David Holdich - Academia.edu (original) (raw)
Papers by David Holdich
Frontiers in Marine Science, 2016
Problem statement Isopods have direct development and lack larval stages, therefore their dispers... more Problem statement Isopods have direct development and lack larval stages, therefore their dispersal ability and gene flow between populations may be potentially lower compared to other marine invertebrates with planktonic larvae. This may contribute to increase the genetic isolation of populations and eventually facilitate speciation events. A sound discrimination of species is crucial in biodiversity assessment, but the establishment of species boundaries requires a combination of multiple approaches (e.g. molecular, morphological and ecological data). However, numerous studies in marine invertebrates, including isopods (e.g. Hurtado et al. 2010), have been reporting the occurrence of cryptic species, which can only be detected through the use of molecular markers, since they can hardly be distinguished through morphological features. Standardized universal molecular markers such as DNA barcodes, the cytochrome c oxidase gene (COI-5P) (Hebert et al. 2003) have been highly successful in the detection of potential and actual cryptic species in a vast diversity of animals, including crustaceans (Costa et al. 2007). In fact, the existence of cryptic species are suspected within most of the large genera in the family Sphaeromatidae such as Cymodoce, Dynamenella and Dynamene (Poore and Bruce 2012). Dynamene is a species-poor genus of sphaeromatid isopods, characterized by the presence of the bidentate process in adult males, a structure unique to this genus. The shape of the bidentate process is necessary to identify and distinguish the different Dynamene species. This implies that adult males are required to accurately discriminate the different species and, in their absence, species identifications of females and juveniles are extremely difficult. Members of this genus are very common and abundant among the invertebrate fauna of intertidal and shallow subtidal rocky substrates in Northeast Atlantic Ocean, where they are represented by 3 species only: D. bidentata, D. edwardsi and D. magnitorata (Holdich 1968). In this study, we examine the genetic diversity of Dynamene morphospecies in rigorously identified specimens collected along Macaronesian, European and Moroccan Atlantic rocky shores, using the DNA barcode region (COI-5P). Results: Cytochrome oxidase I (COI-5P) clearly discriminated between the three species, confirming their morphology-based identifications. Among Dynamene morphospecies the pair D. bidentata / D. magnitorata differed less (19%) when comparing with D. edwardsi (26 and 25% respectively). Both D. bidentata and D. magnitorata displayed intra-specific distances below 2%, in spite the specimens originated from relatively distant populations (e.g. Scotland and Morocco). While in D. bidentata and D. magnitorata nearly none or little population structure was observed, D. edwardsi displayed extensive structure, where it is possible to discriminate up to eight geographically circumscribed lineages with complete sorting and exclusive haplotypes, and displaying over 4% of genetic distance between them. All lineages showed little within lineage diversity (less than 1%). Inspection of the deep phylogeny of D. edwardsi reveals four major deeply divergent lineages, displaying genetic distances between
Journal of Natural History, 1970
Unlike most other members of the family Sphaeromatidae the genus Dynamene Leach is virtually rest... more Unlike most other members of the family Sphaeromatidae the genus Dynamene Leach is virtually restricted to the Boreal-Atlanto-Mediterranean-Black Sea region of the northern hemisphere. Seven species are now known (Holdich, 1968 c), only one of which occurs outside that geographical area, in Australia (Baker, 1908). From past records it would appear that this genus also has a limited habitat range, being restricted to intertidal or shallow water algae and crevices (
A new species of the isopod genus Dynamene, D. curalii, is described from the coral habitat of Ke... more A new species of the isopod genus Dynamene, D. curalii, is described from the coral habitat of Keren Island and Lizard Island in Queensland. The only other species of this genus known from the Southern Hemisphere, D. ramuscula Baker from South Australia, is redescribed. It is noted that the two Australian species differ markedly in one of the diagnostic characters when compared with species from Europe and N.W. Africa.
Journal of the Marine Biological Association of the United Kingdom, 2002
The sphaeromatid crustacean isopod genus Campecopea is revised, a new diagnosis given and the gen... more The sphaeromatid crustacean isopod genus Campecopea is revised, a new diagnosis given and the genus Anoplocopea Racovitza, 1907, is placed in synonymy; the type species Campecopea hirsuta is redescribed and Campecopea lusitanica comb. nov. is recorded from the Canary Islands and Azores. The four species of Campecopea differ from each other principally in the presence or absence of a prominent posteriorly directed dorsal process on the male pereonite 6, the ornamentation of the posterior pereonites, the shape of the uropods and also the fine details of the dactylus accessory spine (smooth or serrate). The phylogenetic significance of dorsal process is re-evaluated here, and the character discussed in relation to Campecopea, Dynoides and Clianella, and several other sphaeromatid genera. It is regarded that the interpretation of this character as being of intrinsic generic merit has resulted in the over splitting of several genera and also the creation of paraphyletic genera. It is su...
Invading Nature - Springer Series In Invasion Ecology
... However, in the 'Global Strategy on Invasive Alien Species'... more ... However, in the 'Global Strategy on Invasive Alien Species'(McNeely et al. ... The classification ofCrustacea used in this review is mainly based on that of Martin and Davis (2001). ... For the first time in the Baltic Sea basin, the species was recorded in Poland in 1997, and its range ...
Knowledge and Management of Aquatic Ecosystems, 2009
Une mise à jour sur la menace perpétuelle des écrevisses non indigènes pour les écrevisses europé... more Une mise à jour sur la menace perpétuelle des écrevisses non indigènes pour les écrevisses européennes
Zoological Journal of the Linnean Society, 1989
ABSTRACT
Zoologica Scripta, 1984
ABSTRACT
Marine Biology, 1971
The main food reserve of the intertidal hebivorous isopod Dynamene bidentata is lipid. Only algal... more The main food reserve of the intertidal hebivorous isopod Dynamene bidentata is lipid. Only algal-dwelling juveniles exhibit rhythms of oxygen consumption and defecation related to the tidal cycle. Males undergo the moult to maturity in the algal habitat and are, therefore, able to replenish their food reserves before entering upon a nonfeeding reproductive phase in empty barncle tests and erevices.
Bulletin Français de la Pêche et de la Pisciculture, 2002
The distribution of crayfish in Europe is examined using information from a variety of sources ma... more The distribution of crayfish in Europe is examined using information from a variety of sources mainly for the last two decades. All European countries have at least one indigenous crayfish species (ICS), many with large stocks, and most countries in Western Europe have at least one non-indigenous crayfish species (NICS). Some species termed indigenous in certain countries may have been introduced in relatively recent times. However, the meaning of the term indigenous (native) varies between countries. NICS continue to expand their ranges and present a continued threat to ICS through competition and transmission of diseases, notably crayfish plague. The current high level of interest in crayfish means that many countries are monitoring their crayfish situation and, where necessary, taking steps to renew stocks of threatened indigenous species, some of which are apparently on the verge of extinction. A more regular monitoring programme for European crayfish as a whole is needed if the...
Bulletin Français de la Pêche et de la Pisciculture, 1997
Great Britain and Ireland still contain some of the best stocks of Austropotamobius pallipes in E... more Great Britain and Ireland still contain some of the best stocks of Austropotamobius pallipes in Europe despite the fact that since the early 1980s many populations have been devastated by the effects of crayfish plague and compétitive exclusion by non-native crayfish. Austropotamobius pallipes is the only crayfish native to Great Britain and Ireland but in récent years a number of introductions of foreign crayfish have been made into Great Britain, for aquacultural, culinary and aquarist purposes. This has resulted in four non-native crayfish species becoming established in thewild, wheretwo, Pacifastacus leniusculus and Astacusleptodactylus, have formed large, harvestable populations, particularly in southern England. Although A. pallipes in Great Britain and Ireland is protected by national and European législation, this has not been enough to protect it from crayfish plague and, in Great Britain, from non-native crayfish. In an attempt to protect A. pallipes even further, législation has been implemented in England, Scotland and Wales which bans the keeping of ail non-native crayfish, except wherethey are being prepared for human consumption. P. leniusculus is, however, exempt from this ban in certain parts of southern Great Britain dueto its high prépondérance on crayfish farms and in the wild. Ireland already bans the introduction of non-native crayfish. It is hoped that the new législation, plus the heightened profile which A. pallipes has recently been given in Great Britain, will ensure its future survival.
Bulletin Français de la Pêche et de la Pisciculture, 2006
The white-clawed crayfish, Austropotamobius pallipes, is usually found associated with stony habi... more The white-clawed crayfish, Austropotamobius pallipes, is usually found associated with stony habitats containing obvious refuges in the form of gaps between and under rocks, macrophytes and marginal tree roots, particularly in streams and lakes with clear water and little marginal mud. If the banks are composed of suitable material, then they may also construct and live in burrows. However, the white-clawed crayfish is also found to be abundant in streams, rivers, canals and millraces with deep, anoxic mud and with very little aquatic vegetation. Foraging on the surface of mud may be the only way they can obtain sufficient food in the form of macroinvertebrates and decaying plant matter. Where do crayfish live in this restricted habitat? Dewatering such waterways for essential engineering works, such as desilting, bridge and weir repairs, bank reinforcements, and maintenance of outfalls can provide an excellent opportunity to study the available habitat and the crayfish populations, in addition good estimates of population size and age class distribution can be obtained, although, as with other methods, juveniles tend to be underrepresented. A number of case studies will be given to illustrate the fact that white-clawed crayfish are able to colonize muddy habitats in some numbers. The value of retaining trees with their roots hanging into waterways as a refuge for both crayfish and small fish is highlighted.
Bulletin Français de la Pêche et de la Pisciculture, 2006
Bulletin Français de la Pêche et de la Pisciculture, 2005
As a result of discussions held at the Innsbruck CRAYNET meeting and the answers given to a subse... more As a result of discussions held at the Innsbruck CRAYNET meeting and the answers given to a subsequent questionnaire sent out to the National Coordinators of the 11 countries/regions, it is clear that most European countries aim at trying to protect their indigenous crayfish species (ICS) from overexploitation, habitat modification, pollution, and spread of non-indigenous crayfish species (NICS) and crayfish plague.
Journal of Zoology, 2009
With 12 figures in the text) Laboratory and field investigations show that there are five distinc... more With 12 figures in the text) Laboratory and field investigations show that there are five distinct marsupial stages and eight post-marsupial stages in the development of the isopod Dynamene bidentutu (Adams). Extensive sexual dimorphism is apparent, beginning at stage 6, and females undergo extensive tissue reduction at the moult to the eighth stage. Young stages inhabit and feed on intertidal algae, and the non-feeding adults (stage 7 and 8 females, and stage 8 males) shelter in crevices or empty barnacle tests. Average brood numbers are fairly high (c. 90) and the average marsupial mortality rate is about 36%. Release of young takes place in May/June, after which the females die, and a resident population of males remains in the crevices to be joined by newly maturing females and males from August onwards each year. The occurrence of two breeding seasons in males, but not in females, explains how the sex ratio of males to females as they leave the algal habitat (1 :9) is about half the ratio of males to females in the reproductive habitat (1 :4).
Journal of Zoology, 2009
Page 1. J. Zool., Lond. (1974) 172,469-482 An investigation of the surface of the cuticle and ass... more Page 1. J. Zool., Lond. (1974) 172,469-482 An investigation of the surface of the cuticle and associated sensory structures of the terrestrial isopod, Porcellio scaber DM HOLDICH Department of Zoology, The University, Nottingham ...
Journal of Natural History, 1988
Journal of Experimental Marine Biology and Ecology, 1976
Abstract A comparison is made of the vertical distribution, life cycles, and ecology of two litto... more Abstract A comparison is made of the vertical distribution, life cycles, and ecology of two littoral species of the sexually dimorphic isopod, Dynamene , on rocky shores of northwest France. Adult D. bidentata (Adams) prefer middle and lower eulittoral crevices and empty Balanus perforatus Brug. tests, and juveniles a wide variety of algae in the same zones. Males are capable of killing B. perforatus by wedging themselves tightly in the operculum so preventing feeding and respiration. Adult Dynamene magnitorata Holdich prefer empty Balanus crenatus Brug. tests, channels within sponges such as Halichondria and Hymeniacidon , and spaces within and between ascidian colonies, e.g., Dendrodoa and Didemnum , in the lower eulittoral and upper sublittoral. Juveniles are found in the same zones on a restricted number of algal species, in particular Chondrus crispus Stack. There is little overlap in the habitat preferences of the two species. Availability of food and tolerance of desiccation do not appear to be the primary factors responsible for the maintenance of zonation patterns; temperature would appear to be the main factor involved. The stages of the life cycle of the two species are the same but the sex-ratio in Dynamene magnitorata is much lower, and this is possibly due to the fact that the male may only spend one breeding season in the reproductive habitat as opposed to the two spent by male D. bidentata . Populations of D. bidentata from different latitudes show a tendency to increase the length of their reproductive and growth phases at more southerly locations so that ovigerous females and juveniles occur throughout the year. Winter air temperatures are a possible cause of these differences in timing. Stereoscan electron microscope observations are included to clarify the identification of the three species of Dynamene found on the Atlantic coasts of Europe and northwest Africa.
Frontiers in Marine Science, 2016
Problem statement Isopods have direct development and lack larval stages, therefore their dispers... more Problem statement Isopods have direct development and lack larval stages, therefore their dispersal ability and gene flow between populations may be potentially lower compared to other marine invertebrates with planktonic larvae. This may contribute to increase the genetic isolation of populations and eventually facilitate speciation events. A sound discrimination of species is crucial in biodiversity assessment, but the establishment of species boundaries requires a combination of multiple approaches (e.g. molecular, morphological and ecological data). However, numerous studies in marine invertebrates, including isopods (e.g. Hurtado et al. 2010), have been reporting the occurrence of cryptic species, which can only be detected through the use of molecular markers, since they can hardly be distinguished through morphological features. Standardized universal molecular markers such as DNA barcodes, the cytochrome c oxidase gene (COI-5P) (Hebert et al. 2003) have been highly successful in the detection of potential and actual cryptic species in a vast diversity of animals, including crustaceans (Costa et al. 2007). In fact, the existence of cryptic species are suspected within most of the large genera in the family Sphaeromatidae such as Cymodoce, Dynamenella and Dynamene (Poore and Bruce 2012). Dynamene is a species-poor genus of sphaeromatid isopods, characterized by the presence of the bidentate process in adult males, a structure unique to this genus. The shape of the bidentate process is necessary to identify and distinguish the different Dynamene species. This implies that adult males are required to accurately discriminate the different species and, in their absence, species identifications of females and juveniles are extremely difficult. Members of this genus are very common and abundant among the invertebrate fauna of intertidal and shallow subtidal rocky substrates in Northeast Atlantic Ocean, where they are represented by 3 species only: D. bidentata, D. edwardsi and D. magnitorata (Holdich 1968). In this study, we examine the genetic diversity of Dynamene morphospecies in rigorously identified specimens collected along Macaronesian, European and Moroccan Atlantic rocky shores, using the DNA barcode region (COI-5P). Results: Cytochrome oxidase I (COI-5P) clearly discriminated between the three species, confirming their morphology-based identifications. Among Dynamene morphospecies the pair D. bidentata / D. magnitorata differed less (19%) when comparing with D. edwardsi (26 and 25% respectively). Both D. bidentata and D. magnitorata displayed intra-specific distances below 2%, in spite the specimens originated from relatively distant populations (e.g. Scotland and Morocco). While in D. bidentata and D. magnitorata nearly none or little population structure was observed, D. edwardsi displayed extensive structure, where it is possible to discriminate up to eight geographically circumscribed lineages with complete sorting and exclusive haplotypes, and displaying over 4% of genetic distance between them. All lineages showed little within lineage diversity (less than 1%). Inspection of the deep phylogeny of D. edwardsi reveals four major deeply divergent lineages, displaying genetic distances between
Journal of Natural History, 1970
Unlike most other members of the family Sphaeromatidae the genus Dynamene Leach is virtually rest... more Unlike most other members of the family Sphaeromatidae the genus Dynamene Leach is virtually restricted to the Boreal-Atlanto-Mediterranean-Black Sea region of the northern hemisphere. Seven species are now known (Holdich, 1968 c), only one of which occurs outside that geographical area, in Australia (Baker, 1908). From past records it would appear that this genus also has a limited habitat range, being restricted to intertidal or shallow water algae and crevices (
A new species of the isopod genus Dynamene, D. curalii, is described from the coral habitat of Ke... more A new species of the isopod genus Dynamene, D. curalii, is described from the coral habitat of Keren Island and Lizard Island in Queensland. The only other species of this genus known from the Southern Hemisphere, D. ramuscula Baker from South Australia, is redescribed. It is noted that the two Australian species differ markedly in one of the diagnostic characters when compared with species from Europe and N.W. Africa.
Journal of the Marine Biological Association of the United Kingdom, 2002
The sphaeromatid crustacean isopod genus Campecopea is revised, a new diagnosis given and the gen... more The sphaeromatid crustacean isopod genus Campecopea is revised, a new diagnosis given and the genus Anoplocopea Racovitza, 1907, is placed in synonymy; the type species Campecopea hirsuta is redescribed and Campecopea lusitanica comb. nov. is recorded from the Canary Islands and Azores. The four species of Campecopea differ from each other principally in the presence or absence of a prominent posteriorly directed dorsal process on the male pereonite 6, the ornamentation of the posterior pereonites, the shape of the uropods and also the fine details of the dactylus accessory spine (smooth or serrate). The phylogenetic significance of dorsal process is re-evaluated here, and the character discussed in relation to Campecopea, Dynoides and Clianella, and several other sphaeromatid genera. It is regarded that the interpretation of this character as being of intrinsic generic merit has resulted in the over splitting of several genera and also the creation of paraphyletic genera. It is su...
Invading Nature - Springer Series In Invasion Ecology
... However, in the 'Global Strategy on Invasive Alien Species'... more ... However, in the 'Global Strategy on Invasive Alien Species'(McNeely et al. ... The classification ofCrustacea used in this review is mainly based on that of Martin and Davis (2001). ... For the first time in the Baltic Sea basin, the species was recorded in Poland in 1997, and its range ...
Knowledge and Management of Aquatic Ecosystems, 2009
Une mise à jour sur la menace perpétuelle des écrevisses non indigènes pour les écrevisses europé... more Une mise à jour sur la menace perpétuelle des écrevisses non indigènes pour les écrevisses européennes
Zoological Journal of the Linnean Society, 1989
ABSTRACT
Zoologica Scripta, 1984
ABSTRACT
Marine Biology, 1971
The main food reserve of the intertidal hebivorous isopod Dynamene bidentata is lipid. Only algal... more The main food reserve of the intertidal hebivorous isopod Dynamene bidentata is lipid. Only algal-dwelling juveniles exhibit rhythms of oxygen consumption and defecation related to the tidal cycle. Males undergo the moult to maturity in the algal habitat and are, therefore, able to replenish their food reserves before entering upon a nonfeeding reproductive phase in empty barncle tests and erevices.
Bulletin Français de la Pêche et de la Pisciculture, 2002
The distribution of crayfish in Europe is examined using information from a variety of sources ma... more The distribution of crayfish in Europe is examined using information from a variety of sources mainly for the last two decades. All European countries have at least one indigenous crayfish species (ICS), many with large stocks, and most countries in Western Europe have at least one non-indigenous crayfish species (NICS). Some species termed indigenous in certain countries may have been introduced in relatively recent times. However, the meaning of the term indigenous (native) varies between countries. NICS continue to expand their ranges and present a continued threat to ICS through competition and transmission of diseases, notably crayfish plague. The current high level of interest in crayfish means that many countries are monitoring their crayfish situation and, where necessary, taking steps to renew stocks of threatened indigenous species, some of which are apparently on the verge of extinction. A more regular monitoring programme for European crayfish as a whole is needed if the...
Bulletin Français de la Pêche et de la Pisciculture, 1997
Great Britain and Ireland still contain some of the best stocks of Austropotamobius pallipes in E... more Great Britain and Ireland still contain some of the best stocks of Austropotamobius pallipes in Europe despite the fact that since the early 1980s many populations have been devastated by the effects of crayfish plague and compétitive exclusion by non-native crayfish. Austropotamobius pallipes is the only crayfish native to Great Britain and Ireland but in récent years a number of introductions of foreign crayfish have been made into Great Britain, for aquacultural, culinary and aquarist purposes. This has resulted in four non-native crayfish species becoming established in thewild, wheretwo, Pacifastacus leniusculus and Astacusleptodactylus, have formed large, harvestable populations, particularly in southern England. Although A. pallipes in Great Britain and Ireland is protected by national and European législation, this has not been enough to protect it from crayfish plague and, in Great Britain, from non-native crayfish. In an attempt to protect A. pallipes even further, législation has been implemented in England, Scotland and Wales which bans the keeping of ail non-native crayfish, except wherethey are being prepared for human consumption. P. leniusculus is, however, exempt from this ban in certain parts of southern Great Britain dueto its high prépondérance on crayfish farms and in the wild. Ireland already bans the introduction of non-native crayfish. It is hoped that the new législation, plus the heightened profile which A. pallipes has recently been given in Great Britain, will ensure its future survival.
Bulletin Français de la Pêche et de la Pisciculture, 2006
The white-clawed crayfish, Austropotamobius pallipes, is usually found associated with stony habi... more The white-clawed crayfish, Austropotamobius pallipes, is usually found associated with stony habitats containing obvious refuges in the form of gaps between and under rocks, macrophytes and marginal tree roots, particularly in streams and lakes with clear water and little marginal mud. If the banks are composed of suitable material, then they may also construct and live in burrows. However, the white-clawed crayfish is also found to be abundant in streams, rivers, canals and millraces with deep, anoxic mud and with very little aquatic vegetation. Foraging on the surface of mud may be the only way they can obtain sufficient food in the form of macroinvertebrates and decaying plant matter. Where do crayfish live in this restricted habitat? Dewatering such waterways for essential engineering works, such as desilting, bridge and weir repairs, bank reinforcements, and maintenance of outfalls can provide an excellent opportunity to study the available habitat and the crayfish populations, in addition good estimates of population size and age class distribution can be obtained, although, as with other methods, juveniles tend to be underrepresented. A number of case studies will be given to illustrate the fact that white-clawed crayfish are able to colonize muddy habitats in some numbers. The value of retaining trees with their roots hanging into waterways as a refuge for both crayfish and small fish is highlighted.
Bulletin Français de la Pêche et de la Pisciculture, 2006
Bulletin Français de la Pêche et de la Pisciculture, 2005
As a result of discussions held at the Innsbruck CRAYNET meeting and the answers given to a subse... more As a result of discussions held at the Innsbruck CRAYNET meeting and the answers given to a subsequent questionnaire sent out to the National Coordinators of the 11 countries/regions, it is clear that most European countries aim at trying to protect their indigenous crayfish species (ICS) from overexploitation, habitat modification, pollution, and spread of non-indigenous crayfish species (NICS) and crayfish plague.
Journal of Zoology, 2009
With 12 figures in the text) Laboratory and field investigations show that there are five distinc... more With 12 figures in the text) Laboratory and field investigations show that there are five distinct marsupial stages and eight post-marsupial stages in the development of the isopod Dynamene bidentutu (Adams). Extensive sexual dimorphism is apparent, beginning at stage 6, and females undergo extensive tissue reduction at the moult to the eighth stage. Young stages inhabit and feed on intertidal algae, and the non-feeding adults (stage 7 and 8 females, and stage 8 males) shelter in crevices or empty barnacle tests. Average brood numbers are fairly high (c. 90) and the average marsupial mortality rate is about 36%. Release of young takes place in May/June, after which the females die, and a resident population of males remains in the crevices to be joined by newly maturing females and males from August onwards each year. The occurrence of two breeding seasons in males, but not in females, explains how the sex ratio of males to females as they leave the algal habitat (1 :9) is about half the ratio of males to females in the reproductive habitat (1 :4).
Journal of Zoology, 2009
Page 1. J. Zool., Lond. (1974) 172,469-482 An investigation of the surface of the cuticle and ass... more Page 1. J. Zool., Lond. (1974) 172,469-482 An investigation of the surface of the cuticle and associated sensory structures of the terrestrial isopod, Porcellio scaber DM HOLDICH Department of Zoology, The University, Nottingham ...
Journal of Natural History, 1988
Journal of Experimental Marine Biology and Ecology, 1976
Abstract A comparison is made of the vertical distribution, life cycles, and ecology of two litto... more Abstract A comparison is made of the vertical distribution, life cycles, and ecology of two littoral species of the sexually dimorphic isopod, Dynamene , on rocky shores of northwest France. Adult D. bidentata (Adams) prefer middle and lower eulittoral crevices and empty Balanus perforatus Brug. tests, and juveniles a wide variety of algae in the same zones. Males are capable of killing B. perforatus by wedging themselves tightly in the operculum so preventing feeding and respiration. Adult Dynamene magnitorata Holdich prefer empty Balanus crenatus Brug. tests, channels within sponges such as Halichondria and Hymeniacidon , and spaces within and between ascidian colonies, e.g., Dendrodoa and Didemnum , in the lower eulittoral and upper sublittoral. Juveniles are found in the same zones on a restricted number of algal species, in particular Chondrus crispus Stack. There is little overlap in the habitat preferences of the two species. Availability of food and tolerance of desiccation do not appear to be the primary factors responsible for the maintenance of zonation patterns; temperature would appear to be the main factor involved. The stages of the life cycle of the two species are the same but the sex-ratio in Dynamene magnitorata is much lower, and this is possibly due to the fact that the male may only spend one breeding season in the reproductive habitat as opposed to the two spent by male D. bidentata . Populations of D. bidentata from different latitudes show a tendency to increase the length of their reproductive and growth phases at more southerly locations so that ovigerous females and juveniles occur throughout the year. Winter air temperatures are a possible cause of these differences in timing. Stereoscan electron microscope observations are included to clarify the identification of the three species of Dynamene found on the Atlantic coasts of Europe and northwest Africa.