MarLIN Glossary

Glossary items


The place in which a plant or animal lives. It is defined for the marine environment according to geographical location, physiographic features and the physical and chemical environment (including salinity, wave exposure, strength of tidal streams, geology, biological zone, substratum, 'features' (e.g. crevices, overhangs, rockpools) and 'modifiers' (e.g. sand-scour, wave-surge, substratum mobility). (Cf. 'environment').

habitat complex

Major divisions of the environment based on physiographic conditions, (such as exposure and substratum) which represent major differences in biological character (e.g. exposed littoral rock, infralittoral muddy sands). They are equivalent to selection units for intertidal Sites of Special Scientific Interest (Connor et al., 1997a & b).

Habitats Directive

The abbreviated term for Council Directive 92/43/EEC of 21 May 1992 on the Conservation of Natural Habitats and of Wild Fauna and Flora (Commission of the European Communities 1992). Known until about autumn 1994 informally as the "Habitats and Species Directive".


Another term for saline (q.v.).


A horizontal boundary layer in the water-column, at which salinity changes sharply with depth.


Plural of 'hapteron'.


Root like structure of macroalgae holdfasts.


A sheltered port, i.e. a place in which a vessel can lie in a position of more or less shelter from the elements with a view to the loading or discharge of cargo.

heavy metal

A generic term for a range of metals with a moderate to high atomic weight, for example cadmium, mercury, lead. Although many are essential for life in trace quantities, in elevated concentrations most are toxic and bioaccumulate, and so are important pollutants.


Organisms which feed on plants, including phytoplankton.


Having both male and female reproductive organs in the same individual (in animals) or flower (in plants) (Lincoln et al. , 1998). Capable of producing both ova and spermatozoa either at the same time (permanent or synchronous) or sequentially (cf. protandry, protogyny, gonochoristic) (adapted from Barnes et al. , 1993).


Capable of producing both ova and spermatozoa either at the same time (permanent) or sequentially (cf. protandry, protogyny, gonochoristic) (Barnes et al. , 1993).

hermit crab

A small lobster-like crustacean which inhabits the empty shells of other animals.

hermit crabs

Plural of 'hermit crab'.


An attachment structure that anchors macroalgae to the substratum, which resembles a collection of roots (see hapteron) but has no nutrient gathering role.


A term used for organisms that freely inhabit fresh water, seawater and brackish water, or which establish populations in all these environments (from Lincoln et al. , 1998).


A term used for animals that are confined to a benthic existence throughout their life cycle (from Lincoln et al., 1998).


A term used of aquatic organisms that remain pelagic throughout the entire life cycle (from Lincoln et al., 1998).


Plankton with a completely pelagic life cycle (cf. meroplankton) (from Baretta-Bekker et al. , 1992).


The Class Holothuroidea (Phylum Echinodermata), are characterized by elongate, cylindrical animals, tapered at each end with a mouth surrounded by retractile feeding tentacles (Hayward et al., 1996), e.g. %Holothuria forskali%.


In Shetland - a small sheltered tidal lagoon (from Scott & Palmer, 1987) (cf. 'vadill').


The feeding polyp of a hydroid colony, bearing stinging tentacles and the mouth (adapted from Cornelius, 1995).


Plural of 'hydranth'.

hydraulic dredge

Bottom sampling equipment for collecting benthic sediment and organisms, towed along the seabed using pumped water to draw material up a tube. Such dredges are also used for the commercial collection of benthic organisms, e.g. cockles.


Organic compounds containing mainly hydrogen and carbon; the basic constituents of fossil fuels.


Plural of 'hydrocaulus'.


The main stem, usually composed of a tubular coenosarc protected by a chitinous tube, the perisarc (adapted from Cornelius, 1995).

hydrodynamic regime

The particular conditions of water movements at one particular site, including wave action, tidal streams and residual currents (based on Earll, 1992).


See 'hydrography'.


The scientific study of seas, lakes & rivers (cf. 'hydrology').


A general term for members of the cnidarian Class Hydrozoa, and includes 'sea firs' and 'white weeds'.


Plural of 'hydroid'.


The study of the distribution, conservation, use etc. of the water of the earth and its atmosphere (from Makins, 1991) (cf. 'hydrography').


The stolon system of a hydroid colony. In many species it is branched or mesh-like but in some athecates the stolons may fuse to form a mat or other structures (Cornelius, 1995).


Plural of 'hydrorhiza'


An ecological succession beginning in a habitat with abundant water, typically on the submerged sediments of a standing water body (e.g. lake, loch or lagoon), and ending on dry land (based on Lincoln et al., 1998).


The chitinous cup surrounding the hydranth in thecate hydroids, typically large enough to accommodate the hydranth (Cornelius, 1995).


Plural of 'hydrotheca'.


The Class Hydrozoa (of Phylum Cnidaria) are characterized by large colonies of simple polyps. The polyp (or polypoid) form dominates. Polyps vary in structure but take the general form of a central mouth and buccal cavity surrounded by stinging tentacles. Polyps arise from a series of tubular stolons and stalks that are covered in a chitinous sheath. Colonies take the form of mats of stolon, with upright stems that may form delicate branching structures (adapted from Hayward et al. 1996). Exceptions exist, such as the By-the-Wind-Sailor (%Velella velella%) and Siphonophores.


Living above but close to the substratum (Lincoln et al., 1998).


The lower, cooler, non-circulating water in a thermally-stratified body of water, often prone to oxygen depletion (based on Allaby & Allaby, 1990).