Sacculina

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1) Sacculina: Morphology, Parasitism, and Ecological Impact of the Parasitic Barnacle

1.1) Classification of Sacculina

1.2) Habit and Habitat

1.3) Geographical Distribution

1.4) General Characteristics

1.5) Special Features

1.6) Identification

1.7) Life Cycle and Reproduction

1.8) Ecological Role and Impact

Sacculina: Morphology, Parasitism, and Ecological Impact of the Parasitic Barnacle

Sacculina is a genus of parasitic barnacles belonging to the infraclass Rhizocephala. Unlike typical barnacles that live attached to hard substrates, Sacculina species are parasitic castrators of crabs. Their adult form bears little resemblance to conventional barnacles; instead, they exhibit a unique morphology adapted for internal parasitism and host manipulation. Sacculina is renowned for its remarkable life cycle and the profound effects it exerts on its crab hosts, including castration, behavioral modification, and altered physiology, making it a fascinating subject in parasitology and marine biology.

Classification of Sacculina

Taxonomic Rank	Name	Characteristics
Kingdom	Animalia	Multicellular eukaryotic organisms
Phylum	Arthropoda	Segmented animals with jointed appendages
Subphylum	Crustacea	Aquatic arthropods with exoskeleton
Class	Thecostraca	Barnacles and related groups with calcareous plates
Infraclass	Rhizocephala	Parasitic barnacles with root-like internal structures
Family	Sacculinidae	Parasitic castrators of crabs
Genus	Sacculina	Parasitic barnacles that invade crab hosts and manipulate them

Habit and Habitat

Sacculina species are marine parasites primarily infecting various crab species, especially green crabs (Carcinus maenas). Their larval stages are free-swimming and pelagic, searching for suitable crab hosts. Infection occurs when the female larva attaches to a vulnerable part of the crab and injects its soft body inside, losing its shell. The parasite’s internal root-like structures (interna) spread through the host’s body, absorbing nutrients while the external reproductive sac (externa) protrudes from the crab’s abdomen.

These parasites inhabit coastal waters with abundant crab populations, often influencing the dynamics of crab communities.

Geographical Distribution

Sacculina is largely found in the Eastern Atlantic Ocean, where green crabs, the primary hosts, are native. The parasite has not widely spread with its host to new regions. It inhabits temperate and tropical coastal marine waters where crabs thrive.

General Characteristics

Commonly called as root-headed barnacle.
Parasitizes crabs. Zoologists were able to discover its real crustacean nature by studying its embryology having first stage nauplius larva.
Adult loses all arthropodan characters and appears like a fleshy tumour attached to the abdomen of the crab on ventral side and lead a parasitic life on decapod crustaceans.
Through peduncle sends root like processes like mycelium of fungus in each appendages of the crab and in the body to derive nutrition.
Appendages, segmentation, mouth, anus, alimentation absent.
Hermaphroditic.
Their presence causes development of many changes in secondary sexual characters of the host.
Body Form: Adult parasite comprises two main parts—the externa, a sac-like reproductive body outside the crab, and the interna, an extensive root-like system inside the host.
Size: The externa can vary from a few millimeters to centimeters protruding from the host’s body.
Color: The externa is typically reddish or pale.
Morphology: Parasitic and greatly reduced compared to free-living barnacles.
Host Interaction: The interna wraps around crab organs, especially the hepatopancreas, for nutrient absorption.

Special Features

The nauplius larva resembles that of other crustaceans but without alimentation. Nauplius is free swimming and changes to next larval stage called Cypris larva. It attaches itself on decapod crab and then discards its shell and burrows into the host’s body. It becomes a mere mass of cells that passes in the crab’s blood stream, attaches to host’s intestine and grows. When the crab molts, part of parasite protrudes in the abdomen’ as an ovary packed with egg. The remaining part of Sacculina becomes branched ramifying throughout the host’s body and absorbing nourishment. The host neither grows nor breeds because its sex organs degenerate due to crab. This is called as parasitic castration.
Parasitic Castration: Suppresses the reproductive system of its crab host, often sterilizing it completely.
Behavioral Manipulation: Infected crabs exhibit altered behavior, including caring for the parasite’s externa as if it were their own eggs.
Sexual Manipulation: Male crabs infected by Sacculina may feminize, developing female-like characteristics and behaviors.
Complex Lifecycle: Involves free-swimming larvae, injection into host, internal parasite growth, and external reproductive sac.
Impact on Host Physiology: Infected crabs cease molting and lose the ability to regenerate claws, affecting survival.

Identification

Sacculina can be identified by:

Presence of the externa sac on the underside of crab abdomen.
Root-like interna structures revealed via imaging or dissection.
Behavioral symptoms in infected crabs, such as egg care in males.
Host species and infection signs.
Molecular identification from larvae or parasite tissues.

Life Cycle and Reproduction

The female Sacculina larva finds a crab host and injects its vermigon larva into the crab’s body after shedding its hard shell. Inside, the parasite grows the interna, spreading root-like tendrils, absorbing nutrients, and controlling the host’s reproductive system.

Externally, the parasite grows an externa sac where it produces eggs. Male Sacculina larvae locate the female externa and inject sperm to fertilize the eggs internally. The crab is manipulated to care for the parasite’s eggs, nurturing the next generation of parasites.

Ecological Role and Impact

Population Control: Significantly reduces crab reproductive output and can alter local crab population dynamics.
Ecosystem Influence: By manipulating crab behavior and physiology, Sacculina indirectly affects predator-prey relationships and community structure.
Biodiversity Effects: Infection rates can reach high levels, influencing the diversity and abundance of crab species in affected areas.
Biological Model: Provides insight into host-parasite co-evolution, manipulation, and ecosystem interactions.

References