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Homework answers / question archive / JOURNAL OF PLANKTON RESEARCH j VOLUME 29 j SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Entomology for the copepodologist HORST KURT SCHMINKE* INSTITUT FU?R BIOLOGIE UND UMWELTWISSENSCHAFTEN, UNIVERSITA?T OLDENBURG, POSTFACH 2503, D-26111 OLDENBURG, GERMANY *CORRESPONDING AUTHOR: schminke@uni-oldenburg

JOURNAL OF PLANKTON RESEARCH j VOLUME 29 j SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Entomology for the copepodologist HORST KURT SCHMINKE* INSTITUT FU?R BIOLOGIE UND UMWELTWISSENSCHAFTEN, UNIVERSITA?T OLDENBURG, POSTFACH 2503, D-26111 OLDENBURG, GERMANY *CORRESPONDING AUTHOR: schminke@uni-oldenburg

Sociology

JOURNAL OF PLANKTON RESEARCH j VOLUME 29 j SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Entomology for the copepodologist HORST KURT SCHMINKE* INSTITUT FU?R BIOLOGIE UND UMWELTWISSENSCHAFTEN, UNIVERSITA?T OLDENBURG, POSTFACH 2503, D-26111 OLDENBURG, GERMANY *CORRESPONDING AUTHOR: schminke@uni-oldenburg.de Received October 27, 2005; accepted in principle August 14, 2006; accepted for publication October 24, 2006; published online December 6, 2006 Copepods are often called ‘insects of the seas’. Is this justified? Today insects are regarded as the most successful group of animals. Measures of absolute success are phylogenetic age (survival through time), dominance (relative abundance, proportion of total biomass, role in energy flow, impact on ecosystems and coexisting organisms), speciosity, geographic range, and breadth of adaptive radiation. Measured by these criteria copepods are no less successful than insects. What about relative success? There must be intrinsic features in the structure and mode of life of insects which make them more successful relative to other animal groups. According to the literature these features are small size, metamorphosis, wings, and mouthparts. If the capacity to fly is equated with the capacity to swim copepods share all these intrinsic features being equal with insects also in relative success. Entomologists believe insects to be unmatched by other groups in most features of evolutionary success. Yet, they outdo copepods only in one respect: number of species. Reasons for this are greater spatial heterogeneity and architectural complexity (of vegetation) on land than in the sea as well as the fact that insects were among the first groups on land relatively unaffected by other groups, whereas copepods had to evolve in an already crowded world. I N T RO D U C T I O N Insects are a fantastic group of animals. They are part of our daily life. We are intrigued by them because of their colours, their flight, their swarms, their songs, their mimicry, their colonial life, their chemical communication, their luminescence and their constructions. We depend on them because they are essential for the functioning of terrestrial ecosystems as plant consumers, decomposers, seed dispersers pollinators, predators, parasites and parasitoids. We are affected by insects as parasites and vectors of diseases and as pests of crops and stored food, but they also provide us with useful products such as honey, silk, colours and pharmaceutically valuable compounds. Insects are ubiquitous on land. Ecologically speaking, it is hardly an exaggeration to say that the land is all theirs. They are not only dominant but also highly successful, and in fact, they are regarded as by far the most successful group of animals on earth today. But what does it mean to be successful and dominant? Is there a difference between these two properties or do they rather amount to the same thing? Wilson (1990) points out that success and dominance correlate with each other, but qualitatively are quite different phenomena. Success is an evolutionary term, dominance an ecological one. Not infrequently, people tend to equate success with number of species or speciosity, but success according to Wilson (1990) means survival through time or longevity of clades, a clade being a monophyletic group, i.e. a species and all of its descendents. There are monophyla that are relatively speciose today but as yet rather short-lived, e.g. the teleost fishes which include 95% of extant fish species but which phylogenetically are rather young. Teleosts originated in the Jurassic some 150 My ago. The monophylum is younger than the oldest mammals. At the other end of the spectrum, there are the rabbit-fishes (Holocephali) which date back to the Palaeozoic some 350 My ago, yet are represented today only by about 30 species (Fig. 1). Dominance is an ecological property, which according to Wilson (Wilson, 1990: p19) is best measured at two levels: ‘at any given time, by the relative abundance of the clade in comparison with related clades, and over its entire history, by the ecological and evolutionary impact it has on the coexisting fauna and flora’. Thus dominant groups ‘compose a large part of the biomass, doi:10.1093/plankt/fbl073, available online at www.plankt.oxfordjournals.org # The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Communicating editor: R.P. Harris JOURNAL OF PLANKTON RESEARCH j VOLUME channel much of its energy flow, and exercise a profound effect on ecosystems and the evolution of other organisms’ (Wilson, 1990: p36). Success and dominance are correlated in the same way as success and speciosity because dominance may be an important feature of the longevity of a monophylum, but quite often both properties are decoupled. Take stomatopods and springtails (Collembola) as examples. Both go back well into the Palaeozoic. First fossils of stomatopods date from the Middle-Mississippian (Schram, 1986), some 340 My ago. Collembola are known as fossils since the SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Middle Devonian, some 390 My ago; Rhyniella praecursor Hirst and Maulik, 1926 is one of the oldest known insects (Fig. 2A). Thus, both taxa are among the most successful monophyla of the animal kingdom; yet, in terms of dominance they are not similar. Stomatopods are confined to warm waters and are obligate carnivores smashing or hitting at prey passing by while they lie burried in mud or hidden in burrows. Collembola dominate soil and litter samples taken anywhere in the world and are the most abundant group of insects. Of the estimated 42.3 million arthropods in one hectare of Seram rain forest on Borneo, almost 50% were Collembola (Fig. 2B) followed by mites (Acarina) as the major component of the remaining arthropods (Stork, 1988). Collembola play a pivotal role in soil formation so that soil ecology would suffer grave and possibly even catastrophic consequences should they disappear. The disappearance of stomatopods on the other hand would have less serious ecological repercussions. Apart from success, dominance is also correlated with some other independent properties of a monophylum. These properties are species numbers, breadth of adaptive radiation or the spectrum of niches utilized by the species of a monophyletic group and geographic range or the entire physical space occupied by a group at a particular time (Wilson, 1990). Thus, a successful group should be phylogenetically old, have a continuous record of evolutionary persistence through geological time, should be dominant and speciose today and should occupy a wide physical space as well as a large number of niches. Do insects as a monophyletic group comply with these criteria? Fig. 2. Collembola. (A) Rhyniella praecursor Hirst and Maulik, 1926 (after Mu?ller, 1978); (B) Relative proportions of different arthropod groups of the estimated 42.3 million arthropods in one hectare of Seram rain forest on Borneo (after Stork, 1988). i150 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 1. The fossil record of two monophyla of fish (extract from Mu?ller, 1985). j 29 H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST ABSOLUTE SUCCESS OF INSECTS Fig. 3. Relative proportions of the animal groups of the animal species numbers presently known (modified after Wilson, 1992). i151 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Let us start with speciosity. Numbers of known insect species published in the literature diverge considerably. Here we follow Grimaldi and Engel (Grimaldi and Engel, 2005) who estimate that the number of named insect species amounts to 925 000. It is undisputed that the overwhelming majority of known animal species are insects. They make up about 75% of all animal species known today (Fig. 3). But the known species are apparently only a tiny fraction of those awaiting discovery and description. A well-publicized estimate of 30 million tropical arthropods (Erwin, 1982) has been derived by an indirect chain of argument from studies of beetles in the canopies of tropical trees (Fig. 4). Stork (Stork, 1988) has questioned Erwin’s calculations because they are based on somewhat arbitrary assumptions but even if it is assumed that the host-specificity levels and the proportion of the canopy fauna that beetles comprise are much lower and the relationship between canopy and ground fauna balanced, a figure of 7 to 10 million arthropods is possible. If species from the rest of the world are added to the total of tropical insect species, 10 million appears to be quite a reasonable estimate although Grimaldi and Engel (Grimaldi and Engel, 2005) on the basis of estimates by experts consider a total of about 5 million species to be more likely. Let us turn to success. Insects are at least 400 My old (Grimaldi and Engel, 2005). They made their first appearance in the Early Devonian. Since then, they have experienced a steady increase in family-level diversity interrupted only by a dip at the P/Tr boundary resulting from the mass extinction at the end of the Permian (Fig. 5) which so severely affected many marine animal taxa and the terrestrial tetrapods (Labandeira and Sepkoski, 1993). Not many fossils are known from the first 60 My of insect evolutionary history. The first insects probably lived on plants forming low carpets over the ground since the Silurian (Eisner and Wilson, 1977). In the Carboniferous some 80 My later, a massive radiation began. At that time, the great coal forests had formed. Despite some losses at the end of the Permian, the rise of insects to ascendency continued uninterrupted until today. According to Labandeira and Sepkoski (Labandeira and Sepkoski, 1993), the losses JOURNAL OF PLANKTON RESEARCH j VOLUME Fig. 5. Family level diversity of fossil marine organisms and fossil insects through geologic time. Arrows indicate the five mass extinctions in earth’s history (combined from various authors). consist of 8 out of 27 orders of insects which did not survive the Permian extinction. Another three orders became extinct sometime during the Triassic (Fig. 6). Since then, some 230 My ago, the ordinal diversity of insects was comparable to that of today. If we look at dominance, insects outnumber by far the rest of the terrestrial fauna. Williams (cited in Eisner and Wilson, 1977; Evans, 1966) estimated that there are a trillion (1018) insects alive at any one time. They must amount to a tremendous biomass. In a piece of rain forest near Manaus in Brazil (Fig. 7), insects comprise 86% of the total animal biomass (Wilson, 1990) and the biomass of the ants alone is four times that of all of the vertebrates combined. Insects impact energy flow enormously at several trophic levels as plant consumers, as predators and as decomposers, and most of the other terrestrial fauna are dependent on insects as a source of food. In fact, insects have paved the way for the evolution of other terrestrial organisms. They were a rich source of food when vertebrates ventured onto land. This is still indicated by the food habits of extant land SUPPLEMENT 1 j PAGES i149 – i162 j 2007 vertebrates (Kattmann, 2001). Frogs, lizards and young crocodiles live on insects, and flying insects were the evolutionary pacemakers for birds and bats (Fig. 8). The majority of song birds (Passeres), the youngest and most speciose bird group (comprising 40% of all bird species), feed on insects or in the case when they are granivores feed insects to their young. Flying insects most likely also were the reason that spiders could profitably extend their webs from the ground into the surrounding vegetation, and the co-evolution of insects and flowering plants has been speculated on and studied significantly. The second correlate of dominance is adaptive radiation which may originate in three ways (Sudhaus, 2004): (i) after arrival into new physical space free of competitors (the drosophilid flies of Hawaii are a famous case in point, but insects as a group are also a good example), (ii) after the evolution of key innovations allowing the exploitation of many new niches as a consequence of them (the phylogeny of insects can be described as a sequence of such radiation events after successive inventions of evolutionary novelties with far-reaching potential; such novelties were the flight apparatus, the capacity to fold the wings over the body, complete metamorphosis) and (iii) after a mass extinction so that a few individuals or a group profit from the disappearance of their most serious competitors and antagonists, the classical example being the mammals following the dinosaurs after the mass extinction at the K/T boundary. Not much must be said about the last correlate of dominance, geographical range, because insects occupy the greatest range possible, i.e. all habitable spaces on land. ABSOLUTE SUCCESS OF COPEPODS Looking back on what has been said, there is no doubt that insects are a tremendously successful group by all appropriate measures. But why should this be of interest to copepodologists? Well, copepodologists like to refer to copepods as being ‘the insects of the seas’. In what follows I would like to explore whether this is justified. It is not without irony that in early systems of classification, copepods and other crustaceans were considered to be insects and placed among the Insecta Aptera, the ‘insects without wings’. In 1875, Mu?ller introduced the new term ‘Entomostraca’ as an order of Insecta meaning ‘insects with a shell’ (Damkaer, 2002). However, the Entomostraca, like Aptera before them, included not only what we call copepods today, but also other crustaceans. So we cannot trace the designation of copepods as ‘insects of the seas’ back to these i152 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 4. Steps in Erwin’s estimation of 30 million species of tropical arthropods (after Stork, 1988). j 29 H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 6. Spindle diagrams to show diversities of families within insect orders through geologic time since the Palaeozoic (Pz). Mz, Mesozoic; Cz, Caenozoic. Scale bar in lower right (modified after Labandeira and Sepkoski, 1993). classification schemes. If Copepoda is a taxon comparable to Insecta, the first question to ask is: are they a successful group by virtue of the same criteria used to assess the insects? Let us start again with speciosity. In his Maxilliped Lecture, Humes (Humes, 1994) reported that going through the Zoological Record he had counted 11 302 species of copepods. Adding the species described since then is likely to raise this figure to 12 500. Citing the opinion that at most 15% of all existing animal species are known to science, Humes (Humes, 1994) calculated that if the 11 302 known species represent 15%, then i153 JOURNAL OF PLANKTON RESEARCH j VOLUME 29 j SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Fig. 8. Insects have paved the way for the evolution of other terrestrial organisms. This is still indicated in the food habits of, for example, land vertebrates (Original: G. Gad). i154 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 7. Relative proportions of the total biomass of animal groups represented in Brazilian rain forest near Manaus (after Wilson, 1990, based on data from Fittkau and Klinge, 1973). H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST Humes (Humes, 1994) speculated that the number of copepods associated with only one species of coral (Acropora hyacinthus Dana, 1846) which is widely distributed on the Great Barrier Reef amounts to 10.5 billion. A group present in such prodigious numbers must play a key ecological role and, indeed, whole marine ecosystems would crumble if the copepods disappeared. As main primary consumers they are at the beginning of most food chains in the sea and with their prodigious production of faecal pellets planktonic copepods make the major contribution to the energy transfer from the euphotic zone to the lightless depths where whole biocoenoses are dependent on this transfer. There is hardly a group with a greater impact on the remaining of the marine biota. In his Maxilliped Lecture, Stock (Stock, 1991) produced a table showing evolutionary trends in the more important suborders of copepods. In the table, it can be seen, for example, that four suborders contain species which have adapted to a subterranean life in caves. This indicates that adaptations to life in this particular habitat have evolved independently at least four times. The same applies to freshwater (Boxshall and Jaume, 2000) or the pelagic realm (Bradford-Grieve, 2002) which have repeatedly been colonized. In his Maxilliped Lecture, Ho (Ho, 2001) reported that one-third of the recorded copepod species live associated or in symbiosis with other organisms. These symbionts must have evolved from free-living forms and the adaptations to special life cycles listed in Stock’s (Stock, 1991) table and others enumerated by Ho (Ho, 2001) make clear that in Ho’s (Ho, 2001: p4) words: ‘In the course of their evolution different groups of symbiotic copepods must have attained the same mode of life (living on or in another organism) with different approaches (adaptive evolution)’. Thus, copepod evolution could certainly be described as a succession of numerous waves of adaptive radiation but we lack the phylogenetic tree to elucidate this conclusively. Finally, geographic range: copepods are ubiquitous in water. A film of water is enough for copepods to thrive on leaf litter (Fiers and Ghenne, 2000) and even sea ice is exploited as a habitat (Schminke and Dahms, 1993). Copepods occur wherever there is water as a liquid. R E L AT I V E S U C C E S S O F I N S E C T S AND COPEPODS Summing up, it can be said that Huys and Boxshall (Huys and Boxshall, 1991: p9) expressed elements of this argument when opening their book with the sentences: ‘Copepods are aquatic crustaceans, the i155 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 the hypothetical grand total should be some 75 000 species of copepods. Seifried (Seifried, 2004) used a different approach trying to infer the hypothetical number of copepod species from the percentage of undescribed species in new samples. Basing her calculations on samples of Harpacticoida from the deep sea of the Angola Basin in which 98% of the species are unknown, she arrived at a total of 150 000 for the Harpacticoida alone. If these undiscovered Harpacticoida represent 33% of all undiscovered Copepoda as do the known Harpacticoida within the known Copepoda, a staggering grand total of some 450 000 species of copepods would result. This is speculative as is Humes’ more moderate figure but not more so than Erwin’s estimate of terrestrial arthropods. These exercises merely serve to get an idea of the dimensions. Turning to success there is not much of a fossil record for copepods. Fossil cyclopoids and a fossil species of the recent genus Cletocamptus Schmankewitsch, 1875 are reported from Miocene lake deposits (Palmer, 1960) and a fossil parasitic copepod was found on a Lower Cretaceous fish (Cressey and Boxshall, 1989). The parasitic copepod, a siphonostomatoid, lived 120 My ago and can be attributed to a family of extant copepods, the Dichelesthiidae. This indicates that the roots of copepods must go back considerably further and, indeed, there is circumstantial evidence that copepods may have originated in the Cambrian. Among the so-called Orsten fauna of the Upper Cambrian (510 My ago), there are stem-lineage Crustacea but also a few fossils which can be assigned to groups of recent Crustacea. Bredocaris admirabilis Mu?ller, 1983 and species of the genus Skara are considered Maxillopoda to which the Copepoda belong (Walossek, 1999). The Skaracarida are even regarded as closely related to the Copepoda so that it is reasonable to tentatively conclude that Copepoda may have already existed in the Cambrian (Fig. 9). If we look at dominance, Hardy’s widely cited dictum comes immediately to mind that copepods are the most plentiful multicellular animals on earth, outnumbering the insects, which have more species but fewer individuals. Boxshall (Boxshall, 1998) made a simple calculation. Assuming that in 1 L of sea water there is only one copepod the estimated 1347 million km3 of the world ocean and seas would contain 1.321 copepod individuals, i.e. three orders of magnitude more than insects. This figure could certainly be tripled if benthic and associated copepods were added. If one assumes that one copepod on average has a wet weight of 0.036 mg the biomass of these planktonic copepods would amount to 46.8 109 t, that is, 150 times the biomass of the whole human population on earth today. JOURNAL OF PLANKTON RESEARCH j VOLUME j 29 SUPPLEMENT 1 j PAGES i149 – i162 j 2007 diminutive relatives of the crabs and shrimps. In terms of their size, diversity and abundance, they can be regarded as the insects of the seas’. They also mention size and this brings us to a second important question. So far, we have only learned about the absolute success of both insects and copepods but nothing about their relative success. In the following, we therefore have to ask what has made both groups so successful relative to other organisms? There must be intrinsic features of their anatomy, physiology, behaviour and life cycle which are keys to their success. Entomologists are quite sure what these features are in the case of insects. Southwood (Southwood, 1978: p35) has summarized this succintly in three words: ‘size, metamorphosis and wings’. Other features are occasionally mentioned from which only ‘mouthparts’ are added here. i156 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 9. Phylogeny and stratigraphic occurrence of Crustacea through geologic time. Circles, taxa recorded; hypothesized lines, stippled; lines after clear record, full (after Walossek, 1999). (A) Skara anulata Mu?ller, 1983. (B) Bredocaris admirabilis Mu?ller, 1983. H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST This allows for a lot of evolutionary flexibility and therefore life cycles with differently adapted stages surely are a strong component of success. Most insects can fly. The effective range of a species is enlarged enormously by this ability. Insects fly to capture prey, to avoid predators, to find a mate, and to disperse to new grounds for feeding and breeding. Flight enables them to exploit patchily distributed resources, to utilize temporary ones and so to combine resources scattered in space and time to a composite niche (Southwood, 1978). They can exploit trees and other habitats difficult for others to reach. Wings increase their spread potential so that they can reach new territory in which to speciate. Copepods cannot fly but they can swim. Instead of taking off into the air, they ascend into the water column and enjoy the same advantages that insects have through flight. They ingeniously use ocean currents and strategically time their vertical migrations so as to avoid or reduce overlap of niches. Their spread potential is increased enormously by drift with ocean currents which in time is limited only by the duration of the distributive stages. Flying and swimming open up an extra evolutionary dimension unavailable to those that cannot fly through the air or swim through the water column. Insects are a textbook example of the modification of the same basic elements to evolve mouthparts to suit very specific needs (Fig. 11). Chewing and gnawing, cutting and shredding, spearing and draining, piercing and sucking and siphoning and licking require particular mouthparts. Insects have found specific solutions for each of these functional needs. On a morphological basis, insect mouthparts have been grouped into 34 fundamental classes which each represent an ecological guild (Labandeira and Sepkoski, 1993). The same mouthpart type could have evolved independently more than once such that the types may cross phylogenetic boundaries. Entomologists are convinced that no other animal group can show such an infinite variety of feeding structures. They do not know copepods. No morphologically based classification of mouthpart types has been undertaken so far for copepods, but going through the descriptive literature, one cannot but be convinced that copepods are no less imaginative in evolving exactly the right type of mouthparts for the capture, manipulation, processing and consumption of any particular type of food. This is the ‘Maxilliped Lecture’ so let us review various types of maxillipeds (Fig. 12). These may be regarded as representative for all mouthparts, although admittedly they also serve other functions. The adaptive radiation of mouthparts has certainly been a potent factor in the proliferation of species in both insects and copepods. i157 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 The effective range of an organism, i.e. the physical space an individual utilizes during its life, is approximately related to its body size. Small organisms have smaller effective ranges than larger ones. Therefore, the same environment is more patchy for small organisms than for large ones. Small organisms can adapt to particular patches and therefore have vastly more possibilities for niche formation. Hutchinson (Hutchinson, 1959: p155) summarizes:‘. . . small size by permitting animals to become specialized to the conditions offered by small diversified elements of the environmental mosaic, clearly makes possible a degree of diversity quite unknown among groups of larger animals’. The life span of an organism is also related to its body size. Small organisms tend to have short life spans, short generation times and high mortality rates which together accelerate evolutionary processes. Fast evolutionary rates allow for a quicker morphological ‘fine-tuning’ and hence for a perfection and diversification of, for example, mouthparts (May, 1978). Insects as well as copepods are predominantly of small body size. Among insects, 85% of the known species are holometabolous having a life cycle with a larva and an adult and an extra transitional stage, the pupal stage (Fig. 10A). Larvae and adults are morphologically so different that it is impossible to tell which larva belongs to which adult, and both have their own adaptations and lead a completely different life, the larvae generally being the feeding and growing, the adults the distributive and reproductive stages. Despite obvious differences in the physiology of metamorphosis, the ecological situation is rather similar in Copepoda (Fig. 10B). Nauplius larva and adult are also completely different, have their own adaptations and lead different lives. The transition takes place in the last nauplius of which there are six stages and leads to a copepodid, an incomplete adult, which also has six stages to complete its development (Dahms, 1992). Fryer (Fryer, 1998: p72) reminded us that ‘an early nauplius, a copepodid, and an adult (which may be present at the same time) may occur at different depths, eat different foods and . . . be preyed upon by different predators or eaten to different extents by the same predator. . . . Changes in morphology, as the animals grow, affect swimming and feeding as, via Reynolds number, do changes in size’. Metamorphosis enables species to sequentially occupy different niches to exploit resources which may be separated both in space and in time and which alone would perhaps not suffice for a season or a whole generation. Metamorphosis enables species to spend part of their life in one habitat and the rest in another so that completely different things can be done in both of them (Southwood, 1978). JOURNAL OF PLANKTON RESEARCH j VOLUME j 29 SUPPLEMENT 1 j PAGES i149 – i162 j 2007 We are at the end of our comparison between insects and copepods. The result is that both are equally successful. When investigating the components of relative success, both groups could even be treated together because they share the same components. This unexpected congruence leaves no doubt: copepods are not unjustifiably called the insects of the seas. Entomologists are convinced that insects are unmatched by most features of evolutionary success (Grimaldi and Engel, 2005). Yet, they outdo copepods only in one respect: number of species. Insects count roughly a million, copepods barely 13 000. This leads us to a last question: how do we account for this difference? DIFFERENCE IN SPECIES NUMBERS BETWEEN INSECTS AND COPEPODS The marine fauna is much more diverse in terms of phyla than its terrestrial counterpart, 80% of all animal phyla are represented in the seas versus only 20% on land. Yet, in the number of recorded species, the relation is reversed, 85% of species are found on land, with only 15% in the oceans. This percentage may be too small for two reasons. The number of specialists studying marine invertebrates is very low compared with those studying insects. The relation is 20 (insects) to 1 (marine invertebrates) (Gleich et al., 2000). No wonder that only 13% of published taxonomic surveys are marine oriented (Winston and Metzger 1998). The second reason is the study by Grassle and Maciolek (Grassle and Maciolek, 1992) suggesting that there may be 10 million species of marine macrofauna. Like Erwin’s estimate, this one has been much debated (Briggs, 1991, 1994; May, 1994a, b). If the meiofauna were added, this estimate may not be unrealistic, because the meiofauna of the deep sea is practically unexplored, less than 15 m2 of the vast expanses of the ocean floor having been sampled. However, one has to stick to what is known and that means that only 15% of recorded species are found in the seas. May (May, 1994a) has summarized possible reasons for this contrast in diversity. Since most of the terrestrial species are insects, the reasons for this disparity should also explain the enormous difference in species numbers between insects and copepods. The first argument holds that terrestrial environments are much more heterogeneous than marine ones and that the continental climate is much more variable. Also spatial heterogeneity is far more pronounced not only on a local but also on a biogeographical scale with continental drift producing ever changing constellations. The result is a patchiness in space and time creating ample possibilities for adaptations and speciation. The second argument is that despite its inordinately large volume the pelagic realm is relatively poor in species, most marine species being benthic. The benthic marine environment lacks i158 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 10. Life cycle of (A) a holometabolous insect (compiled after various authors); (B) a harpacticoid copepod (Drescheriella glacialis Dahms and Dieckmann, 1987) (after Dahms, 1991). H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST the architectural complexity which is provided on land by forests (and other vegetation) still covering large parts of the land surface. This spatial complexity of terrestrial plants which offers innumerable opportunities for specialization and diversification is not found in the sea, coral reefs notwithstanding. In the sea, primary production is accomplished by single-celled organisms which do not provide physical support for bigger organisms but only serve as food (Briggs, 1994). These two arguments which May (May, 1994a) cites along with a few more less relevant for our present context apply to the marine and terrestrial faunas as a whole and are ecological reasons. But there also are reasons which have to do with the evolutionary history of both groups concerned. When insects started to colonize the land, they entered virgin territory devoid of competitors and not pre-occupied by others. As terrestrial ecosystems evolved, insects were ready to exploit the new resources. Whatever chances there were, they took them and filled virtually every possible niche. On land, they had been the first so that they could monopolize whatever opportunities plants had to offer, whether these were architectural complexity or co-evolutionary dynamics. Copepods on the other hand were not the first in the sea. They had to struggle hard to carve out a living. Many opportunities had already been pre-empted by others. They had to seek novel challenges. One was to settle on other animals. No other group has been so successful as copepods in overcoming the defences of other animals and to live on them. How difficult it is to hold one’s own in the sea is best demonstrated with insects. Despite their overwhelming success on land, they are practically absent from the sea (Eisner and Wilson, 1977). There are a few of them in the intertidal and there are the species of the water strider Halobates i159 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 11. Insect mouthparts. (A) of a locust; (B) of a fly; (C) of a bee; (D) of a butterfly; (E) of a mosquito; (F) of a bug (after various authors). JOURNAL OF PLANKTON RESEARCH j VOLUME j 29 SUPPLEMENT 1 j PAGES i149 – i162 j 2007 Eschscholtz, 1822 skating on the surface of the oceans far from land but needing floating debris as terrestrial substitute for the deposition of their eggs. Yet, there are no insects among the plankton or the benthos of the seas. Not that insects are prevented from living in aquatic environments. They have successfully invaded freshwater, but by the time insects originated, metazoan life in the sea had already had 400 My to evolve and to accumulate diversity so that insects most likely were excluded. Under these circumstances, it is admirable how much copepods have diversified after all. They did not land on an empty planet, they had to seek success in an already crowded world, and whenever new opportunities arose, they had to compete with others, not only with others of their kin. In other words, insects only had to compete with insects and the result was more insects. Copepods had to compete with others and the result was more copepods only sometimes. Insects could not lose and copepods not always win. This asymmetry makes all the difference. But insects are not the right measure anyway. Copepods may be species-poor in relation to insects but not to other marine animal groups. One-fifth of Crustacea are copepods which rank second after decapods (Schminke, 1996) but are first in the number of families (Martin and Davis, 2001). This diversification was far more difficult to achieve than in the case of insects. To call i160 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 12. Maxillipeds of a species of Copepoda Misophrioida and of various families of Harpacticoida respectively (compiled from various authors). H. K. SCHMINKE j ENTOMOLOGY FOR THE COPEPODOLOGIST copepods ‘insects of the seas’, therefore, is an understatement. It should rather be the other way round. Insects should be called ‘the terrestrial copepods’. As terrestrial beings ourselves, we tend to view the rest of the world from our terrestrial perspective. That is why Crustacea were ‘insects with a shell’ or Entomostraca. It is somewhat ironical that nowadays evidence from molecular as well as morphological sources is accumulating that insects are not terrestrial Tracheata but rather terrestrial Crustacea having their nearest relatives among crustacean groups (Fig. 13). Crustacea and Insecta may form a monophyletic group called Tetraconata or Pancrustacea (Schram and Jenner, 2001; Richter, 2002; Giribet et al., 2005). As aquatic animals, copepods have quite different lessons to teach. Yet, despite a lot of exciting discoveries about them during the last 25 years, copepodologists are still far behind entomologists and ornithologists. To catch up, efforts must be increased to find out more about copepods so that others become interested and find so much new to learn from them that eventually perhaps the title of my present contribution could be reversed such as has already been done long ago by Hutchinson (Hutchinson, 1951) who entitled one of his famous publications ‘Copepodology for the ornithologist’. Entomologists maintain that insects are the greatest evolutionary success story in the history of life on earth (Grimaldi and Engel, 2005). They have overlooked the fact that there is at least a second no less impressive one. REFERENCES Boxshall, G. A. (1998) Preface. Phil. Trans. R. Soc. London B, 353, 669 –670. Boxshall, G. A. and Jaume, D. (2000) Making waves: the repeated colonization of fresh water by copepod crustaceans. Adv. Ecol. Res., 31, 61– 79. Bradford-Grieve, J. M. (2002) Colonization of the pelagic realm by calanoid copepods. Hydrobiologia, 485, 223–244. Briggs, J. C. (1991) Global species diversity. J. Nat. Hist., 25, 1403–1406. Briggs, J. C. (1994) Species diversity: land and sea compared. Syst. Biol., 43, 130–135. Cressey, R. F. and Boxshall, G. A. (1989) Kabaterina pattersoni, a fossil parasitic copepod from a Lower Cretaceous fish, Cladocyclus gardneri Agassiz. Micropaleontology, 35, 150 –167. Dahms, H.-U. (1991) Leben im gla?sernen Labyrinth. Polares Meereis und seine Bewohner. Natur und Museum, 122, 17– 34. Dahms, H.-U. (1992) Metamorphosis between naupliar and copepodid phases in the Harpacticoida. Phil. Trans. R. Soc. London B, 335, 221 –236. Damkaer, D. M. (2002) The Copepodologist’s Cabinet: A Biographical and Bibliographical History. Vol. 240. Memoirs of the American Philosophical Society, Philadelphia. Eisner, T. and Wilson, E. O. (1977) The Insects. Readings from Scientific American. W. H. Freeman and Company, San Francisco. Erwin, T. L. (1982) Tropical forests: their richness in Coleoptera and other arthropod species. Coleopterists’ Bull., 36, 74– 75. AC K N OW L E D G E M E N T S Evans, H. E. (1966) Life on a Little-Known Planet. E. P. Dutton & Co., Inc., New York. When Bob Kabata came to the end of the first Maxilliped Lecture ( published in 1988), he reminded us Fiers, F. and Ghenne, V. (2000) Cryptozoic copepods from Belgium: diversity and biogeographic implications. Belg. J. Zool., 130, 11–19. i161 Downloaded from https://academic.oup.com/plankt/article/29/suppl_1/i149/1468774 by guest on 21 April 2021 Fig. 13. Relationships of Mandibulata (modified after Schram and Jenner, 2001). that he had spoken at the last conference before his retirement. So it was with me in Hammamet. I have contributed to copepodology only a little myself. The main work has been done by my students. In alphabetical order they are: Karin Bro?hldick, Hans-Uwe Dahms, Jan Drewes, Johannes Du?rbaum, Kai George, Thomas Glatzel, Barbara Hosfeld, Pedro Mart??nez Arbizu, Gisela Moura, Sybille Seifried, Gritta Veit-Ko?hler, Elke Willen. It has been a great time with them, full of excitement, with successes, but also with disappointments. It has always been stimulating intellectually with them. For this, I want to thank them very much. 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