200
ON A MARINE
AQUARIUM.
BY D P . DUDLEY W . COLUNGS, M . B . ,
F.Z.S.
THE sea at Southwold is practically almost always opaque and it is only for a very few weeks in the summer that the water becomes clear. The reason is that the sea-bed, for some distance from the shore, consists chiefly of drift sand, part of which is very fine and kept in suspension by the movement of the water. There are few rocks and therefore no clear rocky pools and, except for the wooden pier and the concrete of the harbour, no homes for Anemones or Molluscs such as Musseis or Winkles or Limpets. As a result of these conditions the study of local marine-life is extremely difficult, and possible only in an aquarium where the water can be kept reasonably c'ear. I have such a one : a small tank with plate-glass sides, 24 inches long and 15 X 15 in section. This has enabled me to make the following brief notes on its varying inhabitants. The water is strained and the bottom covered with about two inches of washed sand and a few smooth stones. The Brown Shrimp (Crangon vulgaris), our commonest Crustacean, is a true sand-lover and spends most of its time buried in the sand. It " digs itself in " at an astonishing speed : with its swimmerets it first excavates a shallow trench in the soft sand ; then lifts up its body longitudinally and brings it down suddenly and forcibly, thus deepening the hole and raising a small ridge of sand on either side. It finishes its work, being by now almost covered, by sweeping the ridges of sand over its body by means of its long extended antennae, using them effectively as arms. Usually all that can be now seen are the stalked eyes, which are almost invisible. The whole process takes about two or three seconds. These Shrimps occasionally swim round the tank close to the surface of the water, especially at night. They appear to locate their food by smell entirely : a small piece of fish, dropped anywhere on the bottom, soon brings the Shrimps to their meal. On the other hand, I have watched a Shrimp scuttling about among attractive food, when it seemed only as a current of water conveyed the " odour " from a fragment that the Shrimp hurriedly seized it. Owing to fear of looting apparently, the successful Shrimp holds its portion under its thorax ; and rc-buries itself to enjoy its meal in safetv and comfort. Crangon vulgaris is very tenacious of life : it will live for hours in a dry basket and, when apparently dead, will revive in a few minutes when put into sea-water.
ON A MARINE AQUARIUM
201
The /Esops Prawn (Pandalus annulicornis) is a very interesting species to watch. It is almost completely transparent with very fine, bright-red lines on its body and two very long antennae, which often appear to be in its way. It never burrows, and is fond of swimming round near the surface. At rest, it sits on a stone or clings to the glass, where it is a little rough from the presence of Algae. It is much more delicate than the Brown Shrimp : its mortality rate is high owing, I believe, to its requiring a higher degree of aeration in the water and a comparatively lower temperature. Crabs are awkward occupants, unless they can be kept almost entirely by themselves ; and then, if possible, they should not be larger than one inch across the body. They are all rather pugnacious ; but the worst is the Small Swimming Crab (Portunus depurator), which I carefully avoid : on the only occasion when, by accident, I introduced one, it celebrated its arrival by killing a Shrimp about twice its size and starting to eat it, the whole episode lasting aboutfifteenseconds. I remember, some years ago, putting one into a pail of sea-water containing three Whiting (Gadus merlangus) of about five inches in length : the Crab immediately attacked one and killed it by grasping its head between the eye-sockets with one of its elaws, which are very sharp. The Edible Crab (Cancer pagurus) is the quietest and most sluggish of them all. I have not seen it attempt to catch a Fish or Shrimp ; but it appears to be fond of Musseis. I usually keep a few small Musseis and " water-cleaners " in the tank ; but, while this Crab was there, it broke open their shells and devoured the occupants. The Edible one has, I think, the most powerful grip of any of its fellows. All small Crabs appear to like Barnacles as an article of diet. Curiously enough, a Little Blenny (Blennius pholis) appears to share the same taste : it would sit down close to a Barnacle, like a Cat watching a Mousehole. Nothing happened until the Barnacle opened and started " sweeping in " with its net; then the Blenny snapped and the Barnacle, or the greater part of it, was eaten. The Common Hermit-crab (Eupagurus Bernhardus) inhabits derelict shells varying according to its size from small Winkles to those of fully grown Whelks. Small Hermit-crabs are most interesting if a supply of spare shells is kept handy : I dropped one I had just caught into the tank and it feil close to another, apparently stitable and so desirable, shell which was lying with its mouth upwards. The Crab immediately seized it with one of its claws while, with the other, it examined the shell very carefully both in- and out-side. Then it held the edges of the two shells closely together with one claw and, freeing its body, threw it over into the new shell, much like a boy leaping a fence while holding the top rail by one hand. It wriggled itself about, apparently not sure that the new shell was a good fit. At last it appeared satisfied and then, and not tili then, let go of its old shell which it had held until that moment.
202
ON A MARINE AQUARIUM
In a model-yachting pool of sea-water I found many Threespined Sticklebacks, and a vast number of Ditch Prawns (Palaemonetes varians). I added a supply of both to my tank, where they quickly became at home. These Fish are very active and have large appetites, being quite friendly with the other inhabitants. They well show the power of altering their colour, common to numerous Fish. In the daytime they are very light in tint, sometimes difficult to see : at night they become almost black. These Sticklebacks live indifferently in either fresh or brackish or salt water, and I have seen them make their nests and bring up their young in water containing any degree of salinity. T h e Ditch Prawns are quaint Crustaceans : whitish and almost quite transparent, with black eyes like many similar species. Their most conspicuous organ is the stomach, especially when filled with food. I t is distinctly weird to see what seem to be small pieces of fish or shrimp floating about without apparent means of locomotion. Ditch Prawns do not mind h e a t : when I have been catching some, the water has occasionally been appreciably warm to the hand. This Prawn has also the curious habit of throwing itself into the air when I draw off the aquarium water to replace it with a fresh supply. When only about three inches are left, there is a constant splashing, as the Prawns j u m p out of it. Many of them stick on the wet sides above water-level, where they stay for some seconds before quietly slipping down into the water. When bringing them from the pond, I use three 2-lb. glass jars Standing upright together on newspaper in a basket. O n e day on arriving home I did not find a single Prawn : they had all jumped out and lay on the paper. When returned to the jars, they seemed none the worse. Many small Crustaceans occasionally shed their skins but few survive the operation for, when thus deprived of their protektive covering, they are killed by other of the tank's occupants. It is necessary to aerate the tank's water continuously, with one of the small electric air-pumps now obtainable, in order to keep its inhabitants healthy and active.
A G E N I A L JANUARY.—Members may be interested to hear that during this week I have picked Hasel catkins füll of pollen, with female flower nearly in füll bloom ; Honeysuckle with green frills ; and, in my garden here, quite unsheltered Christmas Roses, Iris stylosa, Marigolds, Yellozv Jasmine, Primroses Primrose zcanda, Alpine Pansies, a Snoivdrop, Cheiranthus and several half-opened pink and white Chauband Carnations.—DOROTHY GREGORY, Burneston, Hintlesham ; 15 Jan. 1948.