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Global distribution of tropical and subtropical copepods
Abstract
Here we show the main distribution characteristics of marine copepods across the subtropical-tropical latitudes and to bathypelagic depths in the Atlantic, Indian and Pacific Oceans (35°N-40°S). The copepod samples were collected from December 2010 to June 2011 during the Malaspina Circumnavigation Expedition. Epipelagic (0-200 m), mesopelagic (200-1,000 m) and bathypelagic strata (1,000-3,000 m depth) were sampled using an opening and closing Hydro-Bios Multinet at the following depths: 0-200, 200- 500, 500-1,000, 1,000-2,000 and 2,000-3,000 m. As expected, copepods were the most abundant contributors to the zooplankton community (84%), with more than 290 taxonomic categories identified. Other marine organisms observed were chaetognaths (5%), siphonophores (3%), ostracods (2%) and euphausiids (1%). The general distribution patterns observed included low abundances, irregular spatial distributions across the three oceans, and a large decrease of abundance as the depth of the water increased. The lowest abundance was found in the southern and western regions of the Pacific Ocean, while the highest abundances were found close to the upwelling systems of the northeastern Pacific Ocean, off the Cape Vert Islands and in the Benguela current. Large differences were not observed among oceans where depth played the most important role in the structure of the copepod communities.
Myelin is an evolutionary innovation by the nervous system that greatly speeds nerve impulse conduction, thus reducing communication delays within an organism and enhancing its information processing capabilities. The discovery of myelin in approximately half of calanoid copepod taxa came as a surprise. The evolution of myelin is usually associated with large organism size and/or complex nervous systems, not small organisms with simplified nervous systems. Myelinate and amyelinate species occur in similar numbers, with both groups being highly abundant and key members of marine planktonic communities. Myelinates and amyelinates have similar size distributions, similar antennule to prosome length ratios, overlapping maximum escape speeds scaled to copepod length, and similar sensory and motor system organization. Nevertheless, the biogeographic distributions and functional ecological groups of myelinate and amyelinate taxa differ markedly, suggesting niche separation based on nervous system architecture. Behavioral differences between the amyelinate and myelinate forms are apparent: not only are myelinate copepodites quicker than amyelinates in responding to a sudden hydromechanical stimulus but they are also better at localizing and escaping away from its source. The enhanced performances conferred by nervous system myelination in calanoids in combination with biogeographic observations supports the conclusion that myelination provides extra protection in habitats characterized by high risk from visual predators. In contrast, amyelinate calanoids may depend on strategies that reduce encounter rates with predators, such as diel vertical migration, dormant eggs and reduced activity levels.
Being present in almost any aquatic system and owing to their ecological role, copepods have been the focus of a large number of studies from taxonomy to global patterns. However, despite the wealth of information available today, our knowledge of their distribution, biology and ecology is still incomplete. Apparently, the more we learn about them, the more we are spurred on to advancing our research on these tiny crustaceans to fully grasp their significance and role. Through the contributions collected, this volume aims at providing new insight in copepod studies and a new foundation for future studies.
The impact of rehabilitation processes on Acartia distributions in the anthropized Berre Lagoon was investigated comparing a study performed in 2010-12 to ones achieved before the rehabilitation period. In 1966, the opening of a hydroelectric powerplant led to the establishment of a strong unidirectional salinity gradient. The invasive copepod Acartia tonsa, introduced in the 1980's, dominated a low-diversity zooplankton community with another common brackish species, the rotifer Brachionus plicatilis. At that time, Acartia clausi was restricted to the adjacent coastal area. Initiated since the mid 1990's, the rehabilitation processes have managed to reduce salinity fluctuations and maintain it above 15. The time and space partitioning of both Acartia species was modified, since A. tonsa and A. clausi coexisted over the whole lagoon. A seasonal succession pattern was then outlined throughout the year, with A. clausi dominant from winter to spring and A. tonsa from summer to autumn. Likewise, a spatial segregation was observed in the entire lagoon, as A. clausi remained in more marine areas. A. tonsa was less abundant, highlighting that a balance seemed established between these two congeneric species. However, environmental variables did not display any clear direct relationship with these distributions, suggesting more complex mechanisms, such as trophic interactions. Nevertheless, these changes in the distribution of Acartia species following the rehabilitation processes constitute a sign of a hoped recovery.
Copepods are a key link in marine food webs and are consumed by a wide range of predators. As a result, copepods have evolved numerous adaptations for avoiding predation. The escape response of calanoid copepods is arguably one of the most important adaptations as it is well-developed in most species and developmental stages and functions against a variety of predatory modes. Copepods have evolved sensitive mechanoreceptors in the antennae to detect the presence of predators, and respond with powerful swimming strokes which can produce speeds in excess of 500 body lengths per second. Copepods also respond with one of the shortest response latencies of all aquatic organisms, and can react to a hydrodynamic disturbance in as little as 2 milliseconds. Yet many predators are capable of capturing copepods with high success. However, success in capturing copepods varies with predatory mode and developmental stages of the copepod. The great abundance of copepods within the marine environment and number of species that rely on them as food indicate the importance of understanding these interactions. Here we discuss the interactions between predators and their copepod prey, the ability of copepods to evade predators, and several of the mechanisms predators employ to capture copepods.
The Continuous Plankton Recorder (CPR) Survey, operated by the Sir Alister Hardy Foundation for Ocean Science (SAHFOS), is the longest running, most geographically extensive marine survey in the world. Since its inception in 1931, the Survey has monitored near surface planktonic communities, including pelagic copepods, providing essential baseline information on the state of the marine environment. Initial observations focused on the North Sea. However, today, in cooperation with sister CPR surveys that have started independent monitoring programmes, the scope extends around the globe, operating across basin scales, providing a truly unique and invaluable dataset for the international community. CPR data have allowed the description of the geographical distribution of almost 700 planktonic taxa across the North Atlantic, North Pacific and Southern Ocean, monitoring their changes over time. Although both phytoplankton and zooplankton are regularly identified in the Survey, a substantial proportion of the routine analysis is dedicated to the group Copepoda (over 300 taxonomic entities) and forms the focus for this chapter. The CPRs multi-decadal time series allows seasonal cycles and natural variation to be disentangled from changes occurring over a much longer period, such as multiannual oscillations and long-term trends of key copepod species. CPR data have provided evidence for northward distributional shifts of indigenous copepod species, range expansions of non-native species and the presence of pathogenic bacteria on copepods' exoskeletons. In this chapter we summarise the main findings on pelagic copepods based on CPR data collected all over the world, highlighting the key policy issues that they have contributed to over time. The strengths and limitations of CPR observations will also be discussed.