A closeup picture of Ascidiacea, commonly known as the Ascidians or Sea squirts and a Spider crab
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BENTHOS REFERS TO THE DIVERSE COMMUNITY OF ORGANISMS THAT INHABIT THE BOTTOM OF AQUATIC ECOSYSTEMS, PLAYING A CRUCIAL ROLE IN NUTRIENT CYCLING AND THE FOOD WEB.
Benthos, derived from the Ancient Greek word "βένθος" (bénthos), meaning "the depths [of the sea]," refers to the collective group of organisms inhabiting the substrate at or near the bottom of aquatic environments.
This ecological community, commonly known as the benthic zone, encompasses a wide range of habitats from shallow tidal pools along the foreshore to the expansive continental shelf and extending down to the profound depths of the abyssal ocean floor.
image credit: Pearson Education, Inc., publishing as Benjamin Cummings
THE BENTHIC COMMUNITY PLAY ESSENTIAL ROLE IN ECOSYSTEM FUNCTIONING AND NUTRIENT CYCLING, AND THEY ARE INTEGRAL COMPONENTS OF THE OVERALL AQUATIC FOOD WEB.
Benthic animals, also known as zoobenthos, are classified into two ecological categories based on their habitat about the substrate. The first category is the infauna, which consists of species that live entirely or partially within the substrate. These organisms are adapted for burrowing and dwelling in the spaces between sediment particles, which provides them with protection from predators and access to nutrients. Common examples of infaunal organisms include various species of worms, clams, and bivalves, as well as polychaetes.
Infauna plays a crucial role in sediment bioturbation, as their burrowing activities help oxygenate and mix sediments, influencing nutrient cycling and sediment stability.
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The second category is the epifauna which comprises animals that live on or are attached to the seafloor's surface. They can be found on various types of substrates but are particularly abundant and diverse on hard substrates. Epifauna populations are most prominent in rocky intertidal areas and coral reefs. Approximately 80% of the larger zoobenthos belong to the epifauna category. Epifauna encompasses a wide range of organisms, from tiny species like diatoms and barnacles to larger creatures such as sea stars, sea squirts, sea urchins, and crabs.
Sea squirt (Atriolum robustum)
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Benthos can be classified based on the size of the organisms present, which allows for a better understanding of the ecological dynamics within the benthic environment. These size-based classifications encompass three main categories: macrobenthos, meiobenthos, and microbenthos.
The smallest size category consists of microfauna, with organisms less than 0.1 mm in size. It includes bacteria, protozoa and unicellular algae. Despite their small size, MICROBENTHOS play a vital role in nutrient cycling and are primary producers at the base of the benthic food web. They are critical for breaking down organic matter and releasing nutrients essential for the growth of larger organisms.
MEIOBENTHOS are slightly larger than microbenthos and include small metazoans such as nematode worms, copepods, and foraminifera. They are often intermediate links in the benthic food chain, serving as prey for larger epifauna and infauna while feeding on microbenthos.
MACROBENTHOS are the largest benthic organisms and include animals like crabs, sea urchins, sea cucumbers, and larger molluscs. These organisms are highly visible and are often important predators or scavengers within the benthic community. They contribute significantly to the energy flow and nutrient cycling in the benthic ecosystem.
Bobbit worm (Eunice aphroditois)
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The Bobbit worm is a highly unusual and predatory marine creature that inhabits warmer oceans around the world. This creature adopts a unique hunting strategy by burying most of its body under the ocean sediment while exposing only its mouth, which is equipped with a pair of large, scissor-like jaws held wide open. In addition to this, the Bobbit worm boasts five antennae protruding from its head, functioning as sensitive tripwires.
When a fish accidentally brushes against one of these antennae, it triggers a swift and lethal response. The Bobbit worm's exceptionally sharp mouthparts strike with incredible speed, and in some instances, this attack can result in the prey being cleanly sliced in two. This predatory approach allows the Bobbit worm to efficiently capture its prey, showcasing its impressive hunting prowess in the depths of the ocean.
Bobbit worm detail
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photo credit: Jam Press
The phylum Priapulida, to which Priapulidae belongs, is a small yet ancient group of marine worms. They are renowned for their distinctive body plan, often earning them the colloquial nickname "penis worms" due to the shape of their proboscis. Priapulids boast an extensive fossil record dating back to the Cambrian period, which makes them invaluable in the field of evolutionary biology. They offer unique insights into the early evolution of complex animal life on Earth, as they have retained certain primitive features and body structures.
The name "Priapulidae" itself is derived from Greek mythology, emphasizing the characteristic proboscis shape that defines these captivating creatures.
photo source: Marie-Lan Nguyen
BENTHOS FACE NUMEROUS THREATS THAT POSE SIGNIFICANT RISKS TO THEIR HEALTH AND BIODIVERSITY.
One of the primary threats is the increasing ocean temperatures, ocean acidification, and alterations in ocean current patterns. These changes can have detrimental effects on the distribution and survival of benthic organisms.
Temperature fluctuations, in particular, can impact the benthos in several ways. Many benthic organisms have adapted to specific temperature ranges, and variations beyond these ranges can lead to thermal stress and, in some cases, death.
(A) Bottom photographs showing seafloor before (left) and after (right) bottom trawling has occurred on deep-sea coral gardens on the continental slope off Norway. Note in the photo on the right the elongated trawl mark on the seafloor, resulting from dragging trawl doors. (B) Side scan sonograph showing elongated and curved tracks made by bottom trawl boards on the seabed of Moreton Bay, Australia. Light-toned areas are elongated trains of sand dunes.
Anthropogenic threats to benthic habitats
photo source: (A) UN Environment/GRID-Arendal (B) Geoscience Australia
Another significant threat to benthic habitats is the excavation of marine bottoms for commercial purposes, such as the creation of navigation channels. This human activity can harm benthic ecosystems, disrupt habitats, and have long-lasting consequences. Additionally, excessive sediment deposition on seafloors can smother benthic organisms and drastically alter the composition of their habitats.
Furthermore, the increasing impact of climate change and intensifying wave patterns pose a specific threat to organisms inhabiting intertidal zones, often referred to as "splash zones." Organisms in these areas have developed specialized adaptations to withstand mechanical forces from waves. However, as wave intensity continues to grow due to climate change, these adaptations can be compromised. Stronger waves can lead to erosion and result in habitat loss for the benthic organisms residing in these critical areas.
In summary, the benthos face multiple challenges, including temperature-related stress, habitat disruption due to human activities, and the increasing strength of ocean waves, all of which threaten the health and biodiversity of benthic ecosystems.