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Papers by Richard Seaby

Journal of the Marine Biological Association of the UK, 1999

Using a 24-year time series of monthly samples, the factors correlated with long-term variation i... more Using a 24-year time series of monthly samples, the factors correlated with long-term variation in the abundance and growth of sole, Solea solea, in Bridgwater Bay in the Bristol Channel, England are identified. This bay offers shallow estuarine habitat used by sole as a nursery area. Sole first enter the bay in July when 2–3 months old and after a residence of 4–5 months, the majority migrate offshore at the beginning of winter to return the following April. By three years of age most have left the bay never to return although occasional large fish up to 480 mm in length are caught. Sole were found to be highly seasonal in their growth and only increased in length during the months of May to August inclusive. In recent years, there has been an approximately exponential increase in sole abundance that is highly positively correlated with seawater temperature during the early part of the season. The average length of fish in September, at the end of their first growing season, showed...

Journal of the Marine Biological Association of the United Kingdom, 1990

The population structure of the common shrimp Crangon crangon (L.) in British waters is investiga... more The population structure of the common shrimp Crangon crangon (L.) in British waters is investigated. Samples were collected from 31 sites along the coasts of England, Wales, Holland, Belgium and France. Using a morphometric analysis it is concluded that English and Welsh waters hold six distinct populations. Animals belonging to the same population are found on the opposing English and continental coasts in the southern North Sea and English Channel which indicates that the avoidance of offshore waters is not isolating the shrimps. The boundaries between the populations are found to correspond with the boundaries between major plankton assemblages. It is suggested that the populations are isolated by the reduced dispersal of the planktonic larvae across fronts where bodies of water with different physical properties meet but incompletely mix.

Netherlands Journal of Sea Research, 1994

ABSTRACT

Journal of Fish Biology, 1999

Journal of the Marine Biological Association of the UK, 2006

The results of a 25-year study of the population dynamics of the common shrimp, Crangon crangon, ... more The results of a 25-year study of the population dynamics of the common shrimp, Crangon crangon, in the Bristol Channel are presented. The population size varied seasonally, with maximum abundance occurring in early autumn at the completion of annual recruitment. The number of recruits changed greatly between years, and was positively correlated with both average water temperature from January to August, and river flow rate, and negatively correlated with the Winter North Atlantic Oscillation Index. A wide range of other physical and biotic variables was found to have no significant impact on C. crangon abundance. The positive relationship between temperature and C. crangon abundance observed for the Atlantic coast during this study is the opposite of that found for southern North Sea populations. Similar contradictory responses have been noted previously for flatfish such as sole, Solea solea. This suggests that global variables may act to produce different outcomes for Atlantic an...

Oecologia, 1996

ABSTRACT The triclads Polycelis tenuis and Dugesia polychroa and the glossiphoniid leeches Glossi... more ABSTRACT The triclads Polycelis tenuis and Dugesia polychroa and the glossiphoniid leeches Glossiphonia complanata and Helobdella stagnalis are abundant on the stony shores of productive British lakes. All species are food limited and there is considerable overlap in the diets of these triclads and leeches. This paper investigates interactions between the two groups using field and laboratory experiments to try to identify the mechanism of their co-existence. Triclad and leech numbers were manipulated inside experimental enclosures, mathced by controls, erected on the stony shore of an eutrophic English lake. Increasing the numbers of P. tenuis and D. polychroa prior to the reproductive season in spring resulted in a significant decrease in the numbers and body size of G. complanata and H. stagnalis compared with control populations in the summer months, and vice versa. However, increases and decreases were temporary with a readjustment of numbers and body size to control levels in the autumn after reproduction had ceased. It is suggested that increasing the numbers of either group elevated the severity of both intra- and interspecific competition for food. The condition of prey may, in part, determine the strength of competition, and this was examined in laboratory experiments in which different densities and ratios of P. tenuis and H. stagnalis were offered either live of recently crushed Asellus aquaticus. In monospecific controls, growth rates of P. tenuis were greater when fed on crushed than live Asellus, but there was no significant difference in the growth of H. stagnalis fed either live or crushed prey. In mixed cultures of predators, P. tenuis and H. stagnalis were the superior competitors when fed on crushed and live Asellus, respectively. However, when competitive pressure was low, at low densities of predators, the presence of H. stagnalis in mixed cultures fed on live prey was beneficial to the growth of P. tenuis. These results are explained in terms of the greater ability of triclads to detect damaged prey, leaking body fluids, due to their sophisticated chemosensory system, and the ability of leeches to capture live prey due to the presence of suckers. It is concluded that co-existence of the two groups in British lakes is assisted by the partitioning of food on a live or damaged basis.

Oecologia, 1995

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, a... more JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact

Journal of Experimental Marine Biology and Ecology, 2011

Results from a 30-year study of fish and crustacean abundance at Hinkley Point, Somerset, England... more Results from a 30-year study of fish and crustacean abundance at Hinkley Point, Somerset, England are reported. Standard community ecology metrics, including annual total species number recorded, alpha diversity and dominance indices, the rank-abundance curve and the assemblage of permanently present species, have all shown notable stability and no trend over the study period. In contrast, community structure has shown clear change which can be related to the fact that the abundances of many species have shown long-term trends. Of the 30 most abundant species, which together comprise more than 99% of the total species number and biomass collected, 17 have shown a long-term trend in log abundance indicative of exponential change. 9 species have shown approximately exponential increases, and 8 exponential decreases in abundance. This remarkable variation in individual species' abundance has been shown for some species to be related to changes in sea water temperature, the North Atlantic Oscillation Index, and salinity. While annual species richness has not increased, the number of species present each month has, on average, increased. This has been caused by changes in seasonal presence, with summer-autumn species extending their presence further into the winter. For fish, the dominant species show no trend, and it is argued they are likely to be under density-dependent control. It may be that while the most abundant species are constrained by resources, the majority of less abundant forms are dynamically unstable and more likely to be responsive to environmental change.

The Journal of Animal Ecology, 1994

ABSTRACT Summary 1. The leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella st... more ABSTRACT Summary 1. The leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella stagnalis, and their prey were manipulated inside experimental enclosures, matched by controls, erected on the stony shore of an eutrophic, English lake. 2. Five separate manipulations were performed: the addition of tubificid oligochaetes, the addition of snails, the increase in numbers of G. complanata and H. stagnalis, the increase in numbers of E. octoculata, and the reduction in numbers of all three leech species. All invertebrates were sampled, using trays buried in the substratum, in early March, prior to leech reproduction, in July and in late October/early November when breeding had ceased. Leech manipulations were executed immediately after the first sampling of invertebrates, when food addition, at fortnightly intervals, also commenced. 3. The addition of tubificids resulted in significantly greater numbers and biomasses of E. octoculata and H. stagnalis in the experimental compared to the control sites in the autumn sample, i.e. after breeding. The same was true for G. complanata when snails were added. When the numbers of leeches were either decreased or increased, no significant differences in numbers or biomass of the manipulated species were obtained in the autumn. 4. Elevation or readjustment of leech numbers appeared to be achieved through changes in juvenile mortality and to a lesser extent through fecundity. 5. Current results, together with previous findings, provide evidence for food limitation, with a high mortality of recruited young (up to 98%) which is linked to food supply. There is also some evidence for interspecific competition for food, though the occurrence of food refuges, the temporal separation of breeding peaks, and differences in feeding behaviour, involving body size and foraging strategies, between the glossiphoniids reduce the intensity of interactions.

Hydrobiologia, 1994

The diets of Polycelis nigra, P. tenuis, Dugesia polychroa, Dendrocoelum lacteum, Glossiphonia co... more The diets of Polycelis nigra, P. tenuis, Dugesia polychroa, Dendrocoelum lacteum, Glossiphonia complanata, Helobdella stagnalis and Erpobdella octoculata in an English lake were examined, using a serological technique, and compared between 1981-82 and 1989-90. Leech, triclad and prey abundances were also recorded. Between the two studies, snail numbers crashed whereas the abundances of Asellus and Gammarus increased. Dugesia and Glossiphonia numbers decreased substantially, whilst Polycelis tenuis and Helobdella abundances increased. In the second study, the snail component in the diet was greatly reduced resulting in a broader food niche, particularly for Dugesia and Glossiphonia, and greater food overlap between the predators with the exception of Dendrocoelum and Erpobdella which do not eat molluscs. It is postulated that the reduced size of the snail refuge, and consequent increase in severity of interspecific competition with other predators, particularly Polycelis and Helobdella, led to the observed decrease in abundances of Dugesia and Glossiphonia. The decline in the last two genera, perhaps coupled with increased crustacean abundance, could have contributed to the numerical increase of the competitively superior Polycelis tenuis and Helobdella.

Freshwater Biology, 1995

1. Three species of leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella stagna... more 1. Three species of leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella stagnalis, and four species of triclads, Polycelis nigra, P. tenuis, Dugesia polychroa and Dendrocoelum lacteum, commonly coexist on stony shores in productive British lakes. All species are food limited and there is much overlap in their diet. For both leech and triclad communities, coexistence of species is through the occurrence of food refuges. Leeches are more successful than triclads at capturing live prey, whereas both groups feed on damaged prey, comprising incapacitated, live or dead animals that are leaking body fluids. If triclads are better than leeches at exploiting damaged prey, this could be a mechanism for their coexistence. 2. Laboratory experiments investigated the comparative speeds al which leeches and triclads responded to crushed prey. Young and adult predators were offered a crushed specimen of the oligochaete Tubifex tubifex, the snail Lymnaea peregra, the crustacean Asellus aquaticus or the chironomid Chironomus sp., and their reaction times recorded. These four prey groups constitute the main diet of the predators in the field. Only D. polychroa and D. lacteum showed a significantly different reaction time between young and adults to crushed prey, and the reason for this is unclear. All predators, except H. stagnalis and D. polychroa, showed a difference in reaction time to the four types of prey, presumably a consequence of differences in both the 'quality' and 'concentration' of the different prey fluids, and there were some differences between predators in their speed of reaction to the same prey type. The following sequence, from fastest to slowest, in general reaction time to prey was obtained: £. octoculata, D. polychroa, P. tenuis, D. lacteum, P. nigra, H. stagnalis and G. complanata. 3. The location of the damaged food by the predators can be explained partly in terms of their foraging behaviour, with E. octoculata, D. polychroa and P. tenuis exhibiting a more seek-out strategy than other species which have a more sit-and-wait behaviour, and partly on the leveJ of sophistication of their chemosensory system used to detect leaked prey fluids. This system is highly developed in triclad species but poorly developed in leeches. 4. In a second type of experiment in which prey, L. peregra, A. aquaticus or Chironomus sp., were offered at different time intervals after crushing to H. stagnalis and P. tenuis, few predators fed on food crushed for 24 h or longer, although a few leeches fed on Chironomus crushed for up to 72 h. 5. It is concluded that coexistence of leech and triclad species on stony shores in lakes is assisted by partitioning of food on a damaged or live basis.

Hydrobiologia

The effect of predator and prey body size on the feeding success of the British lake-dwelling lee... more The effect of predator and prey body size on the feeding success of the British lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis was examined in the laboratory, and any involvement of size difference between the leeches in allowing coexistence in the field assessed. G. complanata breeds in advance of H. stagnalis and maintains a body size advantage throughout their annual life-cycle. In experiments, conducted at 14 C and a photoperiod of 16 hrs L: 8 hrs D, three size classes of leeches of each species were each exposed to each of three size classes of each of five prey species, viz. Tubifex sp., Chironomus sp., Asellus aquaticus, Lymnaea peregra and Potamopyrgus jenkinsi. For each prey species, three different types of experiments were performed: one leech exposed to four prey individuals; four leeches of the same species with sixteen prey; and two leeches of each species with sixteen prey. In the first experiment, all sizes of G. complanata were capable of fee...

Marine Environmental Research, 2004

Experiments have been undertaken exposing larval common shrimp (Crangon crangon) and lobster (Hom... more Experiments have been undertaken exposing larval common shrimp (Crangon crangon) and lobster (Homarus gammarus) and adult copepods (Acartia tonsa) to the key stresses of entrainment within power-station cooling-water systems. The apparatus has enabled the testing of mechanical, thermal, chlorine and realistic pressure effects both alone and in combination, the range of stressors spanning the standard conditions found within a temperate coastal direct-cooled power station. Mechanical stresses affected only lobster larvae, pressure changes affected only the Acartia adults. Residual chlorine caused significant mortality of Acartia and shrimp larvae, but had no effect on lobster larvae even at 1 ppm. The temperature increment significantly affected all three species, with a synergistic effect on chlorine sensitivity in the shrimp larvae, but only temperatures higher than would be experienced in a normallyoperating power station affected the copepods. The majority of individuals of each species would survive passage through a power-station system under normal conditions. It is notable that, within the species tested, generalizations from the responses of one species to those of another are not valid. #