Ensis Directus Research Articles

Effects of a 5-year trawling ban on the local benthic community in a wind farm in the Dutch coastal zone

Abstract As part of a large impact study in a wind farm (OWEZ) in the Dutch coastal zone, the effects of exclusion of bottom trawling on the benthic community were studied by comparison with nearby reference areas which were regularly fished. In addition to a standard boxcorer for common macrofauna, a Triple-D dredge was used to collect longer-lived, more sparsely distributed infauna and epifauna. Multivariate analysis did not reveal any difference between the assemblages in and outside OWEZ with respect to abundance, biomass, and production after a 5-year closure. The Shannon–Wiener diversity index pointed to a significantly higher diversity in OWEZ compared with some of the reference areas. A minority of the bivalve species assumed to be sensitive to trawling showed higher abundances (Spisula solida) or larger sizes (Tellina fabula, Ensis directus) in OWEZ than in some of the reference areas. In general, samples collected with the Triple-D showed more differences between areas than boxcore samples. No evidence was also found that the species composition in OWEZ relative to the reference areas had changed in the period between 1 (2007) and 5 (2011) years after closure. The change observed in all areas between 2007 and 2011 was mainly due to relatively small variations in species abundances. In conclusion, 5 years after the closure of OWEZ to fisheries, only subtle changes were measured in the local benthic community, i.e. a higher species diversity and an increased abundance and lengths of some bivalves. Depleted adult stocks, faunal patchiness, and a limited time for recovery (5 years) might explain that a significant recovery could not be found. The current study shows that designation of large-scale marine protected areas as planned for the North Sea will not automatically imply that restoration of benthic assemblages can be expected within a relatively short period of years.

The growth and dynamics of Ensis directus in the near-shore Dutch coastal zone of the North Sea

Here we present data on the wax and wane of a subtidal Ensis directus population which settled in 2009 off the coast of Egmond (North Holland Coast in The Netherlands). Initial densities decreased from a maximum of 700m−2 in early 2010 to about 50m−2 in June 2013. In this period the average length increased from ~4cm to ~12cm. In 2011–2012 the population was sampled at 3 to 6week intervals and near bottom environmental conditions were monitored continuously. Samples of animals that were collected were used to follow the change in gonadal mass, tissue glycogen content, tissue weight and shell length. On the basis of these data well defined seasonal cycles were observed. The data indicate that the maturation of gonadal tissue already starts early in the year, initially at the expense of somatic tissue. Main spawning takes place in May. After spawning net somatic tissue growth starts after compensation of losses due to spawning. Somatic growth precedes shell growth which starts at water temperatures exceeding 12–14°C. Mortality, growth and production are comparable to those found for populations in close-by intertidal areas. As such there is no indication that this offshore population significantly suffers from nearby beach nourishments along the Dutch Coast.

Climate-induced range shifts of the American jackknife clam Ensis directus in Europe

Mapping the future potential distribution of alien species has become an issue of great concern. Ecological niche models are increasingly used to forecast the spatial range of introduced species in the context of climate warming. Here, we studied the potential spread of the American jackknife clam Ensis directus into European waters. E. directus, a marine bivalve native to the American coasts, was observed in Europe for the first time in the German Bight at the end of the 1970s. Afterwards, the clam quickly colonized the surrounding waters of the North Sea. Although many studies focused on its biology, ecology and colonization, the extent to which E. directus may invade European and Nordic seas remained poorly known. In this study, we used two ecological niche models (ENMs), calibrated on the native area of the mollusk, to evaluate the potential distributional range of the bivalve over European seas. Under current environmental conditions, E. directus should continue to progress towards the southern coasts of France and may also invade new areas in the Adriatic Sea. Projections for the end of the century suggest that the probability of occurrence of E. directus increases from Denmark to France with both ENMs. The Tunisian coasts may also become a new suitable area for the mollusk but the results of the two ENMs differ for this region. Therefore, contrary to what is often observed, a southward range expansion of E. directus is probable, especially as climate will get warmer.

Reproductive phenology of coastal marine bivalves in a seasonal environment

Environmental conditions during the larval phase (food concentration and temperature) impact recruitment success of marine bivalves by affecting growth and survival. We analysed the seasonal match between environmental conditions and larval presence of six coastal bivalve species over eight consecutive years (2006–2013) in the western Wadden Sea, taking species-specific physiological responses to these conditions into account. The larval occurrence of four species overlapped with their optimum in environmental conditions as calculated following the Dynamic Energy Budget theory: the occurrence of larvae of Cerastoderma edule, Mya arenaria and Mytilus edulis matched with their optimum food conditions, while larvae of Crassostrea gigas were mainly found during the temperature optimum of this species. Larvae of Ensis directus and Macoma balthica, however, only occurred months after their most favourable food conditions. The first group probably uses the energy immediately after uptake for the production and release of gametes (‘income breeders’), whereas the second group accumulates and stores energy reserves for later reproduction (‘capital breeders’). Larvae of C. gigas and M. balthica were observed much earlier in the year than expected from previous occurrences and from formerly know threshold temperatures for spawning. These apparent changes in timing of spawning suggest that Pacific oysters and Baltic tellins have recently acclimated to new environmental conditions.

Razor clam to RoboClam: burrowing drag reduction mechanisms and their robotic adaptation

Estimates based on the strength, size, and shape of the Atlantic razor clam (Ensis directus) indicate that the animal's burrow depth should be physically limited to a few centimeters; yet razor clams can dig as deep as 70 cm. By measuring soil deformations around burrowing E. directus, we have found the animal reduces drag by contracting its valves to initially fail, and then fluidize, the surrounding substrate. The characteristic contraction time to achieve fluidization can be calculated directly from soil properties. The geometry of the fluidized zone is dictated by two commonly-measured geotechnical parameters: coefficient of lateral earth pressure and friction angle. Calculations using full ranges for both parameters indicate that the fluidized zone is a local effect, occurring between 1–5 body radii away from the animal. The energy associated with motion through fluidized substrate—characterized by a depth-independent density and viscosity—scales linearly with depth. In contrast, moving through static soil requires energy that scales with depth squared. For E. directus, this translates to a 10X reduction in the energy required to reach observed burrow depths. For engineers, localized fluidization offers a mechanically simple and purely kinematic method to dramatically reduce energy costs associated with digging. This concept is demonstrated with RoboClam, an E. directus-inspired robot. Using a genetic algorithm to find optimal digging kinematics, RoboClam has achieved localized fluidization burrowing performance comparable to that of the animal, with a linear energy-depth relationship, in both idealized granular glass beads and E. directus' native cohesive mudflat habitat.

Growth increment periodicity in the shell of the razor clam <i>Ensis directus</i> using stable isotopes as a method to validate age

Abstract. To evaluate the role of the razor clam Ensis directus in Dutch coastal waters, understanding its population dynamics is important. As such, the age structure of the population forms a key parameter. Accurate age determination in bivalve shells is not always straightforward due to the difficulty of interpreting externally visible growth lines. In the present paper, we aimed at validating the seasonality in growth line formation using visual techniques in combination with stable oxygen and carbon isotope analyses (δ18O and δ13C). High δ18O values in the shell coincided with growth marks on the external surface of the valve and in acetate peels of the shell's cross section. Most shell δ18O samples were assigned to the months from June to September. From November to March no samples were retrieved, indicating that the shell did not grow. The lowest reconstructed temperature (6.3 °C) suggests that ~ 6 °C may be the threshold temperature for growth. Nevertheless, most of the reconstructed values fell above 14.5 °C, indicating that growth occurred mainly in the summer at relatively high temperatures. Shell δ13C profiles followed a more or less seasonal cycle, but no direct relationship could be made between δ13C values and annual growth lines. Although counting external annual growth lines led to a correct estimation of age and consequently of growth rates, we recommend analysing acetate peels of cross sections to support the distinction between annual lines and disturbance lines.

Effect of algae- and silt concentration on clearance- and growth rate of the razor clam Ensis directus, Conrad

Laboratory experiments were carried out with Ensis directus to estimate clearance rate and growth rate as a function of the amount of food and presence of silt. Two food levels were tested: low food availability (6.5μg chla/l) and high food availability (16.5μg chla/l) at four silt concentrations (0, 50, 150 and 300mg/l). Clearance rates of E. directus varied between 0.7 and 5.9l/h/g DW. At a silt concentration of 300mg/l clearance rates were significantly lower (16–56%) than rates at 150mg/l, 50mg/l and 0mg/l silt. This indicates that only the highest silt concentration induced a reduction in clearance rate. The tested food levels did not influence the clearance rate of E. directus. Maximum growth rate of E. directus was 0.24mm increase in shell length and 2% increase in wet weight per day. Long-term (10weeks) exposure to silt concentrations of 300mg/l showed significantly higher growth than the 150mg/l treatment. This indicates that exposure to a high silt concentration did not induce a reduction in growth, but stimulated growth. Long-term (10weeks) exposure to a food level of 6.5μg chla per litre reduced the shell growth of E. directus with 40% compared to growth at 16.5μg chla per litre. The laboratory experiments suggest that E. directus is more sensitive to a reduction in algal concentration than to an increase in silt concentration.

Distinctly variable mudscapes: Distribution gradients of intertidal macrofauna across the Dutch Wadden Sea

The Wadden Sea is a shallow coastal region, with a large area of sedimentary tidal flats that extends from The Netherlands to Denmark and has been declared a site of international importance in the Dutch and German parts (Ramsar status and UNESCO World Heritage Site). Benthic macrofauna are central to the ecosystem functioning of this area, as they recycle nutrients, decompose organic matter and are an important food source for many secondary consumers, like fish and waterbirds. Due to the environmental gradients characteristic of estuarine systems, it is expected that changes in assemblage composition will be observed across the physical and environmental gradients of the Wadden Sea. First, we explored the spatial variation in assemblage composition of benthic macrofauna across the intertidal part of the Dutch Wadden Sea using 3years of biomass data. Then, we identified the relative importance of six environmental variables for explaining and predicting changes in assemblage composition across the intertidal areas of the Wadden Sea using generalised dissimilarity modelling (GDM). In accordance with the environmental gradients across this system, the biomass contributed by a few common species differed from west to east and were distinct in the Dollard. In the west, bivalves Mya arenaria, Cerastoderma edule and Ensis directus contributed a relatively large and equal share of the total biomass, whereas C. edule contributed the sole largest share of the total biomass towards the east. The polychaete Alitta succinea became a large share of the total biomass in the upper Ems and in the Dollard estuary, but contributed little elsewhere. Similar to the observed differences in species composition, the spatial patterns in assemblage composition, as predicted by the GDM models, identified the Dollard as distinct and that the prevalence of assemblage types in the west differed to the east. Median grain size, followed by microphytobenthic biomass, and exposure time were the most important variables describing differences in assemblage composition. That the Wadden Sea forms a heterogeneous landscape where assemblage composition varies across multiple gradients has repercussions for management and monitoring.

Population genetic analysis of Ensis directus unveils high genetic variation in the introduced range and reveals a new species from the NW Atlantic

We report current genetic variation of populations of the razor shell Ensis directus (Conrad 1843) (Mollusca: Bivalvia: Pharidae) in native (North American) and introduced (European) ranges using nuclear and mitochondrial sequence-based markers. We expected less variation within the introduced range, especially considering the frequent mass mortality events observed in Europe since the species was recorded for the first time in 1978. However, we found higher variation in Europe. The possible significance of temporal fluctuations of genetic variation, limited effect of random genetic drift, and multiple introductions are discussed. Interestingly, the multiple-introduction hypothesis contrasts with the gradual colonisation of European coastal waters but is supported by trained clustering analysis and by the intensity of transatlantic shipping. Genetic and morphometric evidence strongly supports that examined individuals from a supposed E. directus population from Newfoundland (Canada) belong to a separate species. This new Ensis is formally described here and named E. terranovensis n.sp.

Localized fluidization burrowing mechanics ofEnsis directus

Muscle measurements of Ensis directus, the Atlantic razor clam, indicate that the organism only has sufficient strength to burrow a few centimeters into the soil, yet razor clams burrow to over 70 cm. In this paper, we show that the animal uses the motions of its valves to locally fluidize the surrounding soil and reduce burrowing drag. Substrate deformations were measured using particle image velocimetry (PIV) in a novel visualization system that enabled us to see through the soil and watch E. directus burrow in situ. PIV data, supported by soil and fluid mechanics theory, show that contraction of the valves of E. directus locally fluidizes the surrounding soil. Particle and fluid mixtures can be modeled as a Newtonian fluid with an effective viscosity based on the local void fraction. Using these models, we demonstrate that E. directus is strong enough to reach full burrow depth in fluidized soil, but not in static soil. Furthermore, we show that the method of localized fluidization reduces the amount of energy required to reach burrow depth by an order of magnitude compared with penetrating static soil, and leads to a burrowing energy that scales linearly with depth rather than with depth squared.

Long-term dynamics and productivity of a successful invader: The first three decades of the bivalve Ensis directus in the western Wadden Sea

Results are reported of a long-term and multi-station monitoring study in the western Wadden Sea on numerical densities, biomass, individual weights, and production of populations of the newly (late 1970s) introduced bivalve Ensis directus. Data are available on 18 fixed sites, sampled twice-annually ever since 1982 (13 truly intertidal stations and 2 in the transition zone to the subtidal) or 1989 (3 subtidal stations). After a slow start in the 1980s, numbers and biomass rapidly increased during the last 2 decades to reach locally high values in recent years: annual averages of tens of g ash-free dry mass m−2 in the subtidal and even of around 100gAFDMm−2 in the transition zone, where it became the dominant species as to biomass and annual production. Temporal and local variability in production (and resulting biomass) could largely be explained by variation in numerical densities of 0-group individuals at the end of the first winter of their life. Highest densities and growth rates were observed in the transition zone. In the true subtidal, recruitment and growth rates were on average lower. In the true intertidal, abundances remained invariably low, as a consequence of relatively low recruitment and in particular poor subsequent survival. Within the “optimal” transition zone along tidal channels, the benthic habitat is highly dynamic, as it is exposed to physical disturbance resulting from both wave action and strong tidal currents. E. directus appears to be one of the few large-bodied benthic invertebrate species that can cope with the unstable sands in this zone. Most of its exceptional success may be explained by the occupation of this probably under-utilized habitat in Europe.

First Record ofMercenaria mercenaria(Bivalvia: Veneridae) andEnsis directus(Bivalvia: Pharidae) on Bay of Biscay, Iberian Peninsula

ABSTRACT We present the first record of two nonnative Bivalvia species originally from North America, Mercenaria mercenaria and Ensis directus, in the waters of the Iberian Peninsula (southwest of the Bay of Biscay). This constitutes the southernmost distribution for these species in the northeast Atlantic to date. We provide notes on the relative abundance, depth range, and substrate preferences of the species in their new habitat. The bivalve E. directus has became the dominant Ensis species along the Cantabric sandy shores it has colonized.

The fate of an immigrant: Ensis directus in the eastern German Bight

We studied Ensis directus in the subtidal (7–16 m depth) of the eastern German Bight. The jack-knife clam that invaded in the German Bight in 1978 has all characteristics of a successful immigrant: Ensis directus has a high reproductive capacity (juveniles, July 2001: Amrumbank 1,914 m−2, Eiderstedt/Vogelsand: 11,638 m−2), short generation times and growths rapidly: maximum growth rates were higher than in former studies (mean: 3 mm month−1, 2nd year: up to 14 mm month−1). Ensis directus uses natural mechanisms for rapid dispersal, occurs gregariously and exhibits a wide environmental tolerance. However, optimal growth and population-structure annual gaps might be influenced by reduced salinity: at Vogelsand (transition area of Elbe river), maximum growth was lower (164 mm) than at the Eiderstedt site (outer range of Elbe river, L ∞ = 174 mm). Mass mortalities of the clams are probably caused by washout (video inspections), low winter temperature and strong storms. Ensis directus immigrated into the community finding its own habitat on mobile sands with strong tidal currents. Recent studies on E. directus found that the species neither suppresses native species nor takes over the position of an established one which backs up our study findings over rather short time scales. On the contrary, E. directus seems to favour the settlement of some deposit feeders. Dense clam mats might stabilise the sediment and function as a sediment-trap for organic matter. Ensis directus has neither become a nuisance to other species nor developed according to the ‘boom-and-bust’ theory. The fate of the immigrant E. directus rather is a story of a successful trans-ocean invasion which still holds on 23 years after the first findings in the outer elbe estuary off Vogelsand.

Identification and Evaluation of the Atlantic Razor Clam (Ensis directus) for Biologically Inspired Subsea Burrowing Systems

In this article, we identify and analyze a subsea organism to serve as a model for biologically inspired burrowing technology to be used in applications such as anchoring, installation of cables, and recovery of oil. After inspecting myriad forms of life that live on or within ocean substrates, the Atlantic razor clam, Ensis directis, stood out as an attractive basis for new burrowing technology because of its low-energy requirements associated with digging (0.21 J/cm), its speed and depth of burrrowing (∼1 cm/s and 70 cm, respectively), and its size and simplicity relative to man-made machines. As anchoring is a prime application for the technology resulting from this work, the performance of an Ensis directus-based anchoring system was compared to existing technologies. In anchoring force per embedment energy, the E. directus-based anchor beats existing technology by at least an order of magnitude. In anchoring force per weight of device, the biologically inspired system weighs less than half that of current anchors. The article concludes with a review of E. directus's digging strategy, which involves motions of its valves to locally fluidize the substrate to reduce burrowing drag and energy, and the successful adaptation of E. directus's burrowing mechanisms into an engineering system: the RoboClam burrowing robot, which, like the animal, uses localized fluidization to achieve digging energy that scales linearly with depth, rather than depth squared, for moving through static soil.

Dynamics of digging in wet soil

Numerous animals live in, and locomote through, subsea soils. To move in a medium dominated by frictional interactions, many of these animals have adopted unique burrowing strategies. This paper presents a burrowing model inspired by the Atlantic razor clam ( Ensis directus), which uses deformations of its body to cyclically loosen and re-pack the surrounding soil in order to locally manipulate burrowing drag. The model reveals how an anisotropic body – composed of a cylinder and sphere varying sinusoidally in size and relative displacement – achieves unidirectional motion through a medium with variable frictional properties. This net displacement is attained even though the body kinematics are reciprocal and inertia of both the model organism and the surrounding medium are negligible. Our results indicate that body aspect ratio has a strong effect on burrowing velocity and efficiency, with a well-defined maximum for given kinematics and soil material properties.

The role of the invasive bivalve Ensis directus as food source for fish and birds in the Dutch coastal zone

The razor clam Ensis directus was introduced to Europe presumably as larvae in ballast water around 1978. Starting in the German Bight it spread northward and southward along the continental coastline. Currently it is the most common shellfish species in the Dutch coastal zone, where it mainly occurs in the Voordelta and off the Wadden Sea islands. The mean density of E. directus in the Dutch coastal zone increased from around 2–5 individuals m −2 in the late ‘90’s to around 12–19 individuals m −2 from 2002 onwards. Diet studies show that E. directus makes up a significant proportion in the current diet of plaice, sole, dab, flounder and dragonet and in the diet of eider and common scoter. In recent years E. directus contributed 20–100% of the total wet weight in fish stomachs. The proportion E. directus in the diet increases with fish length. Based on stomach contents of oiled and beached birds and of faeces samples the recent frequency of occurrence is 85–90% in eider and 26% in common scoter. Also waders, gulls and corvids prey on E. directus but the contribution to the diet is still unquantified. Because of its great burying depth the species is not easily accessible. Fish either profit from massive die-offs that regularly occur, or they extract (probably only the smaller) individuals from the sediment. Sea ducks can extract E. directus from the sediment, while shorebirds and gulls feed on dying E. directus washing up on the shore. E. directus is possibly an important food item for fish and seabirds when they occur in high densities and in the right size classes. Since the availability depends greatly on massive die-offs, shell size, burying depth and water depth, it is probably not a very reliable food source. Judging from the role E. directus currently plays for the higher trophic levels, its introduction must have caused a major change in the food relations in its distribution area.

Increasing species richness of the macrozoobenthic fauna on tidal flats of the Wadden Sea by local range expansion and invasion of exotic species

A 40-y series of consistently collected samples (15 fixed sampling sites, constant sampled area of 15 × 0.95 m2, annual sampling only in late-winter/early-spring seasons, and consistent sieving and sorting procedures; restriction to 50 easily recognizable species) of macrozoobenthos on Balgzand, a tidal flat area in the westernmost part of the Wadden Sea (The Netherlands), revealed significantly increasing trends of species richness. Total numbers of species annually encountered increased from ~28 to ~38. Mean species density (number of species found per sampling site) increased from ~13 to ~18 per 0.95 m2. During the 40 years of the 1970–2009 period of observation, 4 exotic species invaded the area: (in order of first appearance) Ensis directus, Marenzelleria viridis, Crassostrea gigas, and Hemigrapsus takanoi. Another 5 species recently moved to Balgzand from nearby (subtidal) locations. Together, these 9 new species on the tidal flats explained by far most of the increase in total species numbers, but accounted for only one-third of the observed increase in species density (as a consequence of the restricted distribution of most of them). Species density increased particularly by a substantial number of species that showed increasing trends in the numbers of tidal flat sites they occupied. Most of these wider-spreading species were found to suffer from cold winters. During the 40-y period of observation, winter temperatures rose by about 2°C and cold winters became less frequent. The mean number of cold-sensitive species found per site significantly increased by almost 2 per 0.95 m2. Among the other species (not sensitive to low winter temperatures), 6 showed a rising and 2 a declining trend in number of occupied sites, resulting in a net long-term increase in species density amounting to another gain of 1.6 per 0.95 m2. Half of the 50 studied species did not show such long-term trend, nor were invaders. Thus, each of 3 groups (local or alien invaders/winter-sensitive species/other increasing species) contributed to a roughly similar extent to the overall increase in species density.

A simple one-step PCR method for the identification between European and American razor clams species

A specific multiplex polymerase chain reaction (PCR) was applied to differentiate samples of razor clams Ensis arcuatus, Ensis siliqua, Ensis directus, and Ensis macha. Universal primers were used for the amplification of internal transcribed spacer 1 (ITS-1) in each species. The alignment of the obtained sequences was the basis for the specific design of species-specific reverse primers (ITSArSil-R, ITSDir-R, and ITSMa-R) located in the ITS-1 region. A multiplex PCR using each specific primer together with a common forward primer allowed identification of razor clam species by means of the different sizes of the species-specific amplicons separated in an agarose gel electrophoresis. This work provides a simple, reliable and rapid protocol for the accurate identification of Ensis species. The present methodology can be very useful for traceability of the species and to reinforce labelling regulations.

Evaluation of indicators used to detect mussel farm influence on the benthos: Two case studies in the Magdalen Islands, Eastern Canada

The aim of this study was to identify appropriate indicators to determine the influence of mussel aquaculture on the benthic environment. Both sediment [particle size, sediment profile imaging (SPI), % OM] and benthic community (abundance, biomass, number of species, Margalef’s species richness, Shannon–Weiner diversity, Pielou’s eveness, individual body mass, trophic group, a biotic index — AMBI, and community structure) characteristics were evaluated at two mussel farms in Great-Entry (GE) and Havre-aux-Maisons (HAM) lagoons in the Magdalen Islands (Quebec, Canada). Sampling stations were positioned directly beneath the outer-most mussel lines (0 m) and at distances of 3, 6, 9, 15, 30 m and at a control site (at either 300 or 500 m) along a transect leading from each farm. Contrasting patterns were observed. At GE, sediment characteristics and benthic communities did not vary among stations and were characterized by low diversity, abundance and biomass. At HAM, % OM decreased and macrofaunal diversity and abundance increased with increasing distance from the farm. Biomass was low under the mussel line, increased between 3 and 30 m and was low again at 300 m. This was explained by the abundance of the polychaete Pectinaria granulata, which seems to have benefited from a moderate organic loading associated with the mussel farm. The mean individual biomass of the second-order opportunistic deposit feeders P. granulata and Heteromastus filiformis decreased with distance from the farm, whereas that of the pollution-sensitive suspension feeder Ensis directus and deposit feeder Tellina agilis increased with increasing distance from the farm. At HAM, the effects of mussel farming were restricted to the vicinity of the farm, while at GE the pattern was less clear. The GE mussel farm had either little effect on the local environment or else larger-scale but diffuse effects. The study showed that the a priori choice of the sampling stations and indicators may influence the interpretation of the results. Community structure and SPI were the most efficient indices for detecting both small- and broader-scale influences at both studied mussel farms.

BALANULITHS: FREE-LIVING CLUSTERS OF THE BARNACLE BALANUS CRENATUS

Abstract Free-living clusters of barnacles are described as nucleations around shell fragments of Ensis directus. The bivalve E. directus is an invader in European seas that exhibits mass mortalities, during which the animals creep partly out of the sediment. Barnacles can then settle on the inside and the outside of the posterior part of these empty, articulated, semi–in situ shells. This posterior, encrusted portion apparently became separated from the remainder of the valve, enabling the barnacles to continue life as a free-living balanulith.