FAQ - Marine Biology
Frequently asked questions about marine biology, species taxonomy, and the health of marine ecosystems.
What is the difference between plankton, nekton, and benthos?
Plankton comprises organisms living in suspension in the water column without sufficient swimming ability to resist currents (phytoplankton, zooplankton, bacteria). Nekton includes active swimmers that move independently of currents (fish, cephalopods, marine mammals, turtles). Benthos is the community of organisms living in contact with the seafloor, either on the surface (epifauna: corals, sponges) or within the sediment (infauna: polychaetes, bivalves).
What is a Posidonia meadow and why is it important?
The Posidonia oceanica meadow is a marine habitat formed by an angiospermic plant endemic to the Mediterranean (WoRMS AphiaID: 153813). Posidonia meadows perform critical ecological functions: they produce large amounts of oxygen, sequester carbon (up to 83 Mg C/ha/year according to IUCN 2017 estimates), serve as nursery habitat for numerous fish species, stabilise sandy seafloors, and attenuate coastal currents. Classified as a priority habitat by the EU Habitats Directive (code 1120*). Their regression due to anchoring, eutrophication, or ocean warming is one of the main indicators of Mediterranean coastal degradation.
How are marine species classified? Where are official nomenclatures found?
Marine species follow Linnaean binomial nomenclature (genus + specific epithet, both italicised; author + year in parentheses if the combination has been changed from the original genus). The primary reference register is WoRMS - World Register of Marine Species (marinespecies.org), which assigns a unique AphiaID to each taxon. For fish, FishBase (fishbase.org) is the complementary specialised database. Conservation status is encoded in the IUCN Red List (iucnredlist.org). In RAG systems and data packs, always use the AphiaID as a stable identifier to avoid synonymy problems.
What impact does ocean acidification have on marine species?
Absorption of excess atmospheric CO₂ reduces surface water pH (from 8.2 pre-industrial to approximately 8.1 today, corresponding to roughly a 30% increase in acidity). The most affected species are calcifiers: corals (15–25% reduction in calcification rate per pH unit), molluscs, echinoderms, coccolithophores, and pteropods. In non-calcifying species, effects on sensory behaviour (fish), tissue acidosis, and disruption of reproductive cycles are observed. The phenomenon is particularly pronounced in polar waters and coastal upwelling zones.
What is coralligenous habitat and where is it found in the Mediterranean?
Coralligenous habitat is a bio-constructed habitat formed by encrusting calcareous coralline algae (Corallinales), sponges, bryozoans, and other organisms on rocky bottoms between 20 and 200 m depth in the Mediterranean. It is not a true coral (Cnidaria) but a layered biological concretion hosting some of the most biodiverse benthic communities in the Mediterranean (> 1,600 associated species). It is included in Annex I of the Habitats Directive (code 1170 - reefs). The main coralligenous zones are found along Tyrrhenian, Ligurian, Adriatic (especially Puglia–Tremiti Islands), and Sicilian Channel coasts.
What is the difference between elasmobranchs and bony fish?
Elasmobranchs (class Chondrichthyes) include sharks, rays, and chimaeras: they have cartilaginous skeletons, exposed gill slits (5–7 pairs), dermal denticles, no swim bladder, and reproduce mostly by viviparity or ovoviviparity. Bony fish (class Actinopterygii) have bony skeletons, a gill operculum, a lateral line for hydrodynamic sensing, and a swim bladder for buoyancy control. Evolutionarily they diverged approximately 450 million years ago.
Why are kelp forests important and do they exist in the Mediterranean?
Kelp forests are three-dimensional ecosystems structured by large brown algae (order Laminariales), primarily found in cold and temperate waters outside the Mediterranean (NE Atlantic, Pacific). In the Mediterranean, analogous habitats are formed by Cystoseira spp. (order Fucales), now reclassified as Gongolaria spp. following recent WoRMS revisions. These algal formations provide shelter for hundreds of species, attenuate currents, and are sensitive to eutrophication and ocean warming.
How do hydrothermal vents work and what lives there?
Hydrothermal vents are openings in the ocean floor along mid-ocean ridges (typical depth 2,000–3,500 m) where water heated by the Earth's crust (up to 400°C) emerges rich in reduced compounds (H₂S, CH₄, H₂). Biological productivity does not depend on photosynthesis but on chemioautotrophy: bacteria and Archaea oxidise reduced compounds to fix CO₂ into organic matter. Associated fauna includes giant tubeworms (Riftia pachyptila, WoRMS AphiaID: 161680), vent clams (Calyptogena magnifica), blind shrimp, and serpulid polychaetes.
What does "Good Environmental Status" mean in European directives?
GES (Good Environmental Status) is the environmental objective of the Marine Strategy Framework Directive (MSFD, 2008/56/EC). A marine area is in GES when: biological communities maintain their structure, function, and productivity; food webs are complete and balanced; non-indigenous species do not alter the ecosystem; all sediments and waters comply with reference values for contaminants. GES is assessed through 11 Descriptors, including D1 (biodiversity), D2 (non-indigenous species), D3 (fish stocks), D7 (hydrographic changes), D8 (contaminants).
What is the conservation status of Atlantic bluefin tuna in the Mediterranean?
Atlantic bluefin tuna (Thunnus thynnus, WoRMS AphiaID: 158009) is classified as LC (Least Concern) globally by IUCN (2021 reassessment), reflecting partial recovery following the introduction of ICCAT quotas from 2010. The eastern Mediterranean stock has shown significant recovery, while the western Atlantic stock is still slowly recovering. ICCAT manages annual catches via national TACs; illegal, unreported, and unregulated (IUU) fishing remains a concern, especially through ranching (offshore cage aquaculture).
What are invasive marine species and what is the most notable example in the Mediterranean?
Invasive marine species (non-indigenous species, NIS) are organisms introduced outside their native range through anthropogenic vectors: ballast water, biofouling, the Suez Canal (Lessepsian migration), and ornamental trade. The most notable example in the Mediterranean is Caulerpa cylindracea (WoRMS AphiaID: 144471), an Australian green alga forming monospecific carpets. Other significant NIS: Siganus luridus and Siganus rivulatus (rabbitfish from the Red Sea), Lagocephalus sceleratus (toxic puffer fish), Mnemiopsis leidyi (ctenophore from the Black/Caspian Sea).
How does echolocation work in cetaceans?
Odontocetes (dolphins, killer whales, sperm whales) produce ultrasounds (between 100 Hz and 130 kHz) in the nasal complex (nasal sacs + melon). Sound pulses are projected forward through the melon (lens-shaped adipose organ in the forehead), bounce off the environment, and reflected signals are received by the lower jaw (bone conduction to the inner ear). The system allows prey location at > 100 m with millimetre resolution, and is critical for navigation in turbid and deep waters.
What is the difference between the photic zone and the aphotic zone?
The photic zone (or euphotic zone) is the ocean layer receiving sufficient light for photosynthesis: conventionally down to the compensation depth (1% of surface PAR), varying between 50 m (turbid coastal waters) and 200 m (open oligotrophic ocean). Above this threshold, photosynthesis exceeds algal respiration. The aphotic zone is the rest of the water column where photosynthesis is impossible; includes the mesopelagic zone (200–1,000 m), bathypelagic zone (1,000–4,000 m), abyssopelagic zone (4,000–6,000 m), and hadal zone (> 6,000 m).
What is bycatch and how does it impact non-target species?
Bycatch is the unintentional capture of non-target species in fishing operations. It heavily impacts sea turtles (Caretta caretta, Chelonia mydas), cetaceans (Delphinus delphis, Tursiops truncatus), elasmobranchs (blue shark, manta rays), seabirds, and non-commercial fish. In the Mediterranean, gillnets and longlines have the highest bycatch rates for cetaceans and turtles. FADs (Fish Aggregating Devices), TEDs (Turtle Excluder Devices), and shallow-set longlines reduce the impact.
What is the "Redfield cycle" in the ocean?
The Redfield ratio (1934) describes the average molar ratio between carbon, nitrogen, and phosphorus in phytoplankton organic matter: C:N:P = 106:16:1. This ratio reflects the nutritional needs of microalgae and is used as a reference to deduce which nutrient is limiting in an ocean area (by comparing the N:P ratio in the water against 16:1). Deviations from the Redfield ratio indicate specific sources and sinks of nutrients (e.g., denitrification, nitrogen fixation) and guide ocean biogeochemical models.
What are the main threats to coastal marine ecosystems?
The main documented threats are: (1) overfishing altering food webs; (2) coastal pollution (plastic, nutrients, hydrocarbons, heavy metals); (3) eutrophication with hypoxia and harmful algal blooms (HABs); (4) ocean warming and acidification caused by climate change; (5) habitat destruction and fragmentation (dredging, unregulated anchoring on Posidonia); (6) invasive species (NIS); (7) acoustic pollution (shipping, seismic surveys, military sonar) impacting cetaceans and elasmobranchs.