Parasitism is an enduring symbiotic relationship in which the parasite is nutritionally dependent upon the host for at least part of its life cycle, increasing its own fitness in the process and directly impinging upon the biological fitness of the host. Parasite–host interactions form a significant proportion of the biotic interactions in extant global ecosystems, influencing many characteristics of species networks including behavior, population structure, and ecological function. The antagonistic relationship between parasites and hosts has also been proposed as the primary mechanism leading to the evolution and maintenance of sexual reproduction, due to the negative frequency-dependent selection associated with parasitism. Despite its obvious importance, the origins and early evolution of Metazoan parasitism remains enigmatic. Molecular phylogenies predict the emergence of parasitic clades in the Cambrian and putative instances of shell damage, shell scarring and occasional bioclaustration from the early Cambrian represent circumstantial evidence that hint at possible parasitism, but the rarity of well-preserved specimens precludes decisive identification of parasite–host interactions in the earliest Phanerozoic. Possible examples of epibiontism, commensal infestation, and hitchhiking are also known from the early Cambrian, but none of these constitute definitive instances of parasitism with a clear negative biological effect on the host. This absence of clear evidence for parasitism in the earliest animal communities may, in part, be due to a lack of cross-sectional quantitative analyses on Cambrian material of the type that have been demonstrated as necessary to identify and discriminate instances of animal parasitism in deep time.
In a paper published in the journal Nature Communications on 2 June 2020, Zhifei Zhang and Luke Strotz of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, Timothy Topper, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, and of the Department of Palaeobiology at the Swedish Museum of Natural History, Feiyang Chen, again of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, and of the Department of Biological Sciences at Macquarie University, Yanlong Chen and Yue Liang, again of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, Zhiliang Zhang, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, of the Department of Biological Sciences at Macquarie University, Christian Skovsted, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, and Department of Palaeobiology at the Swedish Museum of Natural History, and Glenn Brock, once again of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, of the Department of Biological Sciences at Macquarie University, describe a new species of Lingulid Brachiopod from the early Cambrian Guanshan Konservat-Lagerstätte, and the encrusting kleptoparasites which cover many specimens of this species.
The early Cambrian (Stage 4) Guanshan Konservat-Lagerstätte occurs mostly in the lower 40m of the Wulongqing Formation, which crops out over a geographically wide area in eastern Yunnan, located in southern China. The Guanshan Biota is unusual in being proportionately dominated by Brachiopods, and so, is strongly differentiated from other Cambrian Konservat-Lagertätten such as the Chengjiang, Sirius Passet, Emu Bay Shale and the Burgess Shale, which are Euarthropod-dominated assemblages.
The new species is placed in the genus Neobolus, and given the specific name wulongqingensis, meaning 'from Wulongqing', in reference to the Wulongqing Formation, from which the fossils were extracted. It is an Organophosphatic Linguliform Brachiopod with an adult shell subcircular, no visible pits or pustules on surface, peripherally ornamented with distinct growth lines; metamorphic shell, average of 2396 μm in width and 1907 μm in length (based upon 13 measured specimens); ventral pseudointerarea orthocline to apsacline with wide and triangular pedicle groove; ventral propareas vestigial or indistinguishable; dorsal pseudointerarea forming narrow, crescent-shaped rim; ventral visceral field short, slightly thickened and not extending beyond midvalve; dorsal interior with long median septum extending to or beyond ⅓ valve length. Short spirolophe lophophore present.
Neobolus wulongqingensis is the most numerically abundant taxon in the Wulongqing Formation, with many thousands of specimens forming dense concentrations of monotypic, mostly conjoined shells, clustered closely on bedding plane surfaces. Remarkably, many of the Brachiopod shells are encrusted with elongate, tapering biomineralised tubes. Symbiotic relationships such as this are seldom directly observed in the fossil record because taphonomic biases generally impede the preservation of direct interaction between organisms. The high-fidelity preservation and great abundance of specimens in the Wulongqing Formation provides a rare opportunity to investigate this unique interaction between a Brachiopod host and their associated encrusting tube-dwelling organisms.
Zhang et al. assess differences in biomass between Brachiopod individuals of the species Neobolus wulongqingensis encrusted with tubes and those individuals lacking tubes, with biomass representing a proxy for the biological fitness of an individual. This analyses suggests that the tube-dwelling organisms reduced the biological fitness of the host and, when considered in combination with observations of the preferred growth orientation of the encrusting tubes, these results suggest the interaction between the tube-dwelling organisms and their host Brachiopod represents kleptoparasitism. This instance in a Cambrian epibenthic marine community likely represents the oldest known parasite–host relationship in the fossil record and reveals that parasite–host interactions emerged in conjunction with the rise of the earliest Animal communities during the Cambrian radiation.
The preservation of marginal chaetae, mantle canals, visceral areas, and, rarely, the lophophore in the Brachiopods indicates rapid burial and minimal transport by episodic obrution deposits. Despite this, the soft body of the tube-dwelling Animal is not well-preserved, and its biological affinities are not self-evident. The greyish-white tubes, normally flattened by post depositional compaction, are immediately apparent. The tubes, some with preserved accretionary growth increments, encrust the exterior of both dorsal and ventral valves of Neobolus wulongqingensis with the open apertures exclusively oriented toward the anterior commissure of host Brachiopods, indicating an intimate, life-long, in-vivo association. The tubes exclusively encrust the exterior of the host shell, which occasionally shows signs of minor damage or disruption of shell growth lines, but there is no evidence of boring into the interior of the brachiopod by the tube-dwelling organism. The tubes are not found attached to any other hosts or substrates, such as the Trilobite or Palaeoscolecid exuviae that occasionally occur in the shell beds. Consequently, Zhang et al. interpret this interaction as representing an obligate relationship, as there is no evidence to suggest that the tube-dwelling organisms can adopt a free-living lifestyle in the absence of their Brachiopod host.
Bayesian estimation analysis (a widely used technique for estimating the probability density function of random variables with unknown parameters) demonstrates that a credible difference in biomass exists between Brachiopods with encrusting tubes (205 specimens) compared to those without (224 specimens). There is no overlap in the 95% highest density interval of the posterior distribution for the means of the two groups and the highest density interval for effect size does not overlap with zero. Mean biomass for individuals with encrusting tubes is thus credibly lower than for those without tubes. A null hypothesis significance testing approach also identified a significant difference between encrusted and non-encrusted individuals with a small effect size. Zhang et al. therefore contend that individual Brachiopods encrusted with tubes have reduced fitness when compared with their non-encrusted counterparts. On the basis of the difference in the values for mean biomass between the two groupings, encrustation results in a 26.08% reduction in overall fitness across the entire measured cohort.
Although Zhang et al.'s analyses indicate that Brachiopods with encrusting tubes are reduced in biomass compared to those without, there is no clear relationship between the biomass of host individuals and increasing numbers of encrusted tubes per individual. In some symbiotic relationships, the impact on the host is amplified depending on the number of parasites present, but this relationship can be highly variable. For Zhang et al.'s dataset, the biomass of the Brachiopod host decreases when a single tube is encrusted to the shell surface, but no further decline is associated with an increasing number of tubes. Both proxies for increasing total parasite load also show no correlation with biomass. This suggests the tube-dwelling organism did not directly inhibit the feeding capability of the host, as larger numbers of parasites do not result in decreased fitness. However, a significant relationship exists between the attachment point of the encrusting tubes and the biomass of the brachiopod host, indicating encrustation earlier in ontogeny results in greater reduced biomass relative to hosts that have been infected at later ontogenetic stages, regardless of the number of symbionts present. In living Brachiopods, smaller individuals generally display an increased growth rate compared to larger individuals. It would therefore be expected that the impact on fitness would be greater for host individuals that are settled by parasites during earlier ontogenetic stages. The increase in median attachment distance for larger numbers of symbionts and the larger size of specimens with greater than four tubes establishes that higher infection rates can only occur when Brachiopod hosts have already managed to grow to larger adult sizes and there is sufficient Brachiopod shell surface area to accommodate a larger number of encrusting tubes.This also indicates that the tube-dwelling organisms do not preferentially encrust smaller Brachiopod individuals, as Brachiopods are clearly encrusted in large numbers later in their ontogeny, when they have reached larger sizes.
Zhang et al.'s analyses demonstrate that the tube-dwelling organism directly impinges upon the biological fitness of the host, supporting the assertion that the encrusting tube-dwelling organisms are parasitic, rather than being either mutualistic or commensal with the Brachiopod host. A reduction in host biomass or growth rate has been directly attributed to the presence of a parasite in a variety of extant symbiotic relationships. Parasites typically increase the energetic requirements of infected organisms, as the host must generate sufficient energy to not only maintain its own requirements but also the needs of the parasite. This commonly leads to hosts with decreased biomass when compared with uninfected individuals. This result represents the first definitive and statistically supported instance of parasitism from the Cambrian and indicates that parasite–host systems were well established by Cambrian Stage 4, suggesting this type of interaction probably emerged even earlier during the main pulse of the Cambrian radiation.
Variations in biomass between individuals and assemblages of the same species have also been previously attributed to regional variation in environmental stressors. All specimens of Neobolus wulongqingensis included in this analysis occur in dense aggregations (estimated 60 000 individuals per m²) from the same geographic locality and stratigraphic package with similar sedimentological features subject to similar environmental and depositional conditions. Consequently, the reduced biomass of tube-encrusted Neobolus wulongqingensis individuals cannot be attributed to environmental factors and a parasitic affect is the most strongly supported probable cause.
In all instances, the apertures of tubes are orientated toward the Brachiopod commissure, spanning an arc (plan view) of about 150°. No tubes have been observed orientated toward the hinge line of the Brachiopod. Tubes consistently grow beyond the commissural margin of Neobolus wulongqingensis into, and slightly above but rarely beyond, the Brachiopod chaetal fringe. Critically, the dominant growth direction of the tubes aligns tightly along a vector between 40° and 70° either side of the median plane of symmetry of the Brachiopod; this alignment is most pronounced in shells with a single encrusting tube but the same orientation pattern occurs in shells with all numbers of tubes, strongly supporting a preferential growth direction in the tubes toward the antero-lateral margin of the Brachiopod shell.
Five specimens of Neobolus wulongqingensis from Wuding Quarry preserve a partial spirolophe lophophore. A spirolophe lophophore produces two separate inhalant laminar feeding currents at the antero-lateral edge of the shell margin that match the preferred orientation and growth position of the encrusting tubes on shells of Neobolus wulongqingensis. The preferred orientation of growth demonstrates that the tube-dwelling organisms were not purely utilising the Brachiopod as a hard substrate on which to construct their tubes. This data when combined with the demonstrated empirical cost to the host in the form of reduced biomass, strongly supports kleptoparasitic behavior. Kleptoparasitism is a form of competition, where food that is either already in the possession of the host or which the host has expended energy on obtaining and capture is imminent, is stolen by the parasite. In Zhang et al.'s scenario, this involves the tube-dwelling organisms acting as intercept feeders, stealing a portion of the Brachiopod feeding stream before it reached the chaetal fringe. Erika Iyengar recognised six distinct morphological, behavioral and physiological criteria that characterise living sedentary/sessile kleptoparasitic interactions in a 2002 study of such relationships. At least five of these criteria can be directly applied to the relationship between the encrusting tube-dwelling organism and Neobolus wulongqingensis further reinforcing a kleptoparasitic relationship.
Kleptoparasitism is rarely identified in the fossil record, and no instances of kleptoparasitism, as far as Zhang et al. are aware, have been proposed for Cambrian communities. Detailed empirical investigations of the energetic and nutritional cost of kleptoparasitism to the host, even for extant systems, are few. For this reason, it is currently difficult to assess if the reduction in host fitness (about 26%) Zhang et al. detect for Neobolus wulongqingensis is typical of sessile kleptoparasitic relationships. Brachiopods are particularly vulnerable to exploitation by kleptoparasites, since active filter feeding represents the greatest energy expenditure in the life of Brachiopods, and the time lag between collection and ingestion of nutritionally beneficial particles also provides potential for other organisms to exploit this resource. Combined with the fact that the biotic interaction Zhang et al. document is interpreted as obligate for the parasite, this suggests that the effect observed is likely greater than would be the case in facultative kleptoparasitic associations. Intriguingly, obligate kleptoparasitism is exceedingly rare in modern marine systems, which might suggest that this novel ecological relationship is always rare in benthic communities or has been secondarily lost some time during the Phanerozoic.
Verification of this kleptoparasitic relationship reveals that the heritage of parasite–host interactions can be traced back more than half a billion years to the rise of Bilaterian Animal communities during the Cambrian and further establishes the importance of the early Cambrian as a primary source of ecological novelty. Antagonistic biotic interactions have also been proposed as the drivers of widespread evolutionary phenomena such as the maintenance of sexual reproduction and genetic polymorphism at disease loci. Both of these phenomena are known drivers of biodiversity increase, suggesting that the already established presence of parasitic relationships in Cambrian communities potentially had a fundamental role in the upsurge in evolutionary innovation associated with the Cambrian Radiation.
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