Jellyfish modulate bacterial dynamic and community structure

Jellyfish blooms have increased in coastal areas around the world and the outbreaks have become longer and more frequent over the past few decades. The Mediterranean Sea is among the heavily affected regions and the common bloom-forming taxa are scyphozoans Aurelia aurita s.l., Pelagia noctiluca, and Rhizostoma pulmo. Jellyfish have few natural predators, therefore their carcasses at the termination of a bloom represent an organic-rich substrate that supports rapid bacterial growth, and may have a large impact on the surrounding environment. The focus of this study was to explore whether jellyfish substrate have an impact on bacterial community phylotype selection. We conducted in situ jellyfish-enrichment experiment with three different jellyfish species. Bacterial dynamic together with nutrients were monitored to assess decaying jellyfish-bacteria dynamics. Our results show that jellyfish biomass is characterized by protein rich organic matter, which is highly bioavailable to 'jellyfish-associated' and 'free-living' bacteria, and triggers rapid shifts in bacterial population dynamics and composition. Based on 16S rRNA clone libraries and denaturing gradient gel electrophoresis (DGGE) analysis, we observed a rapid shift in community composition from unculturable Alphaproteobacteria to culturable species of Gammaproteobacteria and Flavobacteria. The results of sequence analyses of bacterial isolates and of total bacterial community determined by culture independent genetic analysis showed the dominance of the Pseudoalteromonadaceae and the Vibrionaceae families. Elevated levels of dissolved proteins, dissolved organic and inorganic nutrient release, bacterial abundance and carbon production as well as ammonium concentrations characterized the degradation process. The biochemical composition of jellyfish species may influence changes in the amount of accumulated dissolved organic and inorganic nutrients. Our results can contribute insights into possible changes in bacterial population dynamics and nutrient pathways following jellyfish blooms which have important implications for ecology of coastal waters.     

Successful invasion of a food web in a chemostat

A food web in a chemostat is considered in which an arbitrary number of competitor populations compete for a single, essential, nonreproducing, growth-limiting substrate, and an arbitrary number of predator populations prey on some or all of the competitor populations. Although any number of predator populations may prey on the same competitor population, each predator population preys on only one competitor population. The dynamics of substrate uptake is modeled by Lotka-Volterra or Michaelis-Menten (Holling type I or II), but the dynamics of competitor uptake is restricted to Lotka-Volterra. Based on certain parameters, the model predicts the asymptotic survival or extinction of each of the different populations and suggests how competitor and/or predator populations could successfully invade the chemostat with or without causing a diverse ecosystem to crash. Similarly, it suggests how the elimination of certain populations could result in a more diverse or less diverse system.     

Environmental control of phase transition and polyp survival of a massive-outbreaker jellyfish

A number of causes have been proposed to account for the occurrence of gelatinous zooplankton (both jellyfish and ctenophore) blooms. Jellyfish species have a complex life history involving a benthic asexual phase (polyp) and a pelagic sexual phase (medusa). Strong environmental control of jellyfish life cycles is suspected, but not fully understood. This study presents a comprehensive analysis on the physicochemical conditions that control the survival and phase transition of Cotylorhiza tuberculata; a scyphozoan that generates large outbreaks in the Mediterranean Sea. Laboratory experiments indicated that the influence of temperature on strobilation and polyp survival was the critical factor controlling the capacity of this species to proliferate. Early life stages were less sensitive to other factors such as salinity variations or the competitive advantage provided by zooxanthellae in a context of coastal eutrophication. Coherently with laboratory results, the presence/absence of outbreaks of this jellyfish in a particular year seems to be driven by temperature. This is the first time the environmental forcing of the mechanism driving the life cycle of a jellyfish has been disentangled via laboratory experimentation. Projecting this understanding to a field population under climatological variability results in a pattern coherent with in situ records.     

Nudibranch predation and dietary preference for the polyps of <Emphasis Type="Italic">Aurelia</Emphasis><Emphasis Type="Italic">labiata</Emphasis> (Cnidaria: Scyphozoa)

There is concern that jellyfish blooms may be increasing worldwide. Some factors controlling population size, such as temperature and food, often have been studied; however, the importance of predators is poorly known. Aeolid nudibranchs feed on cnidarians, but their predation on the benthic polyps of scyphozoan rarely has been documented. To understand the potential of nudibranchs to consume polyps, we tested several predation preference hypotheses with the generalist feeding nudibranch, Hermissenda crassicornis, and polyps of the common moon jellyfish, Aurelia labiata. Of the six prey species tested during feeding experiments, A. labiata polyps and the tunicate Distaplia occidentalis were significantly preferred. Nudibranch size, diurnal cycle, and ingestive conditioning did not significantly influence prey choice. Nudibranchs showed significant positive chemotaxis toward living polyps, hydroids, and tunicates, but not to sea anemones. Nudibranch chemotaxis was significantly more positive to polar extract of A. labiata than of D. occidentalis. Consumption of polyps was correlated with nudibranch size, with mean consumption by large nudibranchs (>0.92 g) of about 31 polyps h⁻¹. Three other nudibranch species also ate A. labiata polyps. Our results emphasize the potential importance of predation for controlling jellyfish benthic polyp populations and consequent jellyfish blooms.     

盐度对不同温度下海月水母螅状体存活和无性繁殖的影响

海月水母是全球近岸海域的主要致灾水母种类之一,其螅状体的繁殖情况与种群数量是影响水母暴发的重要因素.采用实验生态学的方法分别研究了高温(21 ℃)和低温(12 ℃)条件下,不同盐度梯度对螅状体存活与无性繁殖的影响.结果表明: 高温条件下,盐度15～40时螅状体存活率均大于90%,适合出芽生殖盐度范围为20～32,其中28为最适盐度;在低盐(≤15)或高盐(≥36)环境下,螅状体会进行足囊生殖以度过不良环境条件. 低温条件下,20～40盐度组螅状体存活率均大于90%,20～32盐度组适于螅状体出芽生殖,其中28盐度组出芽生殖效率最高;20～40盐度适于横裂生殖,其中28～32盐度组最利于螅状体横裂生殖. 说明海月水母螅状体有较强的盐度耐受性,一定范围内盐度对螅状体无性生殖影响不显著.     

Fish assemblage structure in urban streams of Puerto Rico: the importance of reach- and catchment-scale abiotic factors

Cotylorhiza tuberculata is a common symbiotic scyphozoan in the Mediterranean Sea. The medusae occur in extremely high abundances in enclosed coastal areas in the Mediterranean Sea. Previous laboratory experiments identified thermal control on its early life stages as the driver of medusa blooms. In the present study, new ecological aspects were tested in laboratory experiments that support the pelagic population success of this zooxanthellate jellyfish. We hypothesized that planulae larvae would have no settlement preference among substrates and that temperature would affect ephyra development, ingestion rates and daily ration. The polyp budding rate and the onset of symbiosis with zooxanthellae also were investigated. Transmission electron microscopy revealed that zooxanthella infection occurred by the polyp stage. Our results showing no substrate selectivity by planulae and high polyp budding rates in high temperatures suggest increased benthic polyp populations, which would lead to higher medusa abundances. Rates of transition from ephyrae to medusae and the feeding of early medusa stages also increased with temperature. Continuing changes in coastal ecosystems such as future climate warming and marine construction may lead to increased populations of jellyfish to the detriment of fish globally.     

Timing and size of blooms of the ctenophore Mnemiopsis leidyi in relation to temperature in Narragansett Bay, RI

The ctenophore Mnemiopsis leidyi is at the northern extreme of its geographic range in Narragansett Bay, an estuary on the northeast coast of the United States. Blooms have typically been observed in late summer and fall according to records from 1950 to 1979. We document an expansion of the seasonal range of this important planktonic predator to include springtime blooms during the 1980s and 1990s. This shift to an earlier seasonal maximum is associated with increasing water temperature in Narragansett Bay. Temperatures in spring have risen, on average, 2 °C from 1950 to 1999 with warm years being associated with the positive phase of the North Atlantic Oscillation. During 1999, M. leidyi appeared earlier in spring and was more abundant than during any previous year for which records are available. Changes in the seasonal pattern and abundance of this predator are likely to have important effects on planktonic ecosystem dynamics of Narragansett Bay. These include reduction of zooplankton abundance in spring followed by increases in size and frequency of summer phytoplankton blooms. Earlier blooms of M. leidyi may also reduce survival of eggs and larvae of fish because, as in 1999, they coincide with the period of peak spawning.     

大型水母幼体生长的影响因子研究进展

21世纪以来,中国东、黄海,韩国西海岸以及日本海连年发生大型水母暴发现象,对海洋渔业的生产活动以及海洋生态系统带来巨大的影响。水母暴发形成机制非常复杂,解释其发生机理并有效预报是目前急待解决的问题。大型水母的生活史中有明显的世代交替现象,受精卵,浮浪幼虫,螅状体,足囊,横裂体到碟状体的幼体发育阶段属无性世代,幼蜇发育到成蜇阶段属有性世代。在早期生活史中,螅状体的足囊繁殖与横裂生殖是大型水母无性繁殖的重要方式,对其成体的数量形成至关重要。综述了国内外有关温度、盐度、光以及营养条件对大型水母早期发育阶段的影响研究进展,研究表明温度是影响螅状体发育以及足囊繁殖和横裂生殖的最主要的环境因子;盐度、光和营养条件在适温范围内,均对螅状体和横裂生殖有一定的影响,其上下限随水母种类和发育阶段有所变化。展望了大型水母早期幼体研究的发展趋势,如环境因子对不同种类的大型水母幼体生长机理的影响、多个环境因子对幼体的综合作用、动态的环境因子与大型水母幼体之间的关系等。     

水母旺发的诱因及对海洋环境的影响

水母旺发已成为一种新型海洋生态灾害,使海洋生态系统的结构和功能、海洋生态环境受到严重破坏.本文总结了水母旺发的可能诱因,重点探讨了水母旺发对海水生源要素、溶解氧、酸碱度以及生物群落的影响.结果表明: 水母旺发与其自身生理结构与生活史密切相关,其具有的身体结构简单、生长迅速、繁殖性强、世代间隔时间短及耐不良环境等特点,使其遇合适环境可迅速繁殖.水母旺发的主要直接诱因可能与海水温度变化有关,海水升温可导致水母食物增多,促进水母生殖,尤其对暖温性水母,更易引起水母聚集繁殖,形成旺发.富营养化、气候变化、过渡捕捞、生物入侵、栖息地改变也是水母旺发的重要影响因素.水母旺发可显著影响生源要素的形态和生物地球化学循环,水母排放NH₄⁺和PO₄^3-速率分别为59.1～91.5 μmol N·kg^-1·h^-1和1.1～1.8 μmol P·kg^-1·h^-1,可为浮游植物提供8%～10%和21.6%的N和P,释放溶解有机碳速率为1.0 μmol C·g^-1·d^-1.水母腐烂时总氮和总磷的释放速率可达4000 μmol N·kg^-1·d^-1和120 μmol P·kg^-1·d^-1,溶解有机碳的释放速率为30 μmol C·g^-1·d^-1;水母腐烂可影响水体的酸碱度与溶解氧含量,导致水体严重酸化与缺氧/无氧,pH降幅为1.3,平均耗氧量可达32.8 μmol·kg^-1·h^-1.水母旺发还可引起某些鱼类与浮游动物生物量的减少与重新分布,浮游微生物增加,间接导致浮游植物增加,引起海洋初级生产的异常.
     

Analyzing Beach Recreationists’ Preferences for the Reduction of Jellyfish Blooms: Economic Results from a Stated-Choice Experiment in Catalonia,Spain

Jellyfish outbreaks and their consequences appear to be on the increase around the world, and are becoming particularly relevant in the Mediterranean. No previous studies have quantified tourism losses caused by jellyfish outbreaks. We used a stated-choice questionnaire and a Random Utility Model to estimate the amount of time respondents would be willing to add to their journey, in terms of reported extra travel time, in order to reduce the risk of encountering jellyfish blooms in the Catalan coast. The estimation results indicated that the respondents were willing to spend on average an additional 23.8% of their travel time to enjoy beach recreation in areas with a lower risk of jellyfish blooms. Using as a reference the opportunity cost of time, we found that the subsample of individuals who made a trade-off between the disutility generated by travelling longer in order to lower the risk of jellyfish blooms, and the utility gained from reducing this risk, are willing to pay on average €3.20 per beach visit. This estimate, combined with the respondents’ mean income, yielded annual economic gains associated with reduction of jellyfish blooms on the Catalan coast around €422.57 million, or about 11.95% of the tourism expenditures in 2012. From a policy-making perspective, this study confirms the importance of the economic impacts of jellyfish blooms and the need for mitigation strategies. In particular, providing daily information using social media applications or other technical devices may reduce these social costs. The current lack of knowledge about jellyfish suggests that providing this information to beach recreationists may be a substantially effective policy instrument for minimising the impact of jellyfish blooms.     

Floater survival affects population persistence. The role of prey availability and environmental stochasticity

We develop two individual-based models using a large and detailed data set (information gathered over more than a century) on a population of a longlived and territorial predator, the Spanish imperial eagle. We investigated the relationship between survival and predator pressure, prey behaviour and patch availability (i.e. settlement areas). Survival of dispersing individuals was highly dependent on the number of available settlement areas, mediated by prey availability. Changes in prey behaviour due to predation pressure (e.g. shifting from diurnal to nocturnal activity) can decrease their availability for predators even if the density significantly exceeds the predator needs. Environmental stochasticity had a strong influence on population viability when it occurred in a synchroneous way between breeding and settlement areas, and an increase in floater mortality negatively influenced stability and dynamics of the breeding segment of populations in reproductive areas. Our simulations demonstrated the link between the dynamics in settlement and breeding areas: factors affecting floater survival also influence whole population dynamics. Moreover, model outputs provided insights into the relationship between environmental stochasticity and population dynamics.     

Obligate mutualism with a predator: Stability and persistence of three-species models

We present a general model for three interacting populations, where one population, called a mutualist, benefits a predator in its interaction with the prey. Biologically, there are four different ways in which the mutualist could benefit the predator: by enhancing prey growth rate, by enhancing the rate of prey capture, by providing an alternative food supply for the predator, and by enhancing the efficiency of utilization of prey, once they are ingested. We discuss examples of each type of interaction. We restrict our model to those situations in which the predator cannot survive on the prey in the absence of the mutualist. Therefore, if mutualism exists, it is obligate for the predator. Other conditions of the model include the dynamics of the prey and the mutualist alone and together in the absence of the predator. Given additional reasonable restrictions on the model, we determine the conditions for persistence, where persistence is defined as the continued existence of all three populations without any of them going extinct. There are two ways in which survival may arise in these models. Under one set of conditions, which is equivalent to the predator being able to invade a prey-mutualist system when rare, persistence will occur for any set of positive critical population sizes. Alternatively, survival will occur if there is an asymptotically stable interior equilibrium. However, the conditions for this are complex, and survival may occur only for initial populations in a limited region around the equilibrium.     

Associations of large jellyfish distributions with temperature and salinity in the Yellow Sea and East China Sea

Climate change may contribute to the increasing frequency and intensity of jellyfish blooms around the world. To test the null hypotheses that distributions did not differ among species of jellyfish or according to temperature salinity, we sampled large jellyfishes using bottom trawl surveys during 2006–2007 in the Yellow Sea (YS) and East China Sea (ECS). The total biomass of large jellyfish in the YS was low in April 2006 in cool waters, increased with warming waters, peaked in early September 2006 (22,891 ± 25,888 kg km^–2), and then decreased with cooling to minimal biomass during March 2007. During its peak early September 2006, Nemopilema nomurai was relatively eurythermal and distributed throughout the YS. Cyanea spp. occurred in warmer waters and attained maximum biomass in May 2007 in the ECS. Ulmaridae, which preferred colder temperatures, reached maximum biomass in October 2006 and occurred mainly in the central YS. Aequorea spp. usually occurred in colder waters, with maximum biomass in May 2007 mainly north of 30°N. Our analyses suggest that environmental preferences of the large jellyfish may enable prediction of jellyfish population sizes and distributions in Chinese waters, which is essential in order to address ecological problems caused by large jellyfish blooms in East Asia Waters.     

Degradation of the Adriatic medusa <Emphasis Type="Italic">Aurelia</Emphasis> sp. by ambient bacteria

The decomposition of jellyfish after major bloom events results in the release of large amounts of nutrients, which can significantly alter nutrient and oxygen dynamics in the surrounding environment. The response of the ambient bacterial community to decomposing jellyfish biomass was evaluated in two marine ecosystems, the Gulf of Trieste (northern Adriatic Sea) and Big Lake (Mljet Island, southern Adriatic Sea). The major difference between these two ecosystems is that Aurelia sp. medusae occur throughout the year in the oligotrophic Big Lake, whereas in the mesotrophic Gulf of Trieste, they occur only seasonally and often as blooms. Addition of homogenized jellyfish to enclosed bottles containing ambient water from each of these systems triggered considerable changes in the bacterial community dynamics and in the nutrient regime. The high concentrations of protein, dissolved organic phosphorous (DOP), and PO₄ ³⁻ immediately after homogenate addition stimulated increase in bacterial abundance and production rate, coupled with NH₄ ⁺ accumulation in both ecosystems. Our preliminary results of the bacterial community structure, as determined with denaturing gradient gel electrophoresis, indicated differences in the bacterial community response between the two ecosystems. Despite divergence in the bacterial community responses to jellyfish homogenate, increased bacterial biomass and growth rates in both distinctive marine systems indicate potentially significant effects of decaying jellyfish blooms on microbial plankton.     

Dynamics of a stochastic predator-prey model with habitat complexity and prey aggregation

Interplay between predator and prey is a complex process in ecosystems due to its nature. The population dynamics can be affected by many extrinsic and intrinsic factors. In this paper, we make an attempt to uncover the effects from environmental disturbances when populations are subject to habitat complexity and aggregation effect. We firstly propose a stochastic predator-prey model with habitat complexity and aggregation efficiency for prey. We then mathematically analyze the model, to demonstrate the existence, uniqueness and the stochastically ultimately boundedness of the global positive solution, and to establish sufficient conditions for the existence of ergodic stationary distribution of the solution. We also establish sufficient conditions under which either only predator population dies out or the entire predator-prey model becomes extinct. Our theoretical and numerical results indicate that: (1) the environmental noises are disadvantage for the survival of biological populations; (2) when the density of prey is greater than one, prey aggregation can heighten the capability of predator species to capture prey and reduce the effect of environmental fluctuations, while when the density of prey is less than one, the results are opposite; (3) habitat complexity is propitious to the survival of prey population and may seriously threaten the persistence of the predator population.     

Dynamics of predator and modular prey: effects of module consumption on stability of prey–predator system

In traditional models of predator–prey population dynamics, it is usually assumed that consumed prey are immediately removed from the population. However, in plant–herbivore interactions, damaged plants are generally alive after attacks by herbivores. This can result in successive or simultaneous attacks by multiple predators on a single prey item (here, the term prey is expanded to include plants). We constructed a mathematical model with two time scales, taking into account predation processes within a generation, considering post‐predation survival and the modularity of prey. We assumed that a prey item can be divided into modules and that it can be fed on by multiple predators or parasitized by multiple parasites at the same time. The model includes two essential factors: the modularity of prey for predators (n) and the detaching/attaching ratio of predators to prey (ε). Based on the formulae, we revealed a general property of realistic dynamics in plant–herbivore and host–parasite interactions. The analysis showed that the model could be approximated by models with the type I, type II or Beddington–DeAngelis functional responses by taking appropriate limits to the situations. When modularity is low or the detaching/attaching ratio is high, population dynamics tend to be stabilized. These stabilizing effects may be related to interference competition among predator individuals or increases in free prey modules and free predator individuals. In the limit of high modularity, the ratio of the attached prey modules to the total prey modules becomes negligible and the dynamics tend to be destabilized. However, if quantity and quality of prey modules are negatively correlated, the equilibrium is likely to be stabilized at high modularity as long as it remains feasible. These results suggest that considering post‐predation survival and modularity of prey is crucial to understand predator–prey interactions.     

Effects of pH on asexual reproduction and statolith formation of the scyphozoan, <Emphasis Type="Italic">Aurelia labiata</Emphasis>

Although anthropogenic influences such as global warming, overfishing, and eutrophication may contribute to jellyfish blooms, little is known about the effects of ocean acidification on jellyfish. Most medusae form statoliths of calcium sulfate hemihydrate that are components of their balance organs (statocysts). This study was designed to test the effects of pH (7.9, within the average current range, 7.5, expected by 2100, and 7.2, expected by 2300) combined with two temperatures (9 and 15°C) on asexual reproduction and statolith formation of the moon jellyfish, Aurelia labiata. Polyp survival was 100% after 122 d in seawater in all six temperature and pH combinations. Because few polyps at 9°C strobilated, and temperature effects on budding were consistent with published results, we did not analyze data from those three treatments further. At 15°C, there were no significant effects of pH on the numbers of ephyrae or buds produced per polyp or on the numbers of statoliths per statocyst; however, statolith size was significantly smaller in ephyrae released from polyps reared at low pH. Our results indicate that A. labiata polyps are quite tolerant of low pH, surviving and reproducing asexually even at the lowest tested pH; however, the effects of small statoliths on ephyra fitness are unknown. Future research on the behavior of ephyrae with small statoliths would further our understanding of how ocean acidification may affect jellyfish survival in nature.     

Influence of jellyfish blooms on carbon,nitrogen and phosphorus cycling and plankton production

Due to their boom and bust population dynamics and the enormous biomasses they can attain, jellyfish and ctenophores can have a large influence on the cycling of carbon (C), nitrogen (N) and phosphorus (P). This review initially summarises the biochemical composition of jellyfish, and compares and contrasts the mechanisms by which non-zooxanthellate and zooxanthellate jellyfish acquire and recycle C, N and P. The potential influence of elemental cycling by populations of jellyfish on phytoplankton and bacterioplankton production is then assessed. Non-zooxanthellate jellyfish acquire C, N and P predominantly through predation on zooplankton with smaller contributions from the uptake of dissolved organic matter. C, N and P are regenerated via excretion of inorganic (predominantly ammonium (NH₄ ⁺) and phosphate (PO₄ ³⁻)) and dissolved organic forms (e.g. dissolved free amino acids and dissolved primary amines). Inorganic nutrients excreted by jellyfish populations provide a small but significant proportion of the N and P required for primary production by phytoplankton. Excretion of dissolved organic matter may also support bacterioplankton production but few data are available. In contrast, zooxanthellate medusae derive most of their C from the translocation of photosynthetic products, exhibit no or minimal net release of N and P, and may actively compete with phytoplankton for dissolved inorganic nutrients. Decomposition of jellyfish blooms could result in a large release of inorganic and organic nutrients and the oxygen demand required to decompose their tissues could lead to localised hypoxic or anoxic conditions. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Jellyfish Body Plans Provide Allometric Advantages beyond Low Carbon Content

Jellyfish form spectacular blooms throughout the world’s oceans. Jellyfish body plans are characterised by high water and low carbon contents which enables them to grow much larger than non-gelatinous animals of equivalent carbon content and to deviate from non-gelatinous pelagic animals when incorporated into allometric relationships. Jellyfish have, however, been argued to conform to allometric relationships when carbon content is used as the metric for comparison. Here we test the hypothesis that differences in allometric relationships for several key functional parameters remain for jellyfish even after their body sizes are scaled to their carbon content. Data on carbon and nitrogen contents, rates of respiration, excretion, growth, longevity and swimming velocity of jellyfish and other pelagic animals were assembled. Allometric relationships between each variable and the equivalent spherical diameters of jellyfish and other pelagic animals were compared before and after sizes of jellyfish were standardised for their carbon content. Before standardisation, the slopes of the allometric relationships for respiration, excretion and growth were the same for jellyfish and other pelagic taxa but the intercepts differed. After standardisation, slopes and intercepts for respiration were similar but excretion rates of jellyfish were 10× slower, and growth rates 2× faster than those of other pelagic animals. Longevity of jellyfish was independent of size. The slope of the allometric relationship of swimming velocity of jellyfish differed from that of other pelagic animals but because they are larger jellyfish operate at Reynolds numbers approximately 10× greater than those of other pelagic animals of comparable carbon content. We conclude that low carbon and high water contents alone do not explain the differences in the intercepts or slopes of the allometric relationships of jellyfish and other pelagic animals and that the evolutionary longevity of jellyfish and their propensity to form blooms is facilitated by their unique body plans.     

Jellyfish blooms: are populations increasing globally in response to changing ocean conditions?

By the pulsed nature of their life cycles, gelatinous zooplankton come and go seasonally, giving rise in even the most undisturbed circumstances to summer blooms. Even holoplanktonic species like ctenophores increase in number in the spring or summer when planktonic food is available in greater abundance. Beyond that basic life cycle-driven seasonal change in numbers, several other kinds of events appear to be increasing the numbers of jellies present in some ecosystems. Over recent decades, man's expanding influence on the oceans has begun to cause real change and there is reason to think that in some regions, new blooms of jellyfish are occurring in response to some of the cumulative effects of these impacts. The issue is not simple and in most cases there are few data to support our perceptions. Some blooms appear to be long-term increases in native jellyfish populations. A different phenomenon is demonstrated by jellyfish whose populations regularly fluctuate, apparently with climate, causing periodic blooms. Perhaps the most damaging type of jellyfish increase in recent decades has been caused by populations of new, nonindigenous species gradually building-up to `bloom' levels in some regions. Lest one conclude that the next millennium will feature only increases in jellyfish numbers worldwide, examples are also given in which populations are decreasing in heavily impacted coastal areas. Some jellyfish will undoubtedly fall subject to the ongoing species elimination processes that already portend a vast global loss of biodiversity. Knowledge about the ecology of both the medusa and the polyp phases of each life cycle is necessary if we are to understand the true causes of these increases and decreases, but in most cases where changes in medusa populations have been recognized, we know nothing about the field ecology of the polyps.