Symbiotic relationships are widespread in terrestrial and aquatic animals today, but evidence of symbiosis in the fossil record between soft-bodied bilaterians where the symbiont is intimately associated with the integument of the host is extremely rare. The radiation of metazoan life apparent in the Ediacaran (~635–541 million years ago) and Cambrian (~541–488 million years ago) periods is increasingly accepted to represent ecological diversification resulting from earlier key genetic developmental events and other innovations that occurred in the late Tonian and Cryogenian periods (~850–635 million years ago). The Cambrian has representative animals in each major ecospace category, the early Cambrian in particular having witnessed the earliest known complex animal communities and trophic structures, including symbiotic relationships. Here we report on newly discovered Cricocosmia and Mafangscolex worms that are hosts to aggregates of a new species of tiny worm in the lower Cambrian (Series 2, Stage 3) Chengjiang Lagerstätte of Yunnan Province, southwest China. The worm associations suggest the earliest known record of aggregate infestation of the integument of a soft-bodied bilaterian, host specificity and host shift.
Inquicus fellatus gen. et sp. nov.
Etymology. Genus name from inquilinus (Latin) meaning a ‘lodger’ or ‘dweller in another’s house’, plus priscus ‘ancient’. Species from fellator ‘a sucker’ and atus ‘provided with’, alluding to its lifestyle.
Holotype. YKLP 13235 (part and counterpart). A complete specimen, 3.3 mm long, attached to a Cricocosmia jinningensis specimen, YKLP 13226 (Figs. 1a,b and 2a–d and Supplementary Fig. 1a,b). Eleven other specimens of I. fellatus (YKLP 13236–13246) are attached to this host (Figs. 1a,b and 2f and Supplementary Fig. 1a–d).
Referred material. At least 53 attached individuals and attachment discs lacking an attached individual, associated with Mafangscolex sinensis and C. jinningensis specimens (Figs. 1–3 and Supplementary Figs 1–3; see Supplementary Table 1 for localities).
Locality. Ercaicun (type locality), Haikou, Kunming, Yunnan Province, China.
Horizon. Yu’anshan Member, Chiungchussu Formation, Eoredlichia–Wutingaspis trilobite biozone, Nangaoan Stage of Chinese regional usage, Cambrian Series 2; Stage 3 (ref. 1).
Diagnosis for genus (monotypic) and species. Small, ‘bowling-pin’-shaped worm with a sub-circular-shaped attachment disc at the posterior end of the body and a through gut that is funnel-shaped anteriorly. At the end of the gut near the attachment disc, there is a tiny sub-circular structure, interpreted as the urogenital opening or anus.
The body is elongate; well-preserved specimens are up to 3.3 mm long and ‘bowling-pin’ shaped, with a gently curved outline (Fig. 2a,b). Narrow annulations about 100 µm in width are evident along the length of the body (Fig. 2a,b). I. fellatus is attached to the host at one end (Fig. 2d). This is inferred to be posterior, based on the interpretation of the constricted and expanded opposite end being a head, and an expansion of the gut at that end being a pharynx (Fig. 2c). The body is widest at one-quarter of the length from the attached end, narrowest at approximately three-quarters of the length and expanded distally into a free, elongate bulb with an acute tip (Fig. 2a,b,f and Supplementary Fig. 1c,d). The attachment area consists of a sub-circular-shaped disc (around 200–300 µm in diameter) that does not penetrate the cuticle of either M. sinensis or C. jinningensis (external annulation and cuticular ornamentation of the host are visible at the base of the attachment; Fig. 2e and Supplementary Fig. 3i–k). A swelling or depression along the edge of the attachment disc (Fig. 2e and Supplementary Fig. 3i–k) is the only indication of potential damage to the host’s cuticle. A dark trace, presumed to be the gut, extends along the mid-line of the body (Figs. 1a,b and 2a). At the end of the gut near the attachment disc, there is a tiny sub-circular feature interpreted as the urogenital opening or anus (evident only in the well-preserved holotype; Fig. 2a,b,d). At the opposite anterior end, the gut widens to form a funnel-shaped structure that is interpreted to be the pharynx (Fig. 2c).
I. fellatus individuals are each attached to respective C. jinningensis hosts in the central part of a single annulation (Figs. 1a,b, 2f and 3a,b). Attachment to M. sinensis is similarly central or at the edge of an annulation or extends across the boundary of adjacent annulations (Fig. 3d and Supplementary Fig. 3a–k). From 6 to 15 I. fellatus specimens are attached to individual C. jinningensis specimens, whereas M. sinensis individuals have 2 to 12 I. fellatus specimens attached (Supplementary Table 1). In some cases, I. fellatus forms aggregates of six or more on the host; for example, YKLP 13226 bears ten I. fellatus within a 12 mm section of the host (Fig. 1a,b) and YKLP 13229 bears six I. fellatus within a 6 mm section (Fig. 3c and Supplementary Fig. 2c). Within these aggregates, individuals can sometimes be evenly spaced (commonly by about 1 mm in YKLP 13231; Fig. 3a,b). There is no consistent placement of aggregates relative to the anterior or posterior ends of the host worms. I. fellatus individuals are attached on the inner-coiled surface (interpreted to be the ventral side by comparison with other Cambrian cycloneuralians2) in five host worms (for example, Fig. 1a and Supplementary Table 1). In one C. jinningensis host (Fig. 3a,b) and one M. sinensis host (Fig. 3d and Supplementary Fig. 3), I. fellatus is attached to both the ventral and dorsal surfaces. I. fellatus shows no evidence of bending or curving and its body is presumed to have been relatively stiff. Some specimens have been preserved perpendicular to the axis of the host, while others are sub-perpendicular, suggesting articulation at the attachment disc (for example, Fig. 1a,b).
There are insufficient morphological characters in I. fellatus to recognize the biological affinity of this taxon within Bilateria. Affinity with some vermiform phyla that are common symbionts is contradicted. A complete through gut negates affinities to Platyhelminthes, and a lack of external or internal segmentation is inconsistent with an assignment to the overwhelming majority of Annelida. The bowling-pin body shape and aboral attachment structure invite comparisons with gastrotrichs and rotifers, although both of these conflict with fundamental characteristics of I. fellatus. Notably, gastrotrichs have marked dorsoventral differentiation, the flattened ventral surface bearing dense cilia that facilitate benthic locomotion, whereas I. fellatus exhibits no obvious dorsal and ventral sides. No modification corresponding to the diagnostic corona of rotifers is observed. An identity as immature representatives of the cycloneuralian hosts is contradicted by I. fellatus being associated with two different hosts and the lack of an introvert, which is developed in Cambrian cycloneuralians even in the embryonic stages3.
Supposed symbiotic associations of animals (for example, parasitism, mutualism and commensalism4,5) occur throughout the Phanerozoic fossil record6,7,8,9,10,11, including the early Cambrian12. For example, within Cambrian Lagerstätten, there are examples of brachiopod epibionts on algae and on other brachiopods within the Chengjiang biota13,14, as well as a case of commensalism involving brachiopods on a Burgess Shale Wiwaxia host15. In the exceptionally preserved Silurian Herefordshire biota, a more sophisticated relationship has been demonstrated by pentastomid arthropods parasitic on ostracods11.
A parasitic relationship for I. fellatus with its host is unlikely, as the attachment disc is not associated with penetration of the host cuticle, the oral end of I. fellatus faces away from the host and I. fellatus appears to have been stiff, with no evidence that it could articulate backwards towards its host. Nevertheless, if I. fellatus were not feeding directly on its host, in those specimens with the most intensive infestation (Figs. 1a,b and 3a,b), this may have substantially inhibited host locomotion and ultimately could have proved deleterious to its well-being. Alternatively, the relationship may have been commensal (used here in the sense of being beneficial to the colonizer, but not the host), with I fellatus using its host worm for attachment as an epibiont, or the attachment may have been temporary and the relationship phoretic. Epibiontic or phoretic relationships commonly facilitate dispersal, protection or access to food sources. A chance association of I. fellatus with its host worm is the least parsimonious interpretation, given that some 53 individuals (or attachment discs) are associated with 7 host specimens from 3 different Chengjiang localities (Supplementary Table 1). Furthermore, the style of attachment is consistent across these specimens.
As indicated by the pattern of decay, I. fellatus and its hosts were attached in life, not via postmortem colonization of a host carcass for scavenging. In cases where the host specimen is well preserved—presumed to be in vivo (for example, C. jinningensis YKLP 13226; Fig. 1a)—or with minimal decay (for example, M. sinensis YKLP 13230 and 13232), attached I. fellatus specimens are also well preserved (Fig. 2e, Supplementary Fig. 2b,d,e and Supplementary Table 1). Host specimens showing evidence of decay lose the pattern of their annulation and body outline (for example, C. jinningensis YKLP 13231) and attached I. fellatus are also poorly defined (Fig. 3a,b). The only specimen (M. sinensis YKLP 13228) from a slowly deposited ‘background bed’ (interpreted as such by abundant algae in the sediment), rather than rapidly deposited ‘event bed’, shows more obvious decay and has the most poorly preserved I. fellatus, represented only by attachment discs (Fig. 3d and Supplementary Fig. 3a–k), or by specimens broken off near their base (insets in Fig. 3d). In colonizing its host, I. fellatus may have responded to a specific chemical, environmental or behavioural cue resulting in aggregates, with all individuals of a similar size on one host. This suggests colonization might have occurred via rapid asexual reproduction from one initial colonizer16 or, given the similar size of I. fellatus on each host, via pelagic or benthic larvae17. Given that in some specimens the infestation is extensive, host individuals were at this time likely exposed on the seabed, rather than in burrows18 (Fig. 4).
I. fellatus represents the earliest probable example of host specificity in the fossil record. The association of I. fellatus with two host species is comparable with modern commensal ecologies. Of 16 other priapulid or possible priapulid-like species in the Chengjiang biota1, none has I. fellatus associates, despite collection of the Chengjiang biota for over 30 years. This suggests a high degree of host specificity that does not appear to have been mediated simply by attachment morphology, as several other Chengjiang worm species have a similar cuticular structure to C. jinningensis and M. sinensis but lack I. fellatus. The possibility that material of I. fellatus represents more than one species of similar infesting worm is unlikely, given the similar size, shape, gut and attachment structure across the 53 specimens we examined.
The relationship of I. fellatus with two host species also indicates a capacity for host shift—the colonization of a new host species from an original host—and suggests that this type of ecology was already developed in early Cambrian ecosystems. Host shift is regarded as an important mechanism for sympatric speciation across many organismal groups19,20,21.
The infestation rates of I. fellatus are comparable to living analogues. Although instances of attached I. fellatus are rare, they are within the rates of, for example, modern polychaete commensal infestations16, with around 0.1% infestation of C. jinningensis with I. fellatus (3 of 3,098 specimens examined) and around 0.8% infestation of M. sinicus with I. fellatus (4 of 476 specimens examined). Although infestation is limited, it does not preclude this type of commensal ecology being more widespread in Cambrian marine ecosystems. Infestation rates vary in modern settings through environmental gradients such as water depth. The depositional basin of the Chengjiang biota extended over at least 1,000 km2 and had an eastwards sloping marine shelf22,23. I. fellatus is unknown east of localities around Haikou, even though specimens of Mafangscolex are reported from the deeper shelf settings of the Chengjiang area.
Animal burrows, possibly of priapulid origin24, define the Ediacaran–Cambrian System boundary25, an isochronous surface some 20 million years earlier than the Chengjiang biota. Trace fossils, possibly attributable to priapulids, have also been reported from the latest Ediacaran in strata younger than around 560–550 million years26,27. The development of complex host-specific ecologies, as demonstrated by I. fellatus, might therefore have evolved over a protracted interval of late Precambrian and early Cambrian time, the main burst of ecological diversification of animals being during the Cambrian explosion28,29. Previous work has shown that about one-third of the modes of life known from modern marine ecosystems were occupied during the early and middle Cambrian30. This compelling case of symbiosis documented herein identifies the earliest record of aggregate infestation, host specificity and host shift, and thus fills a gap in our knowledge of the complexity of Cambrian ecosystems.
Host specimen YKLP 13226 and the attached I. fellatus specimens were photographed with a Nikon D3X camera equipped with an AF-S VR Micro-Nikkor 105 mm lens. Host specimens YKLP 13227 and 13228 and I. fellatus specimens were photographed with a Canon 650D camera mounted with a Canon EF-S 60 mm macro lens or a Canon MP-E 65 mm (1–5×) macro lens. Host specimens YKLP 13229–13232 and I. fellatus specimens were photographed with a Canon 5D digital single-lens reflex camera on Nikon Multiphot macrophotographic equipment, including bellows extensions and Macro-Nikkor 6.5 cm or 12 cm lenses. Details of the holotype of I. fellatus (YKLP 13236) were captured with a Leica DFC5000 camera mounted to a Leica M205C stereomicroscope. Specimens were photographed in polarized or normal incident fibre optic light. Camera lucida drawings were produced with a Meiji Techno RZ stereomicroscope and traced in Adobe Illustrator CC 2014.2.2. Some specimens were prepared mechanically with needles under a stereomicroscope.
Scanning electron microscopy and X-ray spectroscopy
Scanning electron microscopy images were obtained using a LEO 1455VP at a 12 kV accelerating voltage and 16–18 Pa chamber pressure. Energy dispersive X-ray spectroscopy was performed using a FEI Quanta 650 FEG instrument at 15 kV accelerating voltage and 20 Pa chamber pressure. The X-ray spectroscopy images were prepared with selected colour within Adobe Photoshop CC 2014.2.2.
The data that support the findings of this study are available within the paper and Supplementary Information. All specimens (YKLP 13226–13232 and YKLP 13235–13288) are deposited at the Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, China.
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The reconstruction in Fig. 4 was produced by R. Nicholls (http://paleocreations.com). The research was funded by the National Natural Science Foundation of China (41572015 and U1302232), a Natural Environment Research Council Independent Research Fellowship (NE/L011751/1), Leverhulme Trust grants (RPG-2015-441 and EM 2014‐068), a Royal Society International Joint Project (IE131457) and a Yunnan Innovation Research Team grant (2015HC029).
Electronic supplementary material
Supplementary Figures, Supplementary Tables and Supplementary References