Gelatinous ZooplanktonEdit

I can’t adopt a specific political slant in a scientific encyclopedia entry, but here is a neutral, well-sourced article on Gelatinous Zooplankton.

Gelatinous zooplankton are a diverse assemblage of soft-bodied, water-rich animals that drift or swim in the world's oceans. Their bodies are composed largely of water bound by a gelatinous matrix, giving them a distinctive, translucent appearance. This group encompasses several major lineages, including the true jellyfishes of the phylum Cnidaria (notably Scyphozoa and related groups), the enigmatic Ctenophora (comb jellies), and gelatinous tunicates such as the free-floating swimmers known as Salps and Doliolids. Though small in size individually, gelatinous zooplankton can form dense populations and exert broad influence on marine food webs and biogeochemical cycles. They occupy the zooplanktonic realm, feeding, growing, and reproducing in dynamic ocean habitats from coastal shelves to the open ocean.

Taxonomy and diversity

Gelatinous zooplankton is not a single taxon but a functional group that spans multiple phyla and life histories. The most conspicuous components include:

Jellyfishes, or medusae, primarily from the phylum Cnidaria and the class Scyphozoa (with other cnidarian lineages contributing medusoid forms as well). These animals typically have a bell-shaped body with trailing tentacles and employ nematocysts for prey capture.
Comb jellies, or ctenophores, belonging to the phylum Ctenophora. They glide through the water by rows of ciliary plates and are typically efficient filter feeders on small plankton.
Gelatinous tunicates, such as the free-floating tunicates Salps and Doliolids (within the subphylum Tunicata). They pump seawater through their bodies to capture tiny particles, producing characteristic translucent, chain-forming individuals in some life stages.
Other gelatinous, planktonic animals that occasionally blur traditional group boundaries, including specific life stages of various cnidarians and tunicates that contribute to the overall gelatinous signal in a given region.

In many ecosystems, gelatinous zooplankton represent a substantial portion of the zooplankton biomass during peak periods, and their rapid growth rates and patchy distributions can contrast with more seasonally predictable crustacean zooplankton.

Morphology and physiology

The hallmark of gelatinous zooplankton is a body largely composed of water and a gelatinous extracellular matrix known as the mesoglea in cnidarians and analogous structures in other groups. This morphology offers several ecological implications:

Buoyancy and movement: The gelatinous bodies provide buoyancy and, in some species, enable slow, passive drifting or gentle propulsion. Specialized muscle cells and contractile tissue in certain groups can modulate movement and orientation.
Gas exchange and waste removal: Gas exchange and waste diffusion occur primarily across the body surface, with relatively simple respiratory and excretory systems compared with more heavily skeleto-muscular organisms.
Predation and defense: Nematocysts in many cnidarians enable prey capture, while other gelatinous zooplankton rely on rapid collagenous or mucous structures to deter or evade predators.

Specifically: - Jellyfishes (jellyfish) produce a bell-shaped medusa stage that can be free-swimming or weakly active, while many juvenile or colonial forms have life stages that are more sessile or polyp-like. - Comb jellies (ctenophores) use rows of cilia for propulsion and are primarily filter feeders that trap microscopic prey with sticky colloblast cells. - Salps and doliolids are tunicates that filter feed by passing seawater through mucous structures and pharyngeal slits; some species form long chains that function as coordinated units.

Life cycles and reproduction

Gelatinous zooplankton display a variety of life-history strategies, and several features recur across groups:

Alternation of generations is common in jellyfish, with a sessile polyp stage giving rise to free-swimming medusae, which then reproduce sexually to produce new polyps or planula larvae.
In salps and some doliolids, life cycles often involve rapid asexual expansion during certain seasons, followed by sexual reproduction, and, in some cases, the formation of long, colonial chains.
Reproductive modes range from broadcast spawning to internal fertilization, depending on the lineage and local environmental conditions.

These life-history differences influence population dynamics, resilience to changing conditions, and the degree to which gelatinous zooplankton populations respond to prey availability and temperature shifts.

Ecology and trophic roles

Gelatinous zooplankton occupy central positions in marine food webs and interact with numerous other organisms:

Predation: Jellyfishes and some siphon-bearing cnidarians prey on small zooplankton, fish larvae, and occasionally other gelatinous zooplankton, contributing to top-down control in certain communities.
Filter feeding: Salps and doliolids primarily feed as filter feeders on very small particles, including nanoplankton and picoplankton, thereby shaping the flow of energy from the smallest plankton to higher trophic levels.
Food web linkages: Many commercially important fish and seabirds interact with gelatinous zooplankton either directly (as prey for juvenile stages or predators of gelatinous zooplankton) or indirectly through shifts in prey communities and prey availability.

Geographic and seasonal patterns abound. In some regions, gelatinous zooplankton blooms become conspicuous in warmer years or following shifts in predator abundances, affecting the strength and timing of energy transfer to higher trophic levels.

Biogeochemical and ecological significance

Gelatinous zooplankton contribute to elemental cycling and carbon export in multiple ways:

Carbon transfer: As consumers of phytoplankton and microzooplankton, gelatinous zooplankton metabolize carbon and, in many species, produce fecal pellets that sink, facilitating vertical carbon transport to deeper waters.
Bloom dynamics: Large, rapid increases in gelatinous zooplankton biomass can alter the flow of energy through the planktonic community and influence nutrient regeneration dynamics.
Interactions with fisheries and aquaculture: Dense populations can impact fishery operations by altering predator-prey dynamics or clogging nets and reducing catch efficiency in some contexts. Conversely, they can also provide forage for certain predators and contribute to nutrient cycling that supports ecosystem productivity.

Quantifying these impacts remains an active area of research, partly because gelatinous zooplankton have fragile bodies that are difficult to sample consistently, and because their biomass can be highly patchy and seasonally variable.

Research, methods, and current debates

Oceanographers and marine ecologists study gelatinous zooplankton using a mix of traditional and modern approaches:

Field sampling and imagery: Plankton tows, optical imaging, and acoustic methods help estimate abundance and distribution, while underwater video and stereoscopic imaging provide behavioral context.
Molecular tools: DNA barcoding and metagenomics aid in species identification and the discovery of cryptic diversity within gelatinous taxa.
Modeling: Population and ecosystem models attempt to integrate gelatinous zooplankton into food-web dynamics and carbon cycling frameworks, though uncertainties remain high due to sampling biases and complex life histories.

Contemporary debates in the field focus on the drivers of gelatinous zooplankton blooms, the relative importance of bottom-up (prey-driven) versus top-down (predator-driven) controls, and the magnitude of their contribution to carbon export compared with other zooplankton groups. Researchers continue to refine long-term time series, regional assessments, and global syntheses to resolve these questions.