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Intro + Plankton
Phytal + Nekton

The Benthos encloses the communities of plants and animals that live permanently in or on soft and rocky bottoms.

  • Psammon (sandy soft substrate): Along exposed gravel and sandy shores of the sublitoral, waves produce an unstable benthic environment. Few plants can attach to the shifting substrate, and therefore few grazing animals are found. When currents deposit sand in quiet coves and bays, the habitat is more stable. Here, the size and shape of the sediment particles and the organic content of the sediment determine the quality of the environment. The size of spaces between particles regulate the flow of water and the availability of dissolved oxygen, thus enabling the establishment of mesofauna. Such communities made up of microscopic plants and animals live in-between the grains of sand along sea shores and at the bottoms of the oceans.
    Most sand and mud animals are detritus feeders, and most detritus is former plant material that is gradually degraded by bacteria and fungi.

Procedure of extraction (50kB)

Gastropods (XXkB)

Lancelets of the phylum acrania, include one characteristic species, the Branchistoma lanceolata. This is a hydro-dynamically shaped organisms that can swim, but usually lies buried in sand with only the anterior-ventral end protruding outward. Its elongated body with tapering ends, is compressed from side to side, has small median fins and a pair of lateral finlike metapleural folds. Swimming and burrowing are accomplished by the contraction of the myomeres. It feeds by pumping water into the mouth through the oral hood, whose edges bear a series of delicate cirri, that act as a strainer and exclude larger particles, while filtering minute suspended food particles out of the water. The mouth is located in the center of a transverse portion known as the velum. Water in the pharynx escapes into an atrium through 100 or more pharyngeal slits. Larger particles continue posteriorly, whereas small ones are deflected into the midgut cecum. As this mass is passed down the gut and rotated by ciliary action it is degraded further by enzymatic reaction. Fecal material is discharged through the posteriorly located anus. Absorbed nutrients are distributed by a circulatory system. Although no heart is present, hemolymph is propelled by the contraction of the arteries. Apart from excretion via diffusion, excretory organs are segmentally arranged in groups of protonephridic solenocytes that lie in the dorsal coelomic canal. The nervous system consists of a tubular nerve cord located dorsally to the notochord.

Brachistoma lanceolata (70kB)


Sandy substrate (XXkB)

Nematods constitute a phylum of widespread and abundant animals. They occur in enourmus numbers in the interstitial spaces of marine bottom sediments. These animals are usually elongated, radially symmetrical with tapered ends. This threadlike, minute size enables them to progress through the sand grains by whipping and undulatory movements. The thick, complex, collagen cuticle is composed of several layers and is shed up to four times in its life. Unlike the exoskeleton of arthropods, the nematode cuticle grows between molts.

Nematods + Microhedylidae (50kB)

The echinoids (sand dollars - phylum echinodermata) feed on detritus particles found between the sand grains. Holothuria (sea cucumbers) which as echinoids belong to the same phylum echinodermata, do so as well. Sea cucumbers are found wedged in cracks and crevices as well as on the flat sandy benthos. Some burrowing sea cucumbers purify the sand that is high in organic detritus which supports a large quantity of bacteria.

Endofauna in situ (65kB)

Most burrowing lamellibranchs (phylum bivalves) inhabit sandy bottoms. To reduce the intake of sediments, siphonal extensions of the mantle that are separated into an inhalant and exhalant openings, are yet another adaptation.

Polychatea are the dominating group among annelids in any sandy substrate. They reflect metamerism (segmentation of the trunk); i.e. neither prostomium (mouth & head), nor the pygidium carrying the anus, are segmented. Polychaets bear paired parapodia on each segment. Although these can vary in shape and size, each is composed of a dorsal notopodium and a ventral neuropodium, both containing a large number of setae (chitinous bristle) that provide traction against the substratum.

The epibentic sponge Suberites domuncula houses a hermit crab Paguristes oculatus which carries the sponge over the sandy substrate. This hermit is a rather sluggish filter feeder, and thus less advantaged by its heavy load than an actively scavenging hermit would be.


Polychaeta (60kB)

Hermit and sponge (30kB)

  • Pelos (muddy soft substrate): Beach sand is fairly coarse and porous, graining and losing water quickly, while the fine particles of mud hold more water and replace it more slowly. Oxygen is not resupplied quickly, and metabolic waste products are removed slowly or not at all, giving rise to anoxic conditions that produces H2S (rotten egg-smell), which is the main reason why pelos does not support an interstitial fauna.
    As in psammon, the community of microscopic organisms of the pelos live on (epi-) or in (endo-) the muddy or soft substrate. Some buried bivalves that live below the oxygen level in the anoxic zone of the pelos use their siphons to obtain food and oxygen from the water at the muddy surface.

Quite a substantial amount of the scanned psammon, consists of shells from mussels and other snails that share a common lethal feature; certain herbivorous gastropods use the radula as a drill to penetrate the shells of their victims before softening and feeding upon the inner tissues with secretions from a gland on the foot or the proboscis.
Annelids not only exist on/in sandy bottoms, but have also conquered the muddy substrates. Chaetopteridae feed by filtering water through a mucous bag. These species live in a large U-shaped tube composed of a parchment-like material. Three piston-like parapodia propel water through the tube. At the rear end of the bag, the food-laden mucus is constantly rolled into a pellet, and when this reaches a certain size the fans stop beating and the pellet is carried forward by the cilia to the mouth.

Mud sampling (90kB)

Extraction procedure (95kB)

Chaetopterus variopedatus (60kB)

  • Hard Substrate: Sedentary organisms use hard substrate as a temporary or permanent site of residence. To maintain growth and reproduction, these organisms evolved special techniques to guarantee their survival; the most common way to to so is achieved by stretching out their tentacles into the passing water currents; the size and strength of these tentacles already provide some clues about the source and type of captured organisms the animal feeds on.
Hard substrate provides the perfect settling ground for the stalkless free moving crinoidea Antedon mediterranea of the phylum echinodermata. They are suspension feeders of planktonic organisms and thus have their oral surface directed upward into the water (compared to sea stars and sea urchins which have it oriented towards the substrate). The rootlike cirri at the aboral end hang free during locomotion, but cling to the hard substrate once at rest. Its pentamerous crown bears five arms that branch close to their bases into ten, each of these with many alternating pinnules.
These serve not only as filtering organs but are also the site where offspring leave the parent organisms. The ciliated grooves along each arm converge at the mouth in the center of the oral surface to which trapped plankton is conveyed to.

Caryophyllidae are the few hard coral building cinidaria present in the Adriatic Sea. They feed on microscopic organisms in which the individual polyps of the colony represent the capturing entities.

Actinia equina (60kB)

Antedon mediterranea (XXkB)

Pinules of (50kB)

Caryophyllia inornata (XXkB)
  • Secondary Hard Substrate: This substrate includes particles of organic and inorganic matter that accumulate in a loose unconsolidated form of biogenous, lithogenous or cosmogenous origin; usually beyond the compensation depth (aphotic or mesopelagic zone). Huge number of micro-organic debris that have slowly accumulated on the ocean floor (e.g. foraminifera silica-made, diatoms of cooler waters w/ silica housing), and others join with detrital products of disintegration of preexisting rocks and of volcanic ejecta, ronoff by rivers, or glaciers; redistribution through waves and currents including relocation of desert sand by wind into the sea.
    Under certain circumstances, such sediments can harden into a compact conglomerate, thus providing a new nesting substrate.
    Other means of substrate formation are displayed by the tube worms of the phylum annelids. Their secretions form a leathery or calcareous tubes in which they live and extend only their feathery tentacles to strain food from the water. It is this tube that provides the substrate required for minute sedentary organisms.
Serpulidae of the phylum polychaeta even can settle as epifauna on certain brown algae, as shown by Spirorbis pagenstecheri which often settles on Padina pavonia.



Spirorbis pagenstecheri (90kB)
Benthic animals are found at all depths and are associated with all substrates. About 80% of the benthic animals belong to the epifauna; these are the animals that live on or are attached to the surface of rocky areas or firm sediments. Animals that live buried in the substrate belong to the endofauna (infauna) and are commonly associated with soft sediments such as sand or mud.
Some animals of the sea floor are sessile as adults (cirripedia, anthozoa, bivalvia), while others are permanently motile (crustacea, echinodermata, and molluscs). Most benthic forms produce motile larvae that spend a few weeks of their live cycles as meroplankton (biphasic life-cycle), allowing the species to avoid overcrowding and to colonize new areas.
As indicated in the map and the surrounding islands of Rovigno, various sites have been dredged to obtain are presentative sample. Off-shore samples of the benthic fauna were made by dredging the sandy sea floor with the institute's own vessel.
  • The epifauna includes all those organism that spend most of their feeding activity on top of the benthic surface.
    Echinoderms, the most prominent phylum of motile deuterostomia are frequently found on the benthos. Asteroids (sea star) bear calcareous spines that may project out of the dermal endoskeleton. In-between these spines are fingerlike projections called papulae which guarantee the respiratory requirements of the animal. To prevent sedentary orgamisms from settling on the outer dermal wall, pedicillaria (ossicles that form a minute jawlike appendage) bite off any sedentary docking tentative in its initial phase. The ampulla and podia (part of the internal vascular system) are used in locomotion. Muscular contraction of the ampulla forces fluid into the podium and at the same time closes a valve in the lateral canal, preventing backflow. Hydraulic pressure and asymmetrical muscle contraction extend the podium and swing it forward. The sucker at the tip is then brought into contact with the substratum. Following adhesion, longitudinal muscles of the podium contract, shortening the podium, forcing water into the ampulla, and pulling the body forward. Each podium performs a little step, and the combined activity of all the podia enables a sea star to grip objects tenaciously and to crawl about. Coordination of podia by the leading arm results in directed motion.

Dredging the sea bed (100kB)

Dredging procedure (30kB)

Seastars (60kB)

Going offshore into the infralittoral, the dredge yields a different picture.
Some Echinoidea (sea urchins) such as the sand dollars and heart urchins are adapted for burrowing in sand. Both groups lack arms and the body are spherical or discoidal. The skeletal ossicles are flattened plates fused together to form a rigid internal shell, and the body surface is covered with moveable spines mounted upon tubercles. Soft bottom echonoids are selective deposit feeders and use the podia to collect food particles.
A yet another intresting organism among the rich catch was the ascidian Microcosmos sulcatus (described later on in this report when talking about the underwater cave) - once cut open, turned out to be a gourmets joy; a real raw delicassy.
Holothuria (sea cucumber) are somewhat less apatizing. They are similar to sea urchins in lacking arms but the oral-aboral axis is greatly lengthened, so that the animal has a worm-like or cucumber-like shape that lies on its side. Many sea cucumbers live on hard substrata. A few live exposed on the surface, while others lodge themselves beneath and between stones. The body wall of connective tissue can shift rapidly from a flaccid to a rigid state. Some species like Leptosynapta minuta even embed calcareous spicules into the skin.
Hard-bottom sea cucumbers move by means of podia; in some species, three ventral ambulacra are kept against the substratum as a sole and the two dorsal ambulacra have reduced podia. Cucumbers that live in sand or mud generally construct burrows with one end or two openings to the surface. The oral end of the body, a circle of tentacles, representing modified podia surrounds the mouth. The tentacles are stretched out to collect plankton or detritus from the surrounding sea water or sea bottom. They are then retracted and stuffed into the mouth, one at a time.
Some sea cucumbers posses a cluster of tubular evaginations from the base of the respiratory trees. These tubules of Cuvier, can be shot out of the anus. They elongate in the process and are adhesive. A disturbing intruder or predator can be entangled in such a way that the mesh of adhesive threads becomes a death trap.
Evisceration instead, is the process in which a sea cucumber will rupture the anterior and posterior end of the gut and discharges it along with some internal organs via the anus. Although eviscerated animals are commonly seen under lab-conditions, sea cucumber do so from time to time also in their natural habitat to expel the waste laden internal organs, which are then regenerated.

Seastars anatomy (65kB)


Sole (XXkB)


Holothuroids and Nemertini (65kB)

Defensive tubules of Holothuria (100kB)

  • The endofauna groups all organisms that spend most of their lives in the substrate, digging and burrowing.
    Ophiurea (brittle stars) like that of sea stars are composed of long slender arms and a central disc. The arms are nearly filled by a series of large vertebral ossicles, with each vertebra covered by flattened shields. The articulation of the arm ossicles and their musculature give the arms great mobility. Brittle stars move by flexing and pushing with their arms rather than by means of the podia. Arms can be broken off at any point if seized by a predator (autotomy). In the Mediterranean, Ophiurea live in burrows on muddy bottoms. Being deposit feeders, they bury themselves into the pelos. To ensure its nutritional requirement, they just stretch the arms through the substrate into the open water to grasps potential prey.
Along rocky shores of the sublittoral live some rock-boring bivalves of the species Rocellaria (=Gastrochaena) dubia and Lithophaga lithophaga. Both corrode calcareous stones, by drilling a smooth whole into the rock. Upon successful settlement onto a suitable site as a veliger larvae, they reside permanently in that substrate. Caving itself is mediated by acidic secretion which is both time-consuming and a strenous task for these organisms.
As filter feeders, they have to make sure that their in- and exhaling siphons are stretched out into the open water. Getting hold of L.lithophaga involves the fragmentation of the rock itself; unfortunately excessive harvest in certain areas brought this species close to extinction. Being a slow growing species (an adult of the size of 60-70mm is roughly 60-70 years old) it should not be collected anymore for at least the next 20 years.
After death of these borers and under natural circumstances, natural succession enables secondary users, mainly mesofauna, to take advantage of the premade shelter.


Burried britle star (60kB)

Amphiura filiformis (XXkB)


Lithophaga lithophaga (85kB)
  • Such mesofauna includes all the macroscopic organisms that occupy niches, gaps, small caves, or in the case of hard substrate utilize structures left from a burrowing animal as secondary users (whereas, microfauna take advantage of the gaps and spaces in-between the sand grains).

A classical representative of the littoral zone on hard substrate are the Bleniidae. These curious fish know extremely well how to utilize their surrounding environment. Most species of blenniidae do not migrate excessively but stay rather close to their overnight resting crib, hiding place, or site of courting. Whenever blennies withdraw into their cave, the tail enters first and the head last. Regarding their nutritional requirements, most blenniidae are carnivorous, but some follow a herbivorous diet as well. According to their emotional their color pattern can change quite abruptly; which can make determination and classification of blennies quite troublesome.

Blenniidae of the sublittoral (90kB)

Blenniidae and Gobiidae (90kB)

Intro + Plankton
Phytal + Nekton