures of both betaine-ammonia and glycine-ammonia solutions. In general, the response curve to the different mixtures tested was bell-shaped. A. mixta australis only showed the normal curve in response to the glycine-ammonia mixture. The platykurtic curve in T. tasmaniae suggests less optimal response to the betaine-HCl-ammonia solution. P. rufa reacted more strongly to the betaine-ammonia than to the glycine-ammonia solutions, and more of its individuals responded to both solutions than the other two species. These results suggest that sensitivity to betaine-HCl- and glycine-rich fresh and decomposing animal tissue is most sharp in P. rufa. Acuteness, however, may be diminished with duration of decomposition as indicated by reduced responses to mixtures with &gt;50% ammonia. High concentrations of either stimulant also had a suppression effect. Similar mechanisms are used by all three to capture fine particles. Particle size selection overlapped in the two larger species, P. rufa and T. tasmaniae, which ingested a full range of sizes of inert beads from 10 to 90 ˜í¬¿m. The smallest species, A. mixta australis, differed from the other two by mainly ingesting the smallest beads. P. rufa and A. mixta australis both form their feeding appendages into 'cages' (primary composed of mouthparts and the secondary of thoracic endopods) to grasp live prey. T. tasmaniae uses mainly the primary cage because its thoracic endopods remained spread, a feature associated with its behaviour of resting on the substrate. P. rufa was the most efficient predator of the euryhaline daphniid prey, Daphniopsis australis. It showed a steeper linear functional response curve for this prey than the two other species which showed similar flat linear curves. All three species showed similar predation success on Artemia sp. nauplii, and similar predation failure for the calanoid copepod Gladioferens pectinatus. In situ feeding selectivity experiments using 14C-labelled Artemia sp. nauplii and phytoplankton, and 3H thymidine-labelled macroalgal detritus showed that all three species preferentially ingested the animal food, i.e. Artemia sp. nauplii. The activity of the digestive enzyme laminarinase was analyzed over one year to assess herbivorousness in the three mysid species. High levels of activity in T. tasmaniae were sustained throughout the entire year and peaked in August. P. rufa showed levels 2.5 to 4 times lower than those in T. tasmaniae, and remained constant through the year. A. mixta australis showed a bimodal pattern with peaks in February and August comparable to those in T. tasmaniae. The minimum levels were similar to those in P. rufa. Significant interaction between species and sampling date suggests that the peak of activity in August (late winter) may be associated with all three species ingesting relatively high levels of macroalgal detritus. The peak in February (late summer) shown by A. mixta australis reflects similar high levels of consumption of both macroalgal and laminarin producing phytoplanktonic particles. Short term (24 h) starvation reduced activity in P. rufa, but not in the other two species. Medium term (9 d) feeding on the non-laminarin producing dinoflagellate, Scrippsiella trochoidea, reduced activity in T. tasmaniae on day 9, and on day 5 in A. mixta australis. However, the latter species returned to the initial levels on day 7. No definite conclusion can be drawn from the P. rufa results, because all animals died after 3 d. Marked reduction in activity was also observed when juveniles of T. tasmaniae and A. mixta australis had grown to maturity using Artemia sp. nauplii as food. Therefore, laminarinase is likely to be substrate specific in T. tasmaniae, but not in A. mixta australis. Growth rates of the juvenile stages of the three species held separately in the laboratory were similar when fed with excess amounts of Artemia sp. nauplii. When all three species were held together and fed with reduced amounts of nauplii, P. rufa grew faster than the other two species which showed similar rates. Of the three species, only a few T. tasmaniae individuals showed slight growth when fed aged algal detritus and the diatom Phaeodactylum sp. Although P. rufa grew best on animal food the other two species are equally dependent on the same type of food. Foregut and mouthparts morphological data indicate three major feeding adaptations: predominantly macrophagous feeding on tough macrophyte particles (type Ia) for T. tasmaniae, predominantly macrophagous feeding on large animal prey/detritus (type lb) for P. rufa, and mainly microphagous and/or suction feeding on smaller animal prey and fine particulate matter (type le) for A. mixta australis. These studies suggest that probable strong competition associated with overlapping feeding habits in the three species is reduced by differences in size and feeding structure, and by microhabitat partitioning (shown by other authors) with P. rufa and A. mixta australis spending more time aggregated in the water column while T. tasmaniae is found on, or a few centimetres above, the sandy substrate. The foreguts of 45 more species, three from Tasmania and 42 from different biogeographical zones of the globe, were surveyed to test the hypothesis that feeding adaptation is more important than phylogeny in shaping foregut structure. The spheroidal shape shown in the initial three Tasmanian species, and the tubular form were the two main foregut types observed. From evidence on diet and feeding behaviour, a series of four basic feeding adaptations were recognized: the three types (Ia, lb, le) found in the initial three Tasmanian species, and the mainly suction feeding exemplified by the tubular foregut type (type II). Cladistic analysis suggests that the type I adaptations appear to have evolved several times within clade. The specialized suction feeding seems to have been derived from type Ia. These data suggest that the need to collect food seems to have been such a strong selective force that adaptations of the feeding apparatus have masked phylogenetic relationships within these structures in mysids.