Burrow Diameter Research Articles

A COMBINED TRACE- AND BODY-FOSSIL APPROACH REVEALS HIGH-RESOLUTION RECORD OF OXYGEN FLUCTUATIONS IN DEVONIAN SEAS

Black shales deposited in epeiric seas preserve dynamic, bottom-water oxygen conditions as inferred from high-resolution trace- and body-fossil data. Nearly all paleoproxies are imperfect; however, this study utilizes the strength of paleobiological proxies to capture variability in bottom-water oxygen levels through aerobic and anaerobic, as well as dysaerobic (reduced but non-zero oxygen) conditions. Trace- and body-fossil data were collected at high resolution from twelve localities through central and western New York State, United States, exposing Devonian black shales. The combination of relative amount of bioturbation, estimated as ichnofabric index (ii), maximum burrow diameter measurements, and body-fossil species diversity allows relative bottom-water oxygen levels to be interpreted on a millimeter scale. Overall, trace-fossil size and diversity through the dysaerobic zone are much reduced compared with those described from younger strata. The decreased depth of bioturbation compared with younger strata, and resulting reduced overprinting of the sedimentological record by deep burrowing infauna, provide the opportunity for records of high frequency fluctuations in relative oxygen levels to be preserved. The biological signal preserved in these units reveals that bottom-water oxygen levels fluctuated considerably within a narrow stratigraphic range (on a centimeter scale) and that the dysaerobic zone is not temporally stable.

Rapid Assessment of Anthropogenic Impacts of Exposed Sandy Beaches in Ghana using Ghost Crabs (<i>Ocypode</i> Spp.) as Ecological Indicators

We applied a rapid assessment methodology to estimate the degree of human impact of exposed sandy beaches in Ghana using ghost crabs as ecological indicators. The use of size ranges of ghost crab burrows and their population density as ecological indicators to assess extent of anthropogenic impacts on beaches was explored in this study. For each site, three transects were laid perpendicular to the shoreline over a 100 meter distance at 50 m intervals, i.e., at reference points 0, 50 and 100 meter points. Sampling locations were randomly selected along the three transects using a 1 m x 1 m quadrat. Measurements were done twice weekly for a period of four weeks. The results showed that even though the moderately disturbed beach had higher burrow density than the disturbed beach on the average, the difference is not statistically significant (T-test; p > 0.01). However, mean burrow diameter at the moderately disturbed site was statistically found to be significantly larger than for the disturbed site (T-test; p < 0.01). We conclude that burrow sizes is a good estimator for verification of human impacts of exposed sandy beaches. It confirms that burrow density even though an important factor, may not necessarily be a significant estimator of the impacts of human activity on beaches. On this basis, it appears that the estimations of the diameter of burrows of crabs of the Ocypode genera provides a rapid tool for impact verification of sandy beaches and for use in environmental quality monitoring of beach programs in coastal areas.

Measurement techniques for quantification of pumping activity of invertebrates in small burrows

The quantification of exchange processes across sediment-water interfaces needs precise measurements of burrow ventilation of tube-dwelling invertebrates, including small species which can occur in high densities. Since most studies have focused on relatively large organisms, established techniques are usually suitable for burrow diameters clearly > 2 mm. Our aim was to test video analyses, colour tracers, O 2 and flow velocity microelectrodes, thermal flow sensors, particle image velocimetry, and conductive exchange experiments, for quantifying pumping activity in burrows ≤ 2 mm in diameter in muddy sediment. To assure comparability, all measurements were performed with the same species (Chironomus plumosus, 4 th instar larvae). On average we determined a flow velocity of 14.9 mm s -1 , an individual pumping time of 33 min h -1 , an individual pumping rate of 61 ml h -1 (1.7 mm burrow diameter), and a population pumping rate of 1.09 m 3 m -2 d -1 (745 larvae m -2 , 20-23 °C). Our values are higher than mostly reported for other small invertebrates. The differences might be caused by the artificial set-ups used in previous studies, and by the fact that C. plumosus conducts pumping not only to meet its oxygen demand such as other species but also to filter-feed. All seven techniques tested in this study are applicable for burrow diameters of 1.7-2 mm in muddy sediments. We recommend colour tracers and particle image velocimetry to measure flow velocity, and flow velocity microelectrodes and thermal flow sensors to determine the individual pumping time. Conductivity exchange experiments are suitable to directly measure individual pumping rates.

Early Cambrian Teichichnus-dominated ichnofabrics and palaeoenvironmental analysis of the Caerfai Group, Southwest Wales, UK

The early Cambrian Caerfai Group of Pembrokeshire in Southwest Wales yields an intensely bioturbated Teichichnus-dominated ichnofabric, and provides evidence for the early evolution of infaunal organisms. Three units of the Caerfai Group comprise the focus of this study in sections exposed at Caerfai Bay. In the St. Non's Sandstone few primary sedimentary structures are observed, bedding being predominantly massive with subordinate parallel and current ripple cross-lamination. The topmost St. Non's Sandstone contains common soft-sediment deformation phenomena, and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep-fronted delta system. Teichichnus is the dominant trace, with subordinate Planolites, Palaeophycus and possible Rhizocorallium. The overlying Caerfai Bay Shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front. Sandstone debrites are common, as are tuffaceous beds with evidence of reworking by bottom currents. Sandstone and tuff event beds contain common Skolithos and Arenicolites with less frequent Planolites. Interbedded mudstones contain infrequent Teichichnus. The Caerfai Bay Shales coarsen and thicken-upwards into the Caerbwdy Sandstone. Debrite sandstone beds dominate, and bed thickening upsection indicates progradation of the delta system. The dominant trace fossil within the unit is Teichichnus which is constrained to the top few centimetres of individual sandstone beds. Less common are Planolites and Palaeophycus. The size and abundance of Teichichnus in the Caerfai Group are similar to, or exceed those in contemporaneous sections in Avalonia and Baltica. Local environmental factors were probably the primary control on trace fossil distribution. Substrate and salinity are proposed as the main influences on ichnodiversity and size in the Caerfai Group. Teichichnus is most abundant in the St. Non's Sandstone, where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels. Burrow diameter progressively decreases upsection, and bioturbation becomes less pervasive in subsequent units of the Caerfai Group. This indicates that by the Cambrian Stage 2, significant advances into reduced salinity environments had occurred, along with the dispersal of the Teichichnus producer throughout the Avalonia-Baltica seaway. Correlation within Avalonia indicates that a transgression in the Cambrian Stage 2 initiated the deposition of the Caerfai Group marking the onset of the Welsh Basin.

Neighbour density, body size and anti-predator hiding time in the New Zealand mud-crabAustrohelice crassa

Many preys retreat into a refuge as a response to the presence of a predator, a behavioural strategy which guarantees safety but is also costly due to a trade-off between hiding time and time spent in other essential activities. The balance between costs and benefits of hiding, which are influenced by different factors, determine the hiding duration. In a field experiment, the anti-predator behaviour of the mud crabAustrohelice crassawas studied to assess the effects of two factors, body size and neighbour density, on the time spent hiding in burrows following a predator threat. Hiding times, body size (estimated from burrow diameter) and neighbour density (number of other burrows within a 30 cm radius) were measured for 158 individual crabs during a snorkelling survey. Regression analyses showed that hiding time of individual crabs significantly increased with increasing body size, and decreased with increasing neighbour density. These trends are the result of three main selective pressures: size-biased predation risk, dilution of predation in dense clusters of burrows, and more intense competition in dense clusters. Variation among individual crabs probably reflects differences in the balance between costs and benefits for crabs differing in body size and neighbour density.

Burrow morphology and associated animals of the mud shrimpUpogebia yokoyai(Crustacea: Thalassinidea: Upogebiidae)

The burrow morphology of the mud shrimpUpogebia yokoyaiwas investigated on a tidal flat in the Nanakita River mouth in north-eastern Japan usingin situresin casting. A total of 26 burrow casts were recovered, including those of 16 large shrimps and 10 small shrimps. Burrows of large shrimp were relatively simple and Y-shaped with depth exceeding 1.2 m. Although burrow diameter was related to shrimp size, correlation with other burrow measurements was low. Three large casts were connected to others via a narrow horizontal portion potentially reflecting mating behaviour of the shrimp. Burrows of small shrimp were more complex than those of the other upogebiids and were connected to large burrows. In 6.7% of cases, bopyrid isopods were present in the branchial chamber. Three species of gobies were found in the burrows. These data show that burrows ofU. yokoyaiserve not only as a recruitment site for conspecific shrimp, but also as habitat for other animals in the tidal flat.

Bioturbation in Burgess Shale-type Lagerstätten — Case study of trace fossil–body fossil association from the Kaili Biota (Cambrian Series 3), Guizhou, China

Cruziana, Gordia, Planolites, Rusophycus, and Trichophycus are common ichnological elements of the Kaili Biota. New discoveries based on the examination of 323 specimens include eldoniids, echinoderms, trilobites, monoplacophorans, and non-biomineralizing arthropods that are associated with trace fossils. Based on the observed effects of bioturbation on the preservation of five different animal groups, it is clear that infaunal scavengers/deposit feeders were periodically active on the Kaili sea floor and were able to reach historic layers yielding exceptionally preserved fossils. In general, the average level of infaunal activity is absent to moderate (Ichnofabric Index [i.i.] = 1 to 3) in the Kaili substrate; by contrast, the “ Phycodes beds” are completely disturbed by infaunal activity (i.i. = 5). Observed burrow diameter ranges from diminutive (∼ 0.2 mm) to normal (up to 4.2 mm). Computed tomography allows us to visualize the precise geometry of the trace fossil–body fossil association in three dimensions. We concluded that although some Kaili infaunal animals could bore through the biomineralized echinoderms, they did not appear to have scavenged upon these echinoderms based on our three-dimensional reconstruction. Furthermore, Kaili burrowers can reach the historic layers containing exceptionally preserved fossils without altering soft-tissue preservation. On the other hand, we used conventional techniques to reveal that one burrow is filled with fecal pellets (100–200 μm) at its terminal end. Because most burrows are filled with yellow/brown “coarse sediment”, and because there is no compositional difference between the fecal pellets and the surrounding coarse sediment within the burrow, the origin of the yellow/brown “coarse sediment” is interpreted here as parautochthonous and/or autochthonous and as a result of sediment reworking by deposit (or suspension) feeders. The relative scarcity of soft-part preservation in the Kaili Biota compared to the older Chengjiang Biota and the younger Burgess Shale Biota may be the result of post-burial bio-disturbance due to the relatively high intensity of local bioturbation. It is plausible that oxygenation in deeper sediments (facilitated by bioturbation) allowed carcasses in the Kaili biota to undergo a more extensive period of ‘normal’ decay prior to final burial than in other Cambrian Konservat- Lagerstätten.

Non-destructive sampling techniques for the rapid assessment of population parameters in estuarine shore crabs

Retrieving burrowing shore crabs to obtain information regarding their morphological attributes is often difficult and results in significant disturbance and habitat modification. A simple, rapid and non-destructive sampling method was proposed for indirectly estimating crab abundance, morphology and mass through the counting of burrow numbers and their diameter. Measurement of the number of crab burrows was found to correlate with crab abundance via both excavation and video observation techniques. Burrow diameters of individuals were also correlated with carapace width and body mass. Measuring burrow opening diameters along with the number of holes allows an indirect estimate of morphology and mass, which increases the information gained through such rapid sampling techniques and allows assessment of both structural and functional processes at the population level which may be affected through anthropogenic disturbance.

Meiobenthic trace fossils comprising a miniature ichnofabric from Late Permian carbonates of the Oman Mountains

An ichnofabric composed of meiobenthic trace fossils is described from Late Permian shelf carbonates of the Arabian Platform from the Oman Mountains (Saiq Formation, Akhdar Group). It mainly consists of an 8–10 cm deep boxwork with an undulating burrow outline, with an average burrow diameter of about 0.5 mm. The pinch-and-swell-like features lead to bulbous enlargements and alternating blade-like contractions and are diagnostic for Virgaichnus undulatus igen. et isp. nov. In the upper tier of the ichnofabric, V. undulatus is associated with miniature spreiten-like lobes, which originate from a vertical shaft. These circular, elliptical or elongated lobes in the centimetre-range consist of irregularly arranged probes less than 1 mm in diameter and are assigned to Hydrancylus isp. The assessment of burrowing mechanism, burrow complexity, burrow size and geological age indicates worm-like organisms as potential trace makers for V. undulatus. The undulating shape of the burrow segments is in agreement with a peristaltic movement of a non-segmented, highly deformable vermiform body as known from nemerteans. The presented case demonstrates that morphology in combination with size is invaluable for the interpretation of the trace maker and consequently strengthens the importance of meiobenthic trace fossils for palaeoenvironmental and facies analysis.

Transient dwarfism of soil fauna during the Paleocene-Eocene Thermal Maximum.

Soil organisms, as recorded by trace fossils in paleosols of the Willwood Formation, Wyoming, show significant body-size reductions and increased abundances during the Paleocene-Eocene Thermal Maximum (PETM). Paleobotanical, paleopedologic, and oxygen isotope studies indicate high temperatures during the PETM and sharp declines in precipitation compared with late Paleocene estimates. Insect and oligochaete burrows increase in abundance during the PETM, suggesting longer periods of soil development and improved drainage conditions. Crayfish burrows and molluscan body fossils, abundant below and above the PETM interval, are significantly less abundant during the PETM, likely because of drier floodplain conditions and lower water tables. Burrow diameters of the most abundant ichnofossils are 30-46% smaller within the PETM interval. As burrow size is a proxy for body size, significant reductions in burrow diameter suggest that their tracemakers were smaller bodied. Smaller body sizes may have resulted from higher subsurface temperatures, lower soil moisture conditions, or nutritionally deficient vegetation in the high-CO(2) atmosphere inferred for the PETM. Smaller soil fauna co-occur with dwarf mammal taxa during the PETM; thus, a common forcing mechanism may have selected for small size in both above- and below-ground terrestrial communities. We predict that soil fauna have already shown reductions in size over the last 150 years of increased atmospheric CO(2) and surface temperatures or that they will exhibit this pattern over the next century. We retrodict also that soil fauna across the Permian-Triassic and Triassic-Jurassic boundary events show significant size decreases because of similar forcing mechanisms driven by rapid global warming.

Quantification of pumping rate of Chironomus plumosus larvae in natural burrows

This paper investigates and compares experimentally determined water velocity field above natural macrozoobenthos burrows generated by Chironomus Plumosus larva during their bio-irrigation activity. All experiments were carried out using particle image velocimetry and performed in mesocosms filled with sediment burrowed by larvae, and the water velocity fields near the inlets and outlets of the U-shaped burrows were measured. From water velocity data the average volumetric flow rates between 54.6 and 61.1 mm3/s were calculated. Assuming an average burrow diameter of 2.25 mm, the volumetric flow rates suggest the average flow velocities through burrows during the pumping period between 13.7 and 15.4 mm/s. Two additional interesting phenomena could also be shown by analyzing the flow field generated by the larva. The analysis of the amount of tracers used for visualizations revealed that some of the tracer particles added to the water must have been consumed along their path from the inlet toward the outlet, hinting clearly to the so-called filter-feeding action of C. plumosus. The second phenomenon is due to the form of motion C. plumosus generates. By careful flow visualizations it was found that unlike other organisms such as Urechis caupo that use peristaltic body contractions, C. plumosus worms its body sinusoidally catapulting the fluid far into the overlying water body. This action is of ecological advantage for it avoids generating short oxygen circuits for their respiration and filter feeding.

Burrow morphologies of the ghost crabs &lt;i&gt;Ocypode ceratophthalma&lt;/i&gt; and &lt;i&gt;O. sinensis&lt;/i&gt; in foreshore, backshore, and dune subenvironments of a sandy beach in Japan

Burrow morphologies of the ghost crabs Ocypode ceratophthalma and O. sinensis were observed in foreshore, backshore, and dune subenvironments on the Irino Coast, southwestern Japan, with the aim of clarifying their paleoenvironmental significance. The observed diameter of the main shaft, burrow depth, and total burrow length varied from 14 to 40 mm, 12 to 89 cm, and 8 to 143 cm, respectively. Although the burrow diameters are largely similar among the three subenvironments, the burrows increased in length and complexity from the foreshore to dune subenvironments. These findings indicate that burrow morphology can be used as an indicator of ancient sandy beach subenvironments.

Densidade e distribuição espacial do caranguejo Ocypode quadrata (Fabricius, 1787) (Crustacea, Ocypodidae) em três praias arenosas do Espírito Santo, Brasil

Density and spatial distribution of the crab Ocypode quadrata (Fabricius, 1787) (Crustacea, Ocypodidae) on three sandy beaches of Espirito Santo, Brazil. The relative density (number of burrows/m2) and spatial distribution of the crab Ocypode quadrata were evaluated on three sandy beaches of Vila Velha-ES: Itaparica, Itapoa and Praia da Costa. These beaches were sampled on 05/25, 06/13, 06/26 and 07/11/2006. On each day, two 5m width transects were randomly distributed on each beach, perpendicular to the waterline anddivided into 2 x 5m rectangles. Within each transect, all burrows were counted, the diameters were measured (mm), and their locations in relation to the waterline were recorded. Itaparica beach presented a greater mean density of burrows (mean = 0.33, SD = ± 0.26), followed by Praia da Costa (0.16 ± 0.14) and Itapoa (0.08 ± 0.02). Regarding the burrow diameters, Itaparica and Itapoa showed greater values (16.2 ± 8.88 and 16.2 ± 8.91, respectively) than Praia da Costa (12.6 ± 6.40). The density of burrows and the burrow diameters differedsignifi cantly among the beaches. A greater abundance of small burrows occurred near the waterline and a greater abundance of large burrows were situated on the upper beach. The fl ux of people and the passage of the urban cleansing vehicles apparently had an infl uence on the density and spatial distribution standards found for the O. quadrata crabs.

Comparison of Three Burrowing Crayfish Capture Methods Based on Relationships with Species Morphology, Seasonality, and Habitat Quality

Abstract Three techniques used to capture burrowing crayfish were compared for efficiency and efficacy near Big Oaks National Wildlife Refuge. Excavation of burrows, Norrocky burrowing crayfish trap (NBCT), and burrowing crayfish mist nets (BCN) were simultaneously compared for capture efficiency and trap efficacy. Additional research goals were to evaluate patterns in capture rates with changes in seasonal activity, influence of burrow diameter size to capture success, relationships between differences in morphological characteristics of species and capture success, and capture success and habitat quality. Excavation caught significantly more burrowing crayfish than the other two methods (40.7%), but the NBCT (5.2%) was not significantly different from the BCN (4.5%) in trap efficiency. The relative percent efficacy success adjusted for effort of each method was 2.61% for the NBCT and 2.24% for the BCN. Positive correlations between habitat quality and increasing NBCT (rs = 0.414, P = 0.023) and increasi...

Accurate quantification of the influence of benthic macro- and meio-fauna on the geometric properties of estuarine muds by micro computer tomography

This paper describes the novel application of high resolution micro computer tomography (microCT) to the quantification of the properties of marine biogenic structures. CT scanning has been used to examine sediments in the past but the resolution of most previous techniques has been dependent upon commercial medical CT scanners which only have a slice thickness of ≥ 0.625 mm. In addition adequate software has not previously been available to rapidly quantify all the properties of biogenic structures. The microCT technique developed here used a standard core sample of estuarine sediment and new software was developed to calculate the axial variation of the following burrow parameters: number, diameter, volume, surface area and density. The increased resolution has resulted in the first quantification of meiofaunal burrow structures. A test core has shown, as an example, that the total volume of burrows created by macrofaunal organisms decreased from 827 mm 3, within the top 15 mm of the core, to 204.2 mm 3 at a depth 60 –75 mm within the core. Total burrow surface area decreased from 1883 mm 2 to 512 mm 2, for these depth ranges, respectively and burrow diameter ranged from 2.37–2.58 mm, remaining fairly constant between depths. Meiofaunal burrow structures decreased from 1.3–0.1 mm 3 within the top 6 mm of the core with burrow surface area decreasing from 33.52–3.4 mm 2. Again, burrow diameter remained relatively constant, ranging from 0.23–0.25 mm. Quantification to this resolution is required to identify the impact of infaunal organisms on factors such as oxygen penetration, vertical and horizontal (across burrow walls) gradients in redox conditions and chemical/nutrient speciation and flux. The quantification of these burrow properties will improve the ability to examine the interrelationships between chemical, physical and biological processes and their role in ecological functioning. The present study indicates that there is potential for further development of this software to allow more detailed analysis of burrow structures and surface features including parameters such as burrow length, shape and sediment surface roughness.

Chemical composition and ichnofacies of mudstones of the Lower Triassic Osawa Formation in the Ogatsu area, South Kitakami Terrane, northeast Japan

This study aims to reconstruct shelf environment of the Lower Triassic Osawa Formation in the Inai Group, the South Kitakami Terrane (SKT), on the basis of the vertical variations of chemical composition and ichnofacies. The Osawa Formation in the Ogatsu area is divided into the lower part (OL ; intensely bioturbated calcareous siltstones) and upper part (OU ; parallel laminated mudstones). The redox proxies (Zn/TiO2 and Ni/TiO2 ratios) and the benthic activity proxies (ichnofabric index and burrow diameter) suggest that the OL was deposited in a shelf where benthic activity was very high, whereas the OU was deposited in a relatively reductive environment that prevented benthic activity.

Assessment of soil structural differentiation around earthworm burrows by means of X-ray computed tomography and scanning electron microscopy

The burrowing activity of earthworms creates a distinct area around the resulting macropores called the drilosphere, which controls various soil processes. Density and microstructure of the drilosphere were studied and compared with those of the surrounding soil. For this purpose soil cores were separately inoculated with the vertically burrowing earthworm species Lumbricus terrestris. After 70 days some cores were compacted by a hydraulic press (250 kPa) and all cores were analysed by means of X-ray computed tomography. Mean Hounsfield Units were measured for concentric ROI cylinders (ROI = region of interest) of increasing diameters located around vertical macropore sections within selected horizontal slices. Based on these data we estimated stepwise the distribution of bulk density from the inner boundary of the drilosphere to the surrounding soil. In uncompacted soil the bulk density of the drilosphere was increased by 11% over that of the surrounding soil. In cross section, drilosphere and burrow form a concentric area with a total radius up to 2.2 cm. Soil compaction increased the dry bulk density of soil and decreased the diameter of earthworm burrows. Moreover, we found a less dense part of soil between the dense drilosphere and the remaining soil of the compacted core. Scanning electron microscopy revealed that the coarse silt particles of the bulk soil were rearranged to a parallel orientation due to compaction whereas the microstructure of the drilosphere remained unchanged. In any case, the drilosphere revealed a very homogeneous and dense arrangement of silt particles.

Response of the marine infauna to Triassic–Jurassic environmental change: Ichnological data from southern England

The trace fossil record through the Triassic–Jurassic boundary interval is examined at three sites in central and southern England (St. Audrie's Bay, Somerset; Pinhay Bay, Devon; and Long Itchington, Warwickshire). The lower ‘Pre-Planorbis Beds’ of the Blue Lias Formation record low ichnotaxonomic diversity, low bioturbation intensity, small burrow diameters, and an absence of deep tier bioturbation. The stepwise reappearance of ichnotaxa following this interval is similar at the three sites, suggesting similar rates of recovery in the benthic marine ecosystem and highlighting the potential contribution of these trace fossils to stratigraphic correlation. Mass extinction in the Late Triassic is increasingly linked to the emplacement of the Central Atlantic Magmatic Province (CAMP). Timing of the onset of CAMP volcanism in the UK is currently imperfectly known, but potentially occurred within either the Westbury Formation or the Lilstock Formation. If the onset of CAMP was within the Lilstock Formation, the modestly diverse trace fossil assemblage of the Langport Member of the Lilstock Formation suggests that emplacement of CAMP had little lasting effect on the marine benthos in central and southern England. The decline in benthic activity recorded in the ‘Pre-Planorbis Beds’ appears to be unrelated to the onset of CAMP and any associated environmental change. It is more likely related to an episode of marine anoxia. An episode of marine anoxia also provides a suitable causal mechanism for the reduction of infaunal tiering.

Interspecific differences in the burrow morphology between the sympatric mud shrimps, Austinogebia narutensis and Upogebia issaeffi (Crustacea: Thalassinidea: Upogebiidae)

The morphology of burrows constructed by the upogebiid mud shrimps Austinogebia narutensis and Upogebia issaeffi was studied using resin castings of burrows in situ on Mukaishima Island, Seto Inland Sea, Japan, where the two species occurred sympatrically. The burrow structure of both shrimps is a relatively simple Y-shaped pattern, which is typical of the family Upogebiidae. Total burrow length, and length and overall width of the U-shaped section of A. narutensis were greater than those of U. issaeffi, possibly because A. narutensis is the larger species. When the ratios of the burrow measurements to the mean burrow diameter were compared to exclude possible size effects, the burrows of A. narutensis had a wider and shallower U-shaped section than those of U. issaeffi. Because the casts were made where the two species occurred sympatrically, the differences in the burrow morphology were not due to the differences in environmental factors but to the difference in the shrimp species, whether they are adaptive or not.

Long searches for male-defended breeding burrows allow female fiddler crabs, Uca crenulata, to release larvae on time

When individuals benefit from highly selective mate choice and search costs are negligible, they should sample all or at least many potential mates. Most animals sample just a few mates, presumably because search costs override the benefits of lengthy searches. Observations on mate-searching female California fiddler crabs, Uca crenulata, showed that these crabs conducted much longer searches than have been reported for other animals. Moreover, large and small females used different sampling strategies. Searches may be long in this species because females use multiple criteria to select mates and benefit from finding a burrow of the right size. They chose mates that (1) were near to their size (2) had small claws given their body size, and (3) defended long burrows with entrances that matched the size of the female. Females sampled many burrows that did not match their size, indicating that they assessed burrows on contact and not before. An experiment examining why females use male burrow diameter as a mate choice criteria revealed that burrow opening diameter, or a correlate, affected incubation duration and therefore release time of larvae. Larvae were successfully released during high-amplitude nocturnal tides only when females incubated in burrows that allowed the larvae to exit the estuary swiftly and thus reduce predation risk, but not when females incubated in burrows that were too wide or narrow. The effect of burrow aperture on incubation duration may explain why females sampled many male burrows as they searched for a mate and why females of different size classes selected and sampled differently.