A new hydroid from the Upper Cretaceous of Mississippi

A new fossil hydroid is reported as an organic impression on a calcareous gastropod steinkern from the Prairie Bluff Chalk (Maastrichtian), Oktibbeha County, Mississippi. This is the first such hydroid reported from the Upper Cretaceous of the Atlantic Coastal Plain. The fossil organism consists of anastomosing hydrorhizae forming a holdfast, a fascicled hydrocaulus, and elongated, crenulated and ribbed hydrothecae. The fossil is unlike other Mesozoic hydroids that have been reported from Europe and North America; it is described as Mesodendrium oktibbehaensis gen. et sp. nov. and tentiatively referred to an extant family, the Campanulariidae (Calyptoblastina). The complete preservation of the holdfast, hydrocaulus and hydrothecae suggests that this hydroid lived inside gastropod shells. In analogy with Recent symbiotic hydroids inhabiting mollusc shells, the new specimen described here possibly represents the oldest known example of a symbiotic relationship between hydroids and hermit crabs.     

Helpful habitant or pernicious passenger: interactions between an infaunal bivalve,an epifaunal hydroid and three potential predators

The importance of positive interspecific interactions within physically stressful habitats has received increased attention from community ecologists. The exposed sandy beach is an example of a physically rigorous environment where biological interactions have long been considered insignificant. We examined the interaction between the infaunal clam, Donax variabilis, and the hydroid, Lovenella gracilis, on exposed sandy beaches in North Carolina. Epibiotic occupation of Donax by hydroids has been repeatedly observed on ocean beaches but rarely investigated. By providing a stable substrate for attachment, the clam facilitates the persistence of the hydroid in the intertidal beach; however, benefits or costs experienced by the host as a result of this association are unknown. By exposing clams with and without hydroid colonies to multiple types of clam predators, we tested the effectiveness of the hydroid, which possesses stinging nematocysts, in defending its host. The hydroid defended the clam against one common predator, the Florida pompano ( Trachinotus carolinus). Against speckled crabs ( Arenaeus cribrarius) and ghost crabs ( Ocypode quadrata), however, the hydroid offered no protection for its host and instead facilitated predation. The epibiotic hydroid, which projects above the surface of the sand, allowed the crabs to more readily detect clams below the surface. In the field, we evaluated the effect of the hydroid on the tidally synchronized migrations and burrowing speed of the clam. The hydroid, which can form large colonies on the posterior end of the clam, had no effect on Donax burrowing speed, but did reduce the speed of transport of clams by wave swash. Depending on relative predation pressure, the occupation of D. variabilis by L. gracilis can alternately be characterized as beneficial or detrimental to the host.     

Reproductive costs of the symbiotic hydroid Hydractinia symbiolongicarpus (Buss and Yund) to its host hermit crab Pagurus longicarpus (Say)

Studies on the interaction between the hermit crab Pagurus longicarpus and its symbiotic hydroid Hydractinia symbiolongicarpus have focused on positive effects of hydroids on their host hermit crabs (e.g., protection from predators). Yet, these benefits may be balanced with reproductive costs, which are rarely studied. Results from field observations, laboratory trials, and a mesocosm experiment indicate that female hermit crabs in hydroid-colonized shells exhibit depressed ovigery, smaller clutch sizes, and increased clutch failure relative to females in bare shells. Frequent switching between bare and hydroid-colonized shells may alleviate negative effects when the density of hydroids in the environment is low, but at high densities Hydractinia may significantly impact hermit crab reproduction.     

Allogeneic interactions in Hydractinia: is the transitory chimera beneficial?

The colonial marine hydroid, Hydractinia, exhibits four possible outcomes to allogeneic contacts: passive rejection, aggressive rejection, stable fusion and transitory fusion. In the special case of transitory fusion, Hydractinia colonies undergo tissue fusion, followed by tissue death at the original contact area, and colony separation. This type of rejection is different in several aspects from the rejection process that accompanies incompatible encounters. It has been suggested that in transitory fusion, the colonies gain immediate benefits from fusion, mainly due to size increase, without succumbing to costs associated with fusion (germ line parasitism). We report a long-term observation of repeated fusion and separation cycles in clones featuring transitory fusion that revealed a slow-down of specific growth rates following fusion, and recovery in growth rates following separation. Very rapid transfer of stained material between partners in transitory chimeras provides suggestive evidence that protection against germ line parasitism is far from being guaranteed by separation. Our data cast doubt as to whether the benefits considered for transitory fusion are sustainable and support the already made suggestion that fusion with self, rather than fusion with kin, has been the major selective force governing the evolution of allorecognition in colonial invertebrates.     

Absence of within-colony kin discrimination in behavioural interactions of swarm-founding wasps

Within-colony kin discrimination has not been demonstrated conclusively for any social insect, perhaps partly because highly polymorphic genetic markers necessary to assess within-colony relatednesses have only recently become available. We use microsatellite loci to investigate within-colony kin discrimination in behavioural interactions in the neotropical multiple-queen wasp, Parachartergus colobopterus. Within-colony kin discrimination would be particularly advantageous in this species since average genetic relatedness among colony members overall is low (0.32 =/- 0.06), compared to the relatedness value between full sisters of 0.75. Using seven colonies of individually marked females, we recorded behavioural interactions that were cooperative (222 grooming, 2438 feeding), aggressive (511 body or wing biting, 240 mandible biting) or neutral (1676 antennating). We expected cooperative behaviours to favour closer kin and aggressive behaviours to be directed towards more distant kin, but found that none of the behaviours we investigated showed discrimination on the basis of relatedness. We could have detected a difference in relatedness values of as little as between 0.03 and 0.12, depending on the behaviour being analysed. Thus, we found no evidence for kin discrimination in within-colony behaviour in this species.     

Genetic variation in a freshwater bryozoan. I: Populations in the Thames basin, UK

In spite of increasing interest in metapopulation dynamics, the genetic consequences of a metapopulation structure remain poorly understood. Here we examine the metapopulation genetic structure of the colonial, facultatively sexual freshwater bryozoan Cristatella mucedo , in the Thames basin of southern England, UK. Populations from nine sites were sampled and colonies genetically characterized using random amplified polymorphic DNA (RAPD)–PCR. A total of 78 different clones was detected over all sites. Despite the large number of clones, genetic distances among clones both within and among sites were very small. Nonetheless, no clone was present at more than one site even though C. mucedo has an asexual dispersal propagule, and clones strongly clustered by sites. No consistent pattern of clonal structure was evident, with both the number and equitability of clones varying greatly among sites. Although sites were genetically distinct, population genetic regions were absent, and a Mantel test indicated that there was no relationship between geographical distances among sites and genetic distances among populations. Our results indicate that C. mucedo exists as a classical metapopulation in the Thames basin, with dispersal independent of distance and all sites contributing to the genetic diversity of the metapopulation.     

Nutritional physiology and colony form in Podocoryna carnea (Cnidaria: Hydrozoa)

Abstract. We compared growth rates and final morphological states of the athecate colonial hydroid Podocoryna carnea in two nutritional environments: one varying the quantity of food provided at a fixed interval and the second varying the time between feedings of a fixed quantity. In both environments, replicate colonies were either fed uniformly, or fed on only one side and starved on the other. In addition, we fed colonies fluorescence-labeled cultures of Artemia salina and documented the subsequent distribution of label. We found that both the growth rates and the final morphological state varied logarithmically with food supply. Heterogeneous feeding had a marked effect on colony morphology, with a sharp boundary in polyp number, stolon density, and polyp size forming at the fed–unfed interface. The distribution of fluorescence was correlated with sites of colony growth. These results confirm and extend early work on the priority of growth zones in colonial hydroids, and present new challenges for understanding the relationship among energy metabolism, gastrovascular circulation, and colony form.     

Relay-ray way of hydroplasm movement in hydroid colonies

In this study, we continued the investigation of the distribution system of colonial hydroids in the course of its development, starting with its emergence during the planula metamorphosis and ending with the formed colony. The hydroplasmic stream system of two species of colonial hydroids—Perigonimus abyssi G.O. Sars, 1874, and Stauridia producta Wright, 1858—was studied. We found that the main principle by which hydroplasma moves in these colonies, which form no shoots, is the relay-race from one hydranth to another through a stolon fragment connecting them or it moves past the neighbouring hydranth to the next one, etc. We show that the efficiency of the conducting (distribution) system does not depend on the level of complexity of the colonial structure. The results of this study confirm the absence of general colonial processes of integration and self-regulation in hydroid polyps.     

Why small hermit crabs have large shells

Frequent shell exchanges among hermit crabs imply the enigmatic circumstance that large crabs frequently obtain large shells from smaller crabs. This seeming anomaly is explored as a key to the shell resource system. It is hypothesized to reflect how, where, and how often shells become available to the crabs. Shells become available infrequently, as snails die, and are available to the crabs for only a brief time before they become inaccessible. The standing crop of empty shells is almost always low and is irrelevant to rates of shell turnover in the crab population. Crabs are most likely to encounter shells of the wrong size, and the chance of encountering a shell of the desired size decreases as a crab grows. Snails and crabs are usually found on different portions of the shore; thus, crabs must make “foraging trips” for shells. Under this regime of shell supply, a crab will get a suitable shell the fastest when it accepts any fresh shell that is larger than its initial shell. It can then trade with other crabs to improve its shell fit. This behavior will make small crabs into a regular source of large shells for large crabs, and a shell exchange ritual will be strongly favored because both participants will benefit. Shells are an unusual resource because they are the object of both competitive and mutualistic interactions. This ambiguous quality is revealed in the intraspecific and interspecific responses of crabs to each other and to shells.     

Differences in environmental predictability underlie divergent competitive abilities in three congeneric hydroids

Species in which individuals experience predictable and uniform environments should be most finely adapted to their environment. Many hydrozoan species in the genus Hydractinia simultaneously occupy similar microhabitats (gastropod shells inhabited by hermit crabs) but experience considerable differences in their immediate environment (size and species of shells and hosts). In the present study, hydroid species experience differences in environmental predictability and traits that mediate competitive ability (growth form and growth rate). The inferred competitive ability was directly proportional to the extent to which the gastropod environment promotes interactions between small, juvenile colonies, which always end in competitive elimination. Extensive intraspecific variation in competitive ability was explained primarily by crab host species or site. Dense host populations impose more severe disturbance regimes that favour competitively inferior, but disturbance-resistant, phenotypes. Interplay between different types of variation (gastropods and hermit crabs) provides a possible mechanism for the maintenance of intraspecific growth form variation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 322–338.     

Shell fights in the hermit crabPagurus geminus: Computer simulation and experiment

The shell exchange in the hermit crabPagurus gemiuns was examined by computer simulations and experiments, to learn whether it is based on mutualistic interactions with both participants gaining, or on one-sided interactions with the large crab gaining regardless of loss and gain of the partner. The result of the experiment showed a much better fit to that of the one-sided simulation than the mutualistic simulation, suggesting that competitive interactions in which small crabs lose in the resulting shell exchanges are a part of shell fights of this species. While the shell exchange attempt can be regarded as aggressive, a higher probability of mutualistic encounters and of successful mutualistic shell exchanges in hermit crabs is discussed.     

Intraspecific competition and aggression for shells in the hermit crab Pagurus samuel1s

Many studies have investigated shell‐related behaviour in hermit crabs. Few studies, however, have focused specifically on the intraspecies aggression associated with shell competition. We examined intraspecies aggression in hermit crab (Pagurus samuelis) pairs as it relates to competition for a limiting resource, gastropod shells. Pairs of hermit crabs were observed in the laboratory in four different treatments that varied the presence or absence of shells for one or both of the crabs. Measurements of the latency to respond, the number of bouts, and the fight durations were recorded. There was a significant difference among treatments for all three measurements, and naked hermit crabs were much more aggressive than housed hermit crabs. There was no significant difference in aggression between males and females in any of the three treatments. The heightened aggression observed in naked P. samuelis is likely in service of acquiring a protective shell.     

Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea

Green Fluorescent Proteins (GFPs) have been reported from a wide diversity of medusae, but only a few observations of green fluorescence have been reported for hydroid colonies. In this study, we report on fluorescence displayed by hydroid polyps of the genus Cytaeis Eschscholtz, 1829 (Hydrozoa: Anthoathecata: Filifera) found at night time in the southern Red Sea (Saudi Arabia) living on shells of the gastropod Nassarius margaritifer (Dunker, 1847) (Neogastropoda: Buccinoidea: Nassariidae). We examined the fluorescence of these polyps and compare with previously reported data. Intensive green fluorescence with a spectral peak at 518 nm was detected in the hypostome of the Cytaeis polyps, unlike in previous reports that reported fluorescence either in the basal parts of polyps or in other locations on hydroid colonies. These results suggest that fluorescence may be widespread not only in medusae, but also in polyps, and also suggests that the patterns of fluorescence localization can vary in closely related species. The fluorescence of polyps may be potentially useful for field identification of cryptic species and study of geographical distributions of such hydroids and their hosts.     

Ecological relations between hermit crabs and their shell-supplying gastropods: Constrained consumers

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.     

Dominance and effects of strange conspecifics on aggressive interactions in the hermit crab Pagurus longicarpus (say)

Dominance orders in the hermit crab Pagurus longicarpus were observed in the laboratory. Groups of four crabs formed loose dominance hierarchies as determined by repeated display and retreat behaviour. Stronger dominance orders were inversely correlated with the frequency of aggressive interactions. Recognition of individuals, as measured by frequency and intensity of aggressive encounters between familiar crabs and introduced strangers, was not important in maintaining dominance orders. Rather, P. longicarpus recognized the aggressive state of conspecifics, as shown by frequencies of aggressive encounters after individuals of different dominance rankings were exchanged between established hierarchies.     

Interference and exploitation components in interespecific competition between sympatric intertidal hermit crabs

This study was designed to evaluate the effect of interference and exploitation competition in shell partitioning between two hermit crab species (Pagurus criniticornis and Clibanarius antillensis). Field samples revealed that shells of the gastropod Cerithium atratum were the main resource used by both hermit crab species and that Pagurus used eroded or damaged shells in higher frequency than Clibanarius. The exploitative ability of each species was compared between species in the laboratory using dead gastropod (Cerithium) baits to simulate predation events and signalize newly available shells to hermit crabs. Pagurus reached the baits more rapidly than Clibanarius, but this higher exploitative ability did not explain shell utilization patterns in nature. Another experiment evaluated the dominance hierarchy between these two hermit crab species and revealed that Clibanarius was able to outcompete Pagurus for higher quality shells in agonistic encounters. This higher interference competitive ability of Clibanarius in relation to Pagurus may explain field observations. Nevertheless, Pagurus may be responsible to enhance shell availability to other hermit crab species that have lower ability to find and use newly available shells. Differently, the poorer condition of shells used by Pagurus, the higher ability of this species to attend gastropod predation events and its higher consumption rate by shell-breaking crabs (Menippe nodifrons) may increase its predation risks, thus revealing the disadvantages of such an exploitative competitive strategy for hermit crabs.     

Does Sex Trade with Violence among Genotypes in Drosophila melanogaster?

The evolutionary forces shaping the ability to win competitive interactions, such as aggressive encounters, are still poorly understood. Given a fitness advantage for competitive success, variance in aggressive and sexual display traits should be depleted, but a great deal of variation in these traits is consistently found. While life history tradeoffs have been commonly cited as a mechanism for the maintenance of variation, the variability of competing strategies of conspecifics may mean there is no single optimum strategy. We measured the genetically determined outcomes of aggressive interactions, and the resulting effects on mating success, in a panel of diverse inbred lines representing both natural variation and artificially selected genotypes. Males of one genotype which consistently lost territorial encounters with other genotypes were nonetheless successful against males that were artificially selected for supernormal aggression and dominated all other lines. Intransitive patterns of territorial success could maintain variation in aggressive strategies if there is a preference for territorial males. Territorial success was not always associated with male mating success however and females preferred 'winners' among some male genotypes, and 'losers' among other male genotypes. This suggests that studying behaviour from the perspective of population means may provide limited evolutionary and genetic insight. Overall patterns of competitive success among males and mating transactions between the sexes are consistent with mechanisms proposed for the maintenance of genetic variation due to nonlinear outcomes of competitive interactions.     

Individual distance in crustacea: IV. Distance and dominance hierarchies in Pagurus pollicaris

The agonistic acts and distances separating interacting individuals when acts occurred were recorded for specimens of the hermit crab Pagurus pollicaris. Individuals were observed in groups of six under several conditions (during a snail predation, with food in the aquarium, and nothing in the aquarium except the crabs before and after the crabs had access to empty gastropod shells).

The pattern of agonistic interactions within the groups showed a high degree of order, but the lack of changes in behavior over time indicated these crabs have an aggressive order rather than a dominance hierarchy.

The distances separating crabs when acts were executed were significantly affected by (1) the act being executed, (2) the act executed previously by the other crab, (3) the size of the acting crab, (4) the relative size difference of the two interactants, (5) the individual crabs’ identities, and (6) the condition of observation. The individual crab effects on distance were present when crab size was controlled and crabs that reacted at smaller distances tended significantly to elicit reactions at greater distances.     

The influence of some symbionts on the shell-selection behaviour of the hermit crabs, Pagurus pollicarus and Pagurus longicarpus

The influence of some symbionts on the shell-selection by the hermit crabs Pagurus pollicarus and P. longicarpus was examined by placing individual hermit crabs with two similar shells in a choice situation and recording the shell occupied after 12 hr. One shell contained a symbiont species and the other did not. The results indicated that organisms normally found on or in empty shells influence the shell-section by these species of hermit crab. P. pollicarus preferred shells occupied by the sea anemone Calliactis tricolor or by the hydroid Hydractina echinata as opposed to bare shells. P. longicarpus also preferred shells with H. echinata. Both crab species rejected shells with the barnacle Balanus amphitrite. Shells containing the molluscs Crepidula fornicata or C. plana were rejected by the smaller hermit crab P. longicarpus. These molluscs appeared to exert no influence on P. pollicarpus unless they were large or abundant, at which point their weight or occlusion of available space possibly has negative effects on the crab.