Foraminiferan (Protozoa) epizoites on Arctic isopods (Crustacea) as indicators of isopod behaviour?

A total of 38 219 specimens representing 63 species of marine isopods (Crustacea) from deep and shallow Arctic waters were studied in a search for epizoic foraminifers (Protozoa). Foraminifers occurred on 21 species, and their frequency was generally low. A total of 290 foraminifer individuals were found, of which 289 belonged to Cibicides wuellerstorfi, C. refulgens and Cibicides spp. (juveniles) (Cibicidae), while only a single individual belonged to Cornuspira sp. (Cornuspiridae). The foraminifers were most frequent on species of the families Munnidae, Ischnomesidae (suborder Asellota) and on Gnathia stygia (suborder Gnathiidea), but were totally absent from the asellote families Janiridae, Haploniscidae, Nannoniscidae and from the suborder Epicaridea. The foraminifers were mainly located on the legs (Munna acanthifera), the anterior part of the body (Haplomesus quadrispinosus, heteromesus frigidus), or on the head (G. stygia adults). The epizoic foraminifers occur mainly on epibenthic isopods, which do not or only rarely clean themselves. The foraminifers are known to prefer elevated substrata, and in this the habitat of the isopods and the foraminifers coincide. The size of individual isopods was not related to the presence or absence of foraminifers.     

Behavioural and metabolic adaptations of marine isopods to the rafting life style

Rafting on floating objects is a common dispersal mechanism for many marine invertebrates. In order to identify adaptations to the rafting life style, we compared behavioural and metabolic characteristics of two isopods, the obligate rafter Idotea metallica and the facultative rafter Idotea baltica. In laboratory experiments, I. metallica showed low locomotive activity and a tight association to the substratum. Idotea baltica, in contrast, was more active with more frequent excursions in the surrounding water column. Oxygen consumption rates were similar in both species. Idotea metallica fed on zooplankton making this species widely independent of autochthonous food resources of the raft. Feeding rates and digestive enzyme activities were low in I. metallica. Reduced egestion rates may indicate slow gut passage and, thus, efficient resorption of nutrients. Efficient food utilization and the ability to accumulate high amounts of storage products, i.e. lipids, indicate a possible adaptation of I. metallica to low food availability or starvation. The feeding behaviour of I. baltica, in contrast, was more herbivorous and appeared wasteful and inefficient. Low lipid contents in I. baltica also indicate poor storage reserves. Thus, I. baltica requires a permanent access to food.     

Diets of four deep-water scaphopod species (Mollusca) in the North Atlantic and the Nordic Seas

The buccal pouch contents of four deep-water scaphopod (Mollusca) species, Pulsellum affine, Pulsellum teres, Siphonodentalium lobatum and Polyschides olivi, collected in the North Atlantic and the Nordic Seas, were examined. The buccal pouches of the examined scaphopod species contained almost exclusively foraminifers, which agrees with previous studies on shallow and deep-water scaphopods. The scaphopods seemed to be engulfing at least some of the most common foraminifer species, which are distributed in the cold waters of the Nordic Seas or the temperate waters in the northernmost part of the North Atlantic. About half of the food species (30 foraminifer species) occurred in only one scaphopod species, nearly always in very low numbers, and seemed to be an unimportant food source. The remaining and more frequent food species occurred in two or more scaphopod species. A similarity tree was calculated with parsimony and presence–absence coding of the food species in the scaphopods. This resulted in the similarity tree: 100%{P. teres 79%[P. affine 70%(P. olivi and P. lobatum)]}, when branch support was measured with percentage jackknife stability. A general trend was found towards larger food items being consumed by larger scaphopod species, indicated by the positive correlation between the size of the scaphopods and the food species. It is suggested that microdistribution of the foraminifers and the large individual size of the foraminifers may be important factors in avoiding predation by scaphopods.     

Feeding,food preference,and the uptake of food energy by the supralittoral isopod Ligia pallasii

Analyses of gut contents of freshly collected Ligia pallasii (Brandt) showed that the principal foods were encrusting diatoms, insect larvae, occasional members of the same species, and a variety of red and green seaweeds growing in the upper interiidal tidepool habitat. L. pallasii prefers to eat the green seaweed Ulva sp., and the brown alga Nereocystis luetkeana, when given a choice between several seaweeds, although neither of these forms is normally accessible to the isopods. The absorption (assimilation) of food-energy was 78% on a diet of Ulva and 55 to 76% on a diet of N. luetkeana—representative values for an algivorous invertebrate. A correlation analysis on the relationship of feeding preference of L. pallasii with calorific value of 7 potential seaweed foods suggested that feeding preference in this species is related to factors other than energy content of the food. Food preferences of invertebrates are discussed in relation to calorific value, accessibility, and to various nutritional factors.     

Characterizing the resident, fermentative microbial consortium in the hindgut of the temperate-zone herbivorous fish, Hermosilla azurea (Teleostei: Kyphosidae)

The zebraperch, Hermosilla azurea Jenkins and Evermann, a warm-temperate marine fish species with a strictly macroalgal diet, has a relatively long digestive tract with an enlarged hindgut and an associated blind caecum (HC). In zebraperch sampled off Santa Catalina Island, California (33°19′42′′N; 118°18′37′′W) in years 1995 through 2001, direct cell counts, gut epithelium assessment of bacterial attachment, and short-chain fatty acid (SCFA) analyses verified that the zebraperch HC possesses a dense and morphologically diverse, fermentative microbiota. Bacterial cell counts and morphological diversity were significantly higher in HC contents compared to anterior gut regions, suggesting that microbial populations were growing along the digestive tract. Similarly, electron micrographs of the HC epithelium revealed attached microbes, further supporting the possibility that these organisms constitute resident microbiota. Five different SCFAs were detected in all three regions of the digestive tract, but levels were up to three times greater in HC contents. Acetate was consistently the prevailing SCFA in all gut regions. Sequence analysis of bacterial 16S rDNA was used to identify predominant bacterial groups in HC contents. Of the seven main bacterial types identified, Enterovibrio spp. were the dominant bacteria in HC contents followed by species of Bacteroides,Faecalibacterium, and Desulfovibrio. Taken together, our findings show that the zebraperch HC harbors a consortium of microbes that appears to assist in the breakdown of algal polysaccharides in the herbivorous diet of the fish.     

Feeding ecology of the grooved tiger shrimp <Emphasis Type="Italic">Penaeus semisulcatus</Emphasis> De Haan (Decapoda: Penaeidae) in inshore waters of Qatar,Arabian Gulf

The feeding ecology of the green tiger shrimp Penaeus semisulcatus was studied in inshore fishing grounds off Doha, Qatar, using a combination of stable isotope (δ¹³C and δ¹⁵N) analysis and gut contents examination. Samples of post-larvae, juvenile and adult shrimp and other organisms were collected from intertidal and subtidal zones during the spawning season (January–June). Shrimp collected from shallow water seagrass beds were mostly post-larvae and juveniles and were significantly smaller than the older juveniles and adults caught in deeper macroalgal beds. Gut content examination indicated that post-larvae and juvenile shrimp in seagrass beds fed mainly on benthos such as Foraminifera, polychaetes, benthic diatoms and small benthic crustaceans (amphipods, isopods and ostracoda), whereas larger shrimp in the macroalgal beds fed mainly on bivalve molluscs and to a lesser extent polychaetes. In shrimp from both seagrass and algal beds, unidentifiable detritus was also present in the gut (18, 32%). δ¹³C values for shrimp muscle tissue ranged from −9.5 ± 0.26 to −12.7 ± 0.05‰, and δ¹⁵N values increased with increasing shrimp size, ranging from 4.1 ± 0.03 to 7.7 ± 0.11‰. Both δ¹⁵N values and δ¹³C values for shrimp tissue were consistent with the dietary sources indicated by gut contents and the δ¹³C and δ¹⁵N values for primary producers and prey species. The combination of gut content and stable isotope data demonstrates that seagrass beds are important habitats for post-larvae and juvenile P. semisulcatus, while the transition to deeper water habitats in older shrimp involves a change in diet and source of carbon and nitrogen that is reflected in shrimp tissue stable isotope ratios. The results of the study confirm the linkage between sensitive shallow water habitats and the key life stages of an important commercially-exploited species and indicate the need for suitable assessment of the potential indirect impacts of coastal developments involving dredging and land reclamation.     

Trophic diversity of idoteids (Crustacea, Isopoda) inhabiting the Posidonia oceanica litter

The coexistence of three idoteid species in Posidonia oceanica litter raises the question of trophic diversity and their role in the litter degradation process. Hence, diet composition of Idotea balthica, Idotea hectica and Cleantis prismatica was studied using a combination of gut contents and stable isotopes analysis. Gut content observations indicate that P. oceanica dead leaves are an important part of the ingested food for the three species, although their tissues are constituted of only a small to medium fraction of P. oceanica carbon. Our results also underlined the potential role of these species in the degradation of P. oceanica litter by mechanically fragmenting the litter and by assimilating a small to medium fraction of carbon. Moreover, we showed that there were considerable inter- and intra-specific differences in diet composition. Diet differed between juveniles and adults for I. balthica. Crustaceans are an important food source for adults of I. balthica, while I. hectica indicated a major contribution of algal material. C. prismatica showed an intermediate diet. This trophic diversity is probably one of the factors allowing these species to coexist in the same biotope.     

Hepatopancreatic endosymbionts in coastal isopods (Crustacea: Isopoda), and their contribution to digestion

Three isopod species (Crustacea: Isopoda), commonly found in the intertidal and supratidal zones of the North American Pacific coast, were studied with respect to symbiotic microbiota in their midgut glands (hepatopancreas). Ligia pallasii (Oniscidea: Ligiidae) contained high numbers of microbial symbionts in its hepatopancreatic caeca. Numbers of endosymbionts were strongly reduced by ingestion of antibiotics. By contrast, the hepatopancreas of Idotea wosnesenskii (Valvifera: Idoteidae) and Gnorimosphaeroma oregonense (Sphaeromatidea: Sphaeromatidae) did not contain any microbiota. Results of feeding experiments suggest that microbial endosymbionts contribute to digestive processes in L. pallasii, the most terrestrial of the three isopods that we studied. The acquisition of digestion-enhancing endosymbionts may have been an important evolutionary step allowing isopods to colonize terrestrial habitats where relatively indigestible leaf litter is the primary food source. By contrast, the ability to digest phenolic compounds was most developed in one of the more marine species, suggesting that this trait may have evolved independently in isopod species that consume a phenolic-rich diet, whether in marine habitats or on land. Received: 28 August 2000 / Accepted: 8 December 2000     

Relationship between the habitat and diet of three species of atherinids and three species of gobies in a temperate Australian estuary

The distributions and diets of the six most abundant species of teleost in the shallows of a large south-western Australian estuary were examined from samples collected between March 1988 and February 1989. Fish were collected monthly by seine net from over bare sand and from within patchy and dense areas of the aquatic macrophyte Ruppia megacarpa. Their gut contents were compared with samples of the benthos and plankton collected from each of these three habitat types. The densities of the atherinids Leptatherina wallacei and Atherinosoma elongata and of the goby Favonigobius suppositus were greatest in dense R. megacarpa, whereas those of the atherinid L. presbyteroides and the goby F. lateralis were greatest over bare sand. The density of the goby Pseudogobius olorum was greater in patchy R. megacarpa than in the other two habitat types. The gut contents of each of the six species was dominated by crustaceans and/or polychaetes, with detritus also making a major contribution to the diet of P. olorum and F. lateralis. The relative proportions of prey items in the guts of fish in a particular habitat corresponded to the preys' relative occurrence in the environment. This indicates that the fish had been feeding predominantly in one particular habitat prior to capture. Within each habitat type, the six species partitioned the available food, the major components of the diet of each species being different. The gobiid species fed mainly on the benthos and the atherinids typically fed higher in the water column; A. elongata and P. olorum tended to be less selective as to where they foraged. There were no consistent differences in either the dietary breadth or the fullness of the guts of any species among habitat types.     

The imperiled fish fauna in the Nicaragua Canal zone

Large‐scale infrastructure projects commonly have large effects on the environment. The planned construction of the Nicaragua Canal will irreversibly alter the aquatic environment of Nicaragua in many ways. Two distinct drainage basins (San Juan and Punta Gorda) will be connected and numerous ecosystems will be altered. Considering the project's far‐reaching environmental effects, too few studies on biodiversity have been performed to date. This limits provision of robust environmental impact assessments. We explored the geographic distribution of taxonomic and genetic diversity of freshwater fish species (Poecilia spp., Amatitlania siquia, Hypsophrys nematopus, Brycon guatemalensis, and Roeboides bouchellei) across the Nicaragua Canal zone. We collected population samples in affected areas (San Juan, Punta Gorda, and Escondido drainage basins), investigated species composition of 2 drainage basins and performed genetic analyses (genetic diversity, analysis of molecular variance) based on mitochondrial cytb. Freshwater fish faunas differed substantially between drainage basins (Jaccard similarity = 0.33). Most populations from distinct drainage basins were genetically differentiated. Removing the geographic barrier between these basins will promote biotic homogenization and the loss of unique genetic diversity. We found species in areas where they were not known to exist, including an undescribed, highly distinct clade of live bearing fish (Poecilia). Our results indicate that the Nicaragua Canal likely will have strong impacts on Nicaragua's freshwater biodiversity. However, knowledge about the extent of these impacts is lacking, which highlights the need for more thorough investigations before the environment is altered irreversibly.     

Global phylogeography of<Emphasis Type="Italic"> Cassiopea</Emphasis> (Scyphozoa: Rhizostomeae): molecular evidence for cryptic species and multiple invasions of the Hawaiian Islands

The upside-down jellyfish Cassiopea is a globally distributed, semi-sessile, planktonically dispersed scyphomedusa. Cassiopea occurs in shallow, tropical inshore marine waters on sandy mudflats and is generally associated with mangrove-dominated habitats. Controversy over the taxonomy of upside-down jellyfishes precedes their introduction to the Hawaiian Islands during the Second World War, and persists today. Here we address the global phylogeography and molecular systematics of the three currently recognized species: Cassiopea andromeda, C. frondosa, and C. xamachana. Mitochondrial cytochrome c oxidase I (COI) sequences from Australia, Bermuda, Fiji, the Florida Keys, the Hawaiian Islands, Indonesia, Palau, Panama, Papua New Guinea, and the Red Sea were analyzed. Highly divergent COI haplotypes within the putative species C. andromeda (23.4% Kimura 2-parameter molecular divergence), and shared haplotypes among populations of two separate putative species, C. andromeda and C. xamachana from different ocean basins, suggest multiple anthropogenic introductions and systematic confusion. Two deeply divergent Oahu haplotypes (20.3%) from morphologically similar, geographically separate invasive populations indicate long-term (14–40 million years ago) reproductive isolation of phylogenetically distinct source populations and cryptic species. Data support at least two independent introductions to the Hawaiian Islands, one from the Indo-Pacific, another from the western Atlantic/Red Sea. Molecular phylogenetic results support six species: (1) C. frondosa, western Atlantic (2) C. andromeda, Red Sea/western Atlantic/Hawaiian Islands (3) C. ornata, Indonesia/Palau/Fiji (4) Cassiopea sp. 1, eastern Australia (5) Cassiopea sp. 2, Papua New Guinea and (6) Cassiopea sp. 3, Papua New Guinea/Hawaiian Islands.Communicated by P.W. Sammarco, Chauvin     

Wood-based diet and gut microflora of a galatheid crab associated with Pacific deep-sea wood falls

Wood falls in the deep sea have recently become the focus of studies showing their importance as nutrients on the deep-sea floor. In such environments, Crustaceans constitute numerically the second-largest group after Mollusks. Many questions have arisen regarding their trophic role therein. A careful examination of the feeding appendages, gut contents, and gut lining of Munidopsis andamanica caught with wood falls revealed this species as a truly original detritivorous species using wood and the biofilm covering it as two main food sources. Comparing individuals from other geographic areas from substrates not reported highlights the galatheid crab as specialist of refractory substrates, especially vegetal remains. M. andamanica also exhibits a resident gut microflora consisting of bacteria and fungi possibly involved in the digestion of wood fragments. The results suggest that Crustaceans could be full-fledged actors in the food chains of sunken-wood ecosystems and that feeding habits of some squat lobsters could be different than scavenging.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Phytoplankton pigments in the gut concents of planktonic copepods from coastal waters off southern California

Phytoplankton xanthophylls in the gut contents of the copepods Calanus pacificus, Corycaeus anglicus, and Paracalanus parvus, collected from 5 stations off San Onofre, California, in June 1982, were measured by reverse phase, high-performance liquid chromatography (HPLC). The dinoflagellate pigment, peridinin, was usually the most abundant xanthophyll in the guts of all three species of copepods. Evidently, feeding was principally on dinoflagellates (which dominated the phytoplankton biomass). The level of feeding activity, rather than the class of phytoplankton ingested, seemed to differentiate the behaviors of the copepods. Xanthophyll content per unit copepod wet weight was higher in Corycaeus anglicus and Paracalanus parvus than in Calanus pacificus. Chlorophyll a fluorescence of the copepod gut contents was measured in conjunction with the analysis of gut xanthophylls. The xanthophyll content of the gut varied directly with the concentration of chlorophyll a in the gut. Xanthophyll content was not related to the concentration of pheopigments in the gut. Apparently, the xanthophylls that were detected were due to the presence of recently ingested phytoplankton biomass.     

Passing the gut of juvenile flounder, Platichthys flesus: differential survival of zoobenthic prey species

We studied the resistance of benthic prey organisms (Ostracoda, Hydrobia spp., Macoma balthica) to digestion by juvenile flounder, Platichthys flesus (L.), in a series of aquarium experiments. Results showed that some food species are able to survive the gut passage (Ostracoda 53 to 75%, Hydrobia spp. 46 to 92%). This has ecological implications, as increasing eutrophication of the Baltic Sea favours prey species that are nondigestible for the juvenile flounder. Thus, seemingly small changes in food-web structure may have unpredictable effects for the predatory fish. Received: 8 March 1997 / Accepted: 14 March 1997     

Selective feeding of littoral harpacticoids on diatom algae: hungry gourmands?

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm², so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.     

Detrimental effects of the isopod, <Emphasis Type="Italic">Edotia doellojuradoi,</Emphasis> on gill morphology and host condition of the mussel, <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis>

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.     

Microhabitat and resource utilization in cryptic rocky intertidal echinoderms at Aldabra Atoll,Seychelles

On a sheltered upper eulittoral rocky shore at Aldabra Atoll, 5 cryptic, tropical holothurian species Afrocucumis africana (Semper), Holothuria (Selenkothuria) parva Krauss, Polycheira rufescens (Brandt), H. (Thymiosycia) impatiens (Forskaal) and H. (Mertensiothuria) leucospilota Brandt, in decreasing order of abundance, co-occurred on intact beach-rock, whereas the only ophiuroid Ophiocoma scolopendrina (Lamarck) lived mainly on rubble. The 3 commonest holothurian species (A. africana, H. parva and P. rufescens) utilized significantly different microhabitats. All 5 holothurian species differed appreciably in body size, gut contents and feeding behaviour. The differing holochurian body sized could be related to both particle size classes of the gut contents of these species and microhabitats occupied. The amount of suitable microhabitats is probably more important to the number and abundance of cryptic species than the amount of detrital food resources available. The echinoderm assemblage appeared to be stable over time.     

Population dynamics and reproductive biology of the commensal isopod Colidotea rostrata (Crustacea: Isopoda: Idoteidae)

Colidotea rostrata (Benedict, 1898) is the only known commensal idoteid isopod, living on and mimicking the color of two northeastern Pacific sea urchins of the genus Strongylocentrotus. The population dynamics and reproductive biology of C. rostrata on its host urchins were studied at a low rocky intertidal area in southern California (33°40N; 118°30W) from December 1984 to December 1986. Isopod populations remained relatively stable throughout the 2 yr study, with isopods inhabiting an average of 56.1% of the urchins at 6.1±0.6 (x±2 SE) isopods per urchin. Female isopods reach sexual maturity at a length of 6.8 mm. Fecundity in C. rostrata averaged 11.8±0.9 (x±2 SE) embryos per brood, and increased with female body size. Breeding occurs all year long in C. rostrata, with a main reproductive period between the warmer spring and summer months. Newly released mancas and juvenile isopods were present during all months of the study. C. rostrata differs from the free-living Idoteidae in its smaller maximum size, reduced fecundity, 1:1 sex ratio, and low juvenile mortality. These features may represent adaptations to a commensal life style that reflect a reduced mortality pressure on these isopods.