Asian Shore Crab Research Articles

Seasonal lipid storage and dietary preferences of native European vs. invasive Asian shore crabs

The invasive Asian shore crab Hemigrapsus sanguineus and the native European green crab Carcinus maenas share intertidal habitats along European North Atlantic shores and may compete for food. We evaluated the energy-storing capacities of the 2 species and determined their dietary preferences by means of lipid analysis and fatty acid trophic marker indices. Specimens of both sexes and various sizes were sampled in the rocky intertidal of the island of Helgoland (North Sea) in April, June, August, and October 2015. Total lipids of the midgut glands were significantly higher in H. sanguineus than in C. maenas and followed a distinct seasonal cycle in both sexes (ca. 20−50% of dry mass, DM). The lower lipid contents of C. maenas (ca. 20% of DM) remained at a similar level throughout the seasons. The seasonal differences in the females of H. sanguineus may be due to higher reproductive output and, consequently, lipid turnover, but remain unexplained in males. Trophic indices for Bacillariophyceae, Chlorophyta, and especially Phaeophyceae were higher in H. sanguineus than in C. maenas, suggesting a higher degree of herbivory of the invader. In contrast, the Rhodophyta index was higher in C. maenas. Thus, competition for food between the 2 species will probably be low in habitats rich in macroalgae. The ability of H. sanguineus to utilize mainly energy-poor algae but accumulate high-energy reserves may be an advantage for successfully establishing persistent populations in new habitats.

Physiological and behavioral response of the Asian shore crab, Hemigrapsus sanguineus, to salinity: implications for estuarine distribution and invasion.

The invasive Asian shore crab, Hemigrapsus sanguineus, is ubiquitous in the rocky intertidal zone of the western North Atlantic. A likely contributor to this colonization is that H. sanguineus is able to handle a wide range of salinities, and is thus more likely to spread through a greater geographic area of estuaries. This study investigated the salinity effects on this animal by observing survival across a range of salinities, the maintenance of hemolymph osmolality under different salinities, and behavioral preference for and avoidance of salinities. H. sanguineus showed high survival across a broad range of salinities, had little change in hemolymph osmolality over a short-term salinity shock, and behaviorally distinguished between salinities when presented with a choice, under both acclimation salinities of 5 PSU or 35 PSU. Such results suggest H. sanguineus has a hardiness for the rapid changes in salinity that happen in the intertidal zone, yet is capable of physically moving to a more optimal salinity. This enhances their competitiveness as an invader, particularly surviving lower salinities that present challenges during high-precipitation events in rocky intertidal areas, and partially explains this species’ dominance in this habitat type.

Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster.

Invasive species can have profound ecological and evolutionary impacts on native fauna, particularly those from overlapping guilds. Intraguild predation and competition often occur simultaneously, and ontogenetic shifts in competitive strength can dictate the magnitude and direction of species interactions. The recent introduction of the Asian shore crab Hemigrapsus sanguineus to the Atlantic coast of North America has resulted in the potential for considerable intraguild interactions with juvenile American lobsters Homarus americanus, with which it now co-occupies rocky intertidal and shallow subtidal habitats. We present data from 5 yr of monthly (May-October) field sampling revealing a significant decline in lobster density as H.sanguineus density increased in the low intertidal. To investigate potential mechanisms behind this pattern, we conducted three mesocosm experiments designed to examine whether competitive interactions between H.americanus and H.sanguineus are size- or density-dependent. Larger early benthic phase lobsters (16-34mm CL) outcompeted H.sanguineus for both food and shelter resources. These lobsters fed faster and more in the presence of H.sanguineus, effectively defended shelter, were responsible for the majority of agonistic interactions, and, in multiple instances, killed and consumed H.sanguineus. Effects on sheltering and agonstic interactions by these lobsters were independent of H.sanguineus density; however, prior shelter residency increased lobster agonistic behavior towards crabs. In contrast, H.sanguineus outcompeted smaller, early benthic phase lobsters (7.2-11.2mm CL) for shelter, and these interactions were density dependent. Displacement of lobsters from shelter by crabs and cumulative agonistic interactions instigated by crabs towards lobsters scaled positively with H.sanguineus density. Given the similarity of the invasion documented here and others occurring globally, these experiments demonstrate the importance of considering how factors such as ontogeny, density-dependence, and primacy influence the outcomes of interactions between intraguild predators. Disentangling how these factors structure intraguild interactions between invasive and endemic species will both advance our fundamental understanding of community ecology and enhance efforts to conserve and manage natural resources.

Changes in Population Sizes ofHemigrapsus sanguineus(Asian Shore Crab) and Resident Crab Species in Southeastern New England (2010–2016)

The invasion of Hemigrapsus sanguineus (Asian Shore Crab) has been of concern in northeastern North America since the late 1980s. A relatively long-term record (1998–2011) of density estimates in southern New England showed displacement of resident crab species at 3 locations in Massachusetts and Rhode Island. In 2016, I visited the same locations to estimate current crab densities. The springtime Asian Shore Crab density decreased at a coastal location, but increased at an estuarine location to >300 crabs/m2. Resident crabs Carcinus maenas (European Green Crab) and Mud Crabs in the family Panopeidae increased in abundance at all 3 locations.

Invasive versus native brachyuran crabs in a European rocky intertidal: respiratory performance and energy expenditures

The invasive Asian shore crab Hemigrapsus sanguineus is now the second most abundant intertidal crab in the North Sea after the native European green crab Carcinus maenas. To compare their respiratory performance and energy expenditures, we measured standard respiration rates of both species from around the island of Helgoland, North Sea, Germany (54°11′N, 7°53′E) in 2015. Oxygen consumption was recorded in a flow-through setup between 5 and 20 °C. At lower temperatures, H. sanguineus had similar respiration rates as C. maenas, but approximately twofold higher rates at higher temperatures. Numerical models for the calculation of individual respiration rates were established and applied to the entire intertidal populations around Helgoland to compare the energy expenditures of both species. Abundance and biomass data recorded in August 2014 showed that H. sanguineus reached values as high as 21 and 59%, respectively, compared to those of C. maenas. Based on these data and the numerical respiration models, the energy expenditures of both populations were calculated for the whole year 2014. The H. sanguineus population reached, depending on the assumed diet of both species (complete herbivory versus complete carnivory), 86–135% of the expenditure value of C. maenas. As population densities of H. sanguineus are likely to increase in the North Sea, the energy expenditure of the invader and thus its impact on the energy flux in the intertidal habitat will further increase. Whether this will lead to an impact on the local intertidal community depends on species-specific dietary preferences and remains to be investigated.

Larvae of native and non-native crabs behave differently in response to chemical cues from potential fish predators and adult crabs

The behavior of planktonic larval invertebrates can play an important role in predator avoidance. Megalopae, the last larval stage in crab development, are strong swimmers yet are vulnerable to predation by fish. The behaviors of megalopae of the native mud crab Dyspanopeus sayi (Smith, 1869) and the Asian shore crab Hemigrapsus sanguineus (De Haan, 1835) in flowing seawater were analyzed in response to chemical cues from potential fish predators and adult crabs. Fish predators consisted of cunner (Tautogolabrus adspersus [Walbaum, 1792]), tautog (Tautoga onitis [Linnaeus, 1758]), and mummichog (Fundulus heteroclitus [Linnaeus, 1766]). Larvae were reared to early and late stages of megalopal development. Individual megalopae were dropped into a horizontal glass tube containing a stream of flowing artificial seawater (control) or seawater containing chemicals from fish or adult crabs. Behaviors of each megalopa were observed and categorized as swimming, walking, tumbling, or inactive. In control seawater, megalopae of both species swam upstream more than in seawater with fish or crab cues. Mud crab megalopae were inactive in the presence of chemicals from all fish species whereas Asian shore crab megalopae were inactive more in the presence of cues from tautog and cunner than from mummichog, a species unrelated to fish predators in its native range. Late-stage D. sayi megalopae walked on the bottom of the tube in response to confamilial adult cues more than H. sanguineus. Both species can therefore detect chemical cues in flowing water, even early in megalopal development, and H. sanguineus can distinguish cues from different potential fish predators.

Decadal changes in size and intertidal reproductive output by the non-native Asian shore crab ( Hemigrapsus sanguineus )

Decadal changes in size and intertidal reproductive output by the non-native Asian shore crab ( Hemigrapsus sanguineus )

Fish kairomones induce spine elongation and reduce predation in marine crab larvae.

Specialized defense strategies are induced in zooplankton upon detection of predator chemical cues or kairomones. These defenses are well-described for freshwater zooplankters, with morphological defenses being particularly striking, but few studies have reported kairomone-induced morphological defenses in marine zooplankton. Here, we compare morphological responses to kairomones in the larvae of two marine crab species, estuarine mud crabs (Rhithropanopeus harrisii) and Asian shore crabs (Hemigrapsus sanguineus). When reared in the presence of fish kairomones, spine length increased by 2-3% in larval R.harrisii, while no morphological changes were identified in H.sanguineus. In subsequent feeding assays with a co-occurring fish predator (Atlantic silversides, Menidia menidia), consumption of R.harrisii was lower on larvae that had been reared with kairomones. In addition, we found that broods with smaller larvae are more likely to exhibit increases in spine length after kairomone exposure. Hence, the observed morphological response is likely influenced by larval size.

Erratum to: Reconstructing the Invasion History of the Asian shorecrab, Hemigrapsus sanguineus (De Haan 1835) in the Western Atlantic

Erratum to: Reconstructing the Invasion History of the Asian shorecrab, Hemigrapsus sanguineus (De Haan 1835) in the Western Atlantic

Potential impact of the Asian shore crab Hemigrapsus sanguineus on native northeast Pacific crabs

Asian shore crabs (Hemigrapsus sanguineus De Haan) are a dominant invasive species in the northwest Atlantic, where recent evidence suggests that they have been introduced multiple times from their native range of Japan and Korea. Despite favorable environmental conditions and a high level of shipping traffic across the Pacific, this crab has not become established in the northeast Pacific. This may be due to interactions with the native shore crab assemblage, which previous studies have found to outcompete juvenile H. sanguineus for shelter. The present study used a combination of feeding and behavioral experiments to quantify the feeding behavior of native crabs (Pachygrapsus crassipes, Hemigrapsus nudus, Hemigrapsus oregonensis) with and without the presence of H. sanguineus. H. sanguineus ate more than the native crabs combined at both 7 and 13 °C. Crabs in the mixed treatment which included all four species ate less than expected, indicating that at least one species reduced its feeding rate. However, time-lapse photography revealed similar amounts of time feeding in both treatments for all 4 species, suggesting that there were changes in feeding intensity when all crabs were together. The high feeding rates but lack of a competitive advantage for H. sanguineus make it an unlikely but potentially impactful invader in the northeast Pacific.

Species coexistence and the superior ability of an invasive species to exploit a facilitation cascade habitat.

Facilitation cascades generated by co-occurring foundation species can enhance the abundance and diversity of associated organisms. However, it remains poorly understood how differences among native and invasive species in their ability to exploit these positive interactions contribute to emergent patterns of community structure and biotic acceptance. On intertidal shorelines in New England, we examined the patterns of coexistence between the native mud crabs and the invasive Asian shore crab in and out of a facilitation cascade habitat generated by mid intertidal cordgrass and ribbed mussels. These crab species co-occurred in low intertidal cobbles adjacent to the cordgrass–mussel beds, despite experimental findings that the dominant mud crabs can kill and displace Asian shore crabs and thereby limit their successful recruitment to their shared habitat. A difference between the native and invasive species in their utilization of the facilitation cascade likely contributes to this pattern. Only the Asian shore crabs inhabit the cordgrass–mussel beds, despite experimental evidence that both species can similarly benefit from stress amelioration in the beds. Moreover, only Asian shore crabs settle in the beds, which function as a nursery habitat free of lethal mud crabs, and where their recruitment rates are particularly high (nearly an order of magnitude higher than outside beds). Persistence of invasive adult Asian shore crabs among the dominant native mud crabs in the low cobble zone is likely enhanced by a spillover effect of the facilitation cascade in which recruitment-limited Asian shore crabs settle in the mid intertidal cordgrass–mussel beds and subsidize their vulnerable populations in the adjacent low cobble zone. This would explain why the abundances of Asian shore crabs in cobbles are doubled when adjacent to facilitation cascade habitats. The propensity for this exotic species to utilize habitats created by facilitation cascades, despite the lack of a shared evolutionary history, contributes to species coexistence and the acceptance of invasives into a diverse community.

Reconstructing the Invasion History of the Asian shorecrab, Hemigrapsus sanguineus (De Haan 1835) in the Western Atlantic

Numerous marine species have been introduced via anthropogenic vectors; however, many species, including highly abundant ones, have uncertain invasion histories. Here, we include the first large-scale population genetics study of the Asian shorecrab (Hemigrapsus sanguineus) in native (east Asia) and non-native (east USA) regions to help resolve questions of source, vector, and multiple introductions. Using a barcoding gene and six microsatellite loci, we analyzed 731 cytochrome oxidase I sequences and 500 microsatellites across 51 populations over two time periods, 2001–2002 and 2013–2014. Explorations of both genetic and historical data indicated that multiple introductions to USA are probable with the greatest genetic diversity concentrating around Long Island (near New York City). Analyses of ballast water flux and genetic data suggest that introductions from Japan via shipping are the likely source and pathway of introduction to USA. In particular, we found unexchanged ballast arriving to USA (notably New York region) from sources throughout Japan, including southern and northern areas where closer genetic connections to USA were detected. Western Europe was also a prominent source of unexchanged ballast, suggesting possible trans-Atlantic transfers. Altogether, extensive biogeographic studies like this help provide integral evidence for resolving key invasion history questions, as well as a baseline understanding of a species’ probable success and prospect for further spread. In turn, while mandatory ballast water exchange in USA has greatly lessened ballast-borne invasions, our study underscores that hotspots of unexchanged ballast can remain potential sources of novel or supplemental introductions worldwide.

Population development of the invasive crab Hemigrapsus sanguineus (De Haan, 1853) and its potential native competitor Carcinus maenas (Linnaeus, 1758) at Helgoland (North Sea) between 2009 and 2014

The Asian shore crab Hemigrapsus sanguineus (De Haan, 1853) has recently established populations in the North Sea and now occurs within the native ranges of the green crab Carcinus maenas (Linnaeus, 1758). To determine potential competitive effects and to assess the progress of the invasion, species-specific population characteristics (numerical abundances, biomasses, and size distributions) of the two species around the island of Helgoland (German Bight, southern North Sea) were compared for surveys conducted in 2009 and 2014. Sampling sites were chosen based on accessibility and differed in their topography and wave exposure, which allowed testing for the influence of these factors on the establishment success of H. sanguineus. The numerical abundance and biomass of H. sanguineus increased markedly and approached those of C. maenas in 2014. At a sheltered site, H. sanguineus even outnumbered C. maenas, whereas the converse was observed at a site exposed to strong winds and waves. Although such contrasting abundance patterns between the native and the introduced shore crab may be the result of direct interference, the dominance of H. sanguineus at the sheltered site may also be explained by enhanced larval settling rates caused by odors of conspecifics. The results suggest that the invasion of H. sanguineus has not yet reached its equilibrium, and population abundances in the North Sea are expected to further increase in the future.

Increase in density of genetically diverse invasive Asian shore crab (Hemigrapsus sanguineus) populations in the Gulf of Maine.

Hemigrapsus sanguineus, the Asian shore crab, has rapidly replaced Carcinus maenas, the green crab, as the most abundant crab on rocky shores in the northwest Atlantic since its introduction to the United States (USA) in 1988. The northern edge of this progressing invasion is the Gulf of Maine, where Asian shore crabs are only abundant in the south. We compared H. sanguineus population densities to those from published 2005 surveys and quantified genetic variation using the cytochrome c oxidase subunit I gene. We found that the range of H. sanguineus had extended northward since 2005, that population density had increased substantially (at least 10-fold at all sites), and that Asian shore crabs had become the dominant intertidal crab species in New Hampshire and southern Maine. Despite the significant increase in population density of H. sanguineus, populations only increased by a factor of 14 in Maine compared to 70 in southern New England, possibly due to cooler temperatures in the Gulf of Maine. Genetically, populations were predominantly composed of a single haplotype of Japanese, Korean, or Taiwanese origin, although an additional seven haplotypes were found. Six of these haplotypes were of Asian origin, while two are newly described. Large increases in population sizes of genetically diverse individuals in Maine will likely have a large ecological impact, causing a reduction in populations of mussels, barnacles, snails, and other crabs, similar to what has occurred at southern sites with large populations of this invasive crab species.

Conspecific tolerance and heterospecific competition as mechanisms for overcoming resistance to invasion by an intertidal crab

The success of the invasive Asian shore crab, Hemigrapsus sanguineus, stems partly from its ability to exclude established crab species from preferred rocky and cobble intertidal habitat. Here, we assessed preference and competition for habitat types (cobble vs. sand) for H. sanguineus and two competitor species; the previous invasive green crab, Carcinus maenas, and the native rock crab, Cancer irroratus, in New England. In simple laboratory experiments, we paired similarly sized heterospecifics and conspecifics from each species, and also grouped combinations of C. maenas and H. sanguineus in a series of four-individual sets in order to dissect the outcome of intra- and inter-specific competition at different densities. Individually, all three species preferred cobble substrate. With paired conspecifics, H. sanguineus individuals would cohabitate in cobble, whereas C. maenas and C. irroratus individuals each excluded conspecifics from cobble. In heterospecific pairs, H. sanguineus excluded both C. maenas and C. irroratus from cobble. C. maenas and C. irroratus, were equally likely to exclude the other species, but rarely excluded H. sanguineus. In larger assemblages, H. sanguineus preferentially grouped under cobble, whereas C. maenas were more evenly distributed among habitat types. These observations demonstrate that conspecific tolerance and heterospecific competition can be effective, complementary mechanisms for overcoming invasion resistance. Such mechanisms help explain the well-studied success of H. sanguineus following its introduction into New England coastal habitats, and the resulting exclusion of preexisting crab species.

Estuarine fouling communities are dominated by nonindigenous species in the presence of an invasive crab

Interactions between anthropogenic disturbances and introduced and native species can shift ecological communities, potentially leading to the successful establishment of additional invaders. Since its discovery in New Jersey in 1988, the Asian shore crab (Hemigrapsus sanguineus) has continued to expand its range, invading estuarine and coastal habitats in eastern North America. In estuarine environments, H. sanguineus occupies similar habitats to native, panopeid mud crabs. These crabs, and a variety of fouling organisms (both NIS and native), often inhabit man-made substrates (like piers and riprap) and anthropogenic debris. In a series of in situ experiments at a closed dock in southwestern Long Island (New York, USA), we documented the impacts of these native and introduced crabs on hard-substrate fouling communities. We found that while the presence of native mud crabs did not significantly influence the succession of fouling communities compared to caged and uncaged controls, the presence of introduced H. sanguineus reduced the biomass of native tunicates (particularly Molgula manhattensis), relative to caged controls. Moreover, the presence of H. sanguineus favored fouling communities dominated by introduced tunicates (especially Botrylloides violaceous and Diplosoma listerianum). Altogether, our results suggest that H. sanguineus could help facilitate introduced fouling tunicates in the region, particularly in locations where additional solid substrates have created novel habitats.

Scaling the consequences of interactions between invaders from the individual to the population level.

The impact of human‐induced stressors, such as invasive species, is often measured at the organismal level, but is much less commonly scaled up to the population level. Interactions with invasive species represent an increasingly common source of stressor in many habitats. However, due to the increasing abundance of invasive species around the globe, invasive species now commonly cause stresses not only for native species in invaded areas, but also for other invasive species. I examine the European green crab Carcinus maenas, an invasive species along the northeast coast of North America, which is known to be negatively impacted in this invaded region by interactions with the invasive Asian shore crab Hemigrapsus sanguineus. Asian shore crabs are known to negatively impact green crabs via two mechanisms: by directly preying on green crab juveniles and by indirectly reducing green crab fecundity via interference (and potentially exploitative) competition that alters green crab diets. I used life‐table analyses to scale these two mechanistic stressors up to the population level in order to examine their relative impacts on green crab populations. I demonstrate that lost fecundity has larger impacts on per capita population growth rates, but that both predation and lost fecundity are capable of reducing population growth sufficiently to produce the declines in green crab populations that have been observed in areas where these two species overlap. By scaling up the impacts of one invader on a second invader, I have demonstrated that multiple documented interactions between these species are capable of having population‐level impacts and that both may be contributing to the decline of European green crabs in their invaded range on the east coast of North America.

A comparison of reproductive patterns and adult dispersal in sympatric introduced and native marine crabs: implications for species characteristics of invaders

The introduction and translocation of nonindigenous marine species is widespread and can pose severe threats to biodiversity and ecosystem functioning. Predicting which species are potential invaders is of particular interest to ecologists. One approach is to identify characteristics that predispose a species to becoming a successful invader. Since its introduction in the 1980’s, the invasive Asian shore crab, Hemigrapsus sanguineus, has shown a remarkable ability to colonize rocky intertidal habitats along the east coast of the United States. In Long Island Sound H. sanguineus occurs sympatrically with the functionally equivalent, but non-invasive, native Atlantic mud crab, Panopeus herbstii. The presence of both species at the same site allowed us to make a detailed, simultaneous assessment of life history traits and adult dispersibility of co-occurring invading and native crab species. We investigated fecundity and maturation rates, length of breeding season and brood production for both species, and conducted field experiments using mark–recapture techniques to determine mobility patterns. Our results show that the nonindigenous Asian crab has a greater reproductive potential than the native mud crab as evidenced by a longer breeding season, multiple brood production and higher fecundity rates. Field experiments confirmed previous studies indicating H. sanguineus is a highly mobile crab, and further demonstrated that adult Asian crabs are more likely than mud crabs to disperse from their shelter/refuge sites. Recovery rates for native mud crabs were significantly higher than those for Asian crabs in three experimental trials, across sites and years. This work provides new information about life history characteristics of both species and supports the hypothesis that high reproductive potential combined with high adult dispersal ability may be important factors associated with the invasion/establishment success of the Asian shore crab. More study is needed, however, to determine the applicability of these findings to other highly successful marine invaders.

Comparison of parasite diversity in native panopeid mud crabs and the invasive Asian shore crab in estuaries of northeast North America

Comparison of parasite diversity in native panopeid mud crabs and the invasive Asian shore crab in estuaries of northeast North America

Scent of doom improves larvae's vision

Crab mums will never win the ‘Best Zoological Parent’ award: as soon as a female releases her young, she's off, leaving the offspring to head for the depths and fend for themselves. Corie Charpentier from the University of Delaware, USA, explains that the minute creatures only ascend to feed under cover of dark, descending again when the sun rises: ‘The daytime depth is set by the lowest light intensity that they can detect’, Charpentier says. These zooplankton are also sensitive to sudden changes in light intensity – dodging deeper to evade detection when a predator's shadow passes over – and the tiny crustaceans seem to become even more sensitive to light changes after sniffing a predator's odour; ‘Crab larvae descend following smaller increases in downwelling light after exposure to predator chemical cues’, explains Charpentier. However, it wasn't clear how the threatening odours, known as kairomones, affected the larvae's vision. ‘Little work has been done to understand how zooplankton integrate these visual and chemical cues to control swimming behaviour’, Charpentier says, so she and her thesis advisor, Jonathan Cohen, set about finding out how the aroma of fear affects crab larval vision.Fortunately, collecting the larvae was as easy as taking egg-laden crab mums-to-be from nearby estuaries and waiting for them to release their offspring. Charpentier and Cohen also decided to investigate two local species with dramatically different larval lifestyles: dwarf crab (Rhithropanopeus harrisii) larvae live in predator-infested estuaries while invasive Asian shore crab (Hemigrapsus sanguineus) larvae are transported offshore to safer waters. Then, the duo gently wiped mucus from the bodies of a local fish, the mummichog, and diluted the goo to produce ‘eau de predator’ before testing its effects on larval vision.Filming the larvae's responses as she simulated 3 s flashes of light filtering down to depths of 9–12 m, Charpentier saw that the lowest light levels that triggered the larvae's defensive descent were 1.1×1013 photons m–2 s–1 for the dwarf crab larvae and 4.77×1012 photons m–2 s–1 for the Asian shore crab larvae. However, when the youngsters had been bathed for up to 3 h in the menacing fish odour, both species became much more sensitive, plunging to safety at intensities that were only a third as bright as the light that had triggered an escape response before they sniffed the odour.But how had the alarming smell affected the larvae's eyes? Charpentier painstakingly measured electrical activity in the retina of larvae as she exposed the animals to increasingly dimmer flashes of light, and found that the retina's light sensitivity increased after bathing in the kairomone stench. And when she tested the effect of the odour on the speed of the retina's response to light, she was surprised that the sensitised larvae responded as fast, and sometimes even faster, than the larvae that had not experienced the kairomone threat. Charpentier explains that this was unexpected as an increase in light sensitivity is usually accompanied by slower visual responses in most animals.Having confirmed that kairomones affect the visual sensitivity of the larvae's retina, Charpentier took a closer look at the structures that comprised the crustacean's compound eye to see whether they had altered, and she was impressed to discover that each eyelet (rhabdom) became wider, increasing the structure's light sensitivity, while the dwarf crab's eye structures also became shorter. Exposure to kairomone odours released by predatory fish was altering the sensitivity of the larvae's eyes to light by directly affecting the eye structure and retina activity to give them a head start when danger looms.