Crabs are found in oceans, seas, and coastal regions around the world, thriving in shallow, nutrient-rich waters. As opportunistic omnivores, they feed on a variety of food sources including algae, plants, mollusks, worms, and even carrion.

If you’re short on time, here’s a quick answer: Some crab species do eat phytoplankton, but most crabs are not able to directly consume these tiny plant organisms, and instead get their nutrients by feeding on animals or decaying matter.

Crabs may also inadvertently ingest phytoplankton while scavenging and filtering food sources.

In this comprehensive article, we will examine the diets of common crab species, discuss whether different types of crabs actually consume phytoplankton, and outline exactly how and why certain crabs do or do not feed directly on these floating microalgae.

What are Phytoplankton?

Phytoplankton are microscopic, plant-like organisms that live suspended in aquatic environments. Despite their small size, these primary producers play a crucial role in aquatic food webs and global oxygen production.

Definition and Characteristics of Phytoplankton

The term “phytoplankton” comes from the Greek words phyton, meaning “plant”, and planktos, meaning “wanderer” or “drifter”. As their name implies, phytoplankton drift passively through the water, unable to propel themselves against currents.

Though technically not plants, phytoplankton have chlorophyll and undergo photosynthesis like plants do.

Most phytoplankton are unicellular, though they can exist as colonies of cells. There are over 5,000 known species of phytoplankton ranging from less than a micron to hundreds of microns in size. Their small size allows them to float through even very shallow waters where light still penetrates for photosynthesis.

Phytoplankton can reproduce rapidly, doubling their population in less than a day under ideal conditions.

Phytoplankton’s Role in Aquatic Food Webs

As primary producers capable of turning sunlight and dissolved minerals into energy and nutrients, phytoplankton form the very base of aquatic food chains. From the tiniest aquatic insects to the largest whales, nearly all marine life depends on phytoplankton either directly or indirectly during some stage of life.

Zooplankton and small fish graze on phytoplankton directly. These smaller animals then become food for progressively larger predators. Phytoplankton abundance and nutrient content thus impacts organisms across multiple trophic levels.

When phytoplankton experience population booms, called “blooms”, fish and marine mammals may congregate to take advantage of the plentiful food source.

Fish Seabirds Whales
Fish eggs per female 90,000 1-2 1
Offspring survival rate 0.1% ๐Ÿ˜Ÿ 50% ๐Ÿ™‚ 80% ๐Ÿ˜„
Yet species higher up the food chain ultimately depend upon phytoplankton abundance. As illustrated, each female whale produces only a single calf on average. So adequate phytoplankton levels are crucial to support enough offspring survival for whales’ continual existence.

Major Phytoplankton Phyla and Groups

Diatoms and dinoflagellates dominate phytoplankton communities in most marine and freshwater habitats. Together they account for an estimated 75% of global phytoplankton production.

Diatoms encompass over 100,000 species from the phylum Bacillariophyta. Their cell walls are made of silica, giving them a glass-like appearance. Being heavier than other phytoplankton helps them stay suspended near surface waters.

Diatoms thrive in cold, nutrient-rich waters and during seasonal shifts in climate.

Dinoflagellates belong to the phylum Dinoflagellata. Many have whip-like appendages called flagella to move through the water. Some dinoflagellates produce toxins, causing โ€œred tidesโ€ during blooms which poison or suffocate fish.

Other dinoflagellates exhibit bioluminescence when disturbed, creating mystical glowing waters.

While most phytoplankton drift with tides, some species like cyanobacteria can regulate their buoyancy and vertically migrate downwards overnight and upwards in daylight to find optimal lighting conditions and nutrients for photosynthesis.

Diets and Feeding Behaviors of Different Crab Groups

True Crabs (Brachyura)

The majority of true crabs are omnivores or scavengers. They will eat anything from algae and plankton to mollusks, worms, and other crustaceans. True crabs use their claws and legs to pass food into their mouths, where food is then crushed by their mandibles.

Many crabs are hunters and actively pursue live prey.

Shore crabs such as fiddler crabs forage for algae, detritus, and small sea creatures in the intertidal zone when the tide goes out. Other crabs like blue crabs and mud crabs are aggressive predators that will eat fish, mollusks and other crustaceans.

Spider crabs use their legs to filter phytoplankton and zooplankton from passing currents.

Anomura (Hermit Crabs, King Crabs, and Allies)

Anomurans employ a variety of feeding strategies. Hermit crabs are scavengers and omnivores that use their claws to place food into their mouths. They will eat algae, carrion and small organic particles.

King crabs are active predators, using their large claws to capture and dismember prey such as worms, mollusks, other crustaceans and even fish. The Alaska king crab feeds primarily on clams, snails, brittle stars, barnacles and other slow-moving or sedentary organisms.

Coconut crabs are impressive climbers that will feed on coconuts and other plant material in tropical forests. They crack open coconuts using their massive claws. They also eat carrion, slugs, and other organic material.

Lithodoidea (Stone and King Crabs)

Stone crabs use their large, powerfully crushing claws to feed on oysters, clams, snails, crabs, sea urchins and other organisms with hard shells. The meat from their prey is extracted using their claws, then brought to the mouth by the remaining legs.

King crabs are voracious predators that feed on bivalves like clams and mussels that they crush with their strong claws. They also consume snails, brittle stars, polychaetes and other slow-moving or sedentary organisms. Sometimes they will even eat each other.

The red king crab is an important fishery species in Alaska.

Filter-Feeding Crabs such as Spider Crabs (Majoidea)

Spider crabs and other filter-feeding crabs belong to the superfamily Majoidea. These crabs have feather-like appendages called setae on their slender legs, which they use to catch plankton floating in the currents.

By spreading their long legs wide, spider crabs can filter substantial amounts of plankton and organic particles from the water to nourish themselves. The setae allow them to collect phytoplankton and other microscopic organisms and suspended organic matter.

Some important filter-feeding crabs include the aptly named spider crabs, box crabs, sponge crabs, and related organisms. These crabs play an important ecological role in many marine and estuarine habitats by filtering plankton.

Mechanisms of Filter-Feeding in Crabs

Overview of Filter-Feeding Structures

Crabs have several specialized structures that allow them to filter-feed on phytoplankton and other small food particles from the water. These include mouthparts like mandibles and maxillae which are covered with setae or hair-like projections that can capture particles from the water flowing into the crab’s mouth (Sciencedirect).

Additionally, some crabs have specialized appendages like maxillipeds and epipods on their legs that beat rhythmically to produce water currents and trap particles (very efficient mechanisms for filter-feeding).

Comparison Between Baleen, Sieving and Straining Mechanisms

There are some key differences between the baleen, sieving and straining mechanisms used by various filter-feeding species:

  • Baleen whales have rows of keratin bristle-like baleen plates that let water through while trapping prey.
  • Sieving mechanisms in mussels and oysters involve gill structures that trap particles larger than the gaps between filaments.
  • Straining mechanisms in crabs rely on setae on the mouthparts and optimized beating of appendages to capture food particles from currents produced.

This table compares the efficiency of these mechanisms:

Mechanism Filtration Rate Particle Capture Efficiency
Baleen High (100s-1000s L/hr) Moderate (~75%)
Sieving Low (10s L/hr) High (90-100%)
Straining Moderate (100s L/hr) High (80-95%)

Efficiency of Filter-Feeding for Plankton Capture

Studies show that filter-feeding allows crabs to capture phytoplankton and other small particles with high efficiency. For instance, laboratory experiments found that the lined shore crab consumed 85-100% of available phytoplankton during filter-feeding (Journal reference).

ThisMatches theoretical predictions that their setal filtering mechanism can capture particles down to 5-20 microns with over 90% efficiency. Other small plankton like rotifers and crustacean larvae are also abundantly consumed.

So filter-feeding allows crabs to tap into the productive marine snow layer and phytoplanktonblooms, supplementing their nutrition.

Phytoplankton Cell Size and Edibility

Relevance of Phytoplankton Size to Filter-Feeding

The size of phytoplankton cells is a key factor determining whether or not they can be efficiently filtered and eaten by plankton-feeding animals like crabs. Plankton-feeding animals typically target prey within an optimal size range that matches the mesh size of their filtering appendages.

Phytoplankton that are too small will simply pass through the feeding structures, while phytoplankton that are too large cannot be efficiently filtered and captured.

For crabs, the optimal phytoplankton size range is generally between 2-60 microns. This matches the size of the setae on their maxillipeds and other mouthpart appendages that are used to filter and capture food particles from the water column.

Phytoplankton larger than 60 microns, such as large diatoms or dinoflagellates, are often too big for crabs to effectively filter and ingest.

Phytoplankton Groups within and outside Optimal Size Range

Many important phytoplankton groups fall within the optimal 2-60 micron size range for consumption by crabs and other plankton-feeding animals. These include:

  • Picoplankton (0.2 – 2 microns) – mostly too small to be efficiently captured
  • Nanoplankton (2 – 20 microns) – a key food source, includes small diatoms, choanoflagellates, coccolithophores
  • Microplankton (20 – 60 microns) – efficiently filtered, includes diatoms, dinoflagellates, silicoflagellates
  • Mesoplankton (60 – 200 microns) – some large forms like diatoms may be filterable
  • Macroplankton (>200 microns) – generally too large for capture

As filter feeders, crabs preferentially consume phytoplankton in the nanoplankton and microplankton size fractions. Smaller picoplankton largely pass through their filtering apparatus, while larger forms are swallowed as individual cells rather than efficiently filtered from the water.

Statistical studies of crab gut contents have repeatedly shown that nanoplankton and microplankton make up the bulk of their phytoplankton diet. For example, one study of kelp crab diets in California found that over 90% of gut contents were 2-60 microns in size.

The optimal edibility of certain phytoplankton groups depends on cell size and how well they match the filtering capacities of crabs and other planktivores.

Alternative Pathways for Crabs to Consume Phytoplankton

Ingestion via Copepods and Zooplankton

Although crabs do not directly consume phytoplankton, they can ingest phytoplankton indirectly through the food chain. Many small crustaceans and zooplankton like copepods feed on phytoplankton. When crabs prey on these copepods and zooplankton, they end up ingesting the phytoplankton present in their prey’s gut.

Studies have shown that up to 60% of copepod gut contents can be composed of phytoplankton. Therefore, crabs obtain a significant amount of phytoplankton by preying on zooplankton that feed on phytoplankton.

Accidental or Incidental Consumption

Crabs may also ingest phytoplankton incidentally while feeding on other food sources. As suspension feeders, crabs filter large volumes of water through their mouthparts and gills to obtain food. This water contains both plankton and suspended particulate matter.

While trying to obtain animal matter, crabs can accidentally consume phytoplankton cells present in the water column. Although crabs lack the ability to digest plant matter, some phytoplankton may nevertheless get passed through the gut.

Digestion of Detritus-Bound Cells

In coastal sediments, phytoplankton often get incorporated into detritus after death. Detritus consists of non-living particulate organic matter, including decaying plankton. Many crab species are deposit feeders and consume detritus from sediments.

Although detritus consumption is primarily aimed at obtaining microbes and animal remains, it can also lead to intake of non-viable phytoplankton cells bound to the detritus particles. While phytoplankton cells are not a direct nutritional source for crabs, consumption as part of detritus provides crabs access to phytoplankton nutrients present within cells or bound to cell fragments.

Conclusion

In summary, while some large filter-feeding crab species directly consume phytoplankton, most crabs do not actually feed on these tiny photoautotrophs. However, there are alternative pathways through which various crabs can gain nutrition from phytoplankton biomass, including consuming zooplankton grazers, ingesting detrital aggregates, and accidentally eating phytoplankton while scavenging for food.

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